fix(spotify): device-scope Rust Connect control + ignore foreign-device poll state (multi-bot) [corner-case R4-4]

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -30,7 +30,7 @@
 - **本地音频上传播放** — 在搜索页拖拽或选择本地音频上传，上传后可直接播放 / 下一首播放 / 加入队列；管理员可在 设置 → 行为设置 开关此功能，播放结束或停止/清空/替换队列时会清理服务端接收的本地文件
 - **专属链接（单机器人锁定）** — 通过 `/bot/<id>` 专属链接打开 WebUI 时锁定到单个机器人，刷新后保持，适合把某台机器人的控制页分享给特定用户
 - **频道无人时自动暂停** — 机器人所在频道没有其他人时自动暂停播放，有人加入后自动恢复（**默认关闭**，可在设置中开启）
-- **多平台音源** — 网易云音乐 + QQ 音乐 + 酷狗音乐 + 哔哩哔哩（默认内置），YouTube 可选启用（通过 yt-dlp），统一搜索，结果标注来源
+- **多平台音源** — 网易云音乐 + QQ 音乐 + 酷狗音乐 + 哔哩哔哩（默认内置），YouTube 可选启用（通过 yt-dlp），**Spotify（实验性）** 可选启用（需 Premium + 自建开发者应用，默认关闭，详见 [Spotify 音源（实验性）](#spotify-音源实验性)），统一搜索，结果标注来源
 - **真实客户端协议 (TS3/TS6 双协议)** — 机器人在 TeamSpeak 中可见（非 ServerQuery 隐身模式），自动检测并适配 TS3 和 TS6 服务器，支持 TS6 HTTP Query API
 - **YesPlayMusic 风格 WebUI** — 精美界面，支持深色/浅色主题切换
 - **完整播放控制** — 播放/暂停/上一首/下一首/进度跳转/音量调节
@@ -518,6 +518,131 @@ pip install -U yt-dlp
 - 受 YouTube 风控/地域限制，部分视频可能无法播放
 - `yt-dlp` 更新较频繁，如果播放失败，先尝试升级 `yt-dlp` 到最新版本
 
+## Spotify 音源（实验性）
+
+> **⚠️ 实验性功能，启用前请务必读完本节**
+>
+> - **需要 Spotify Premium 账号。** 免费账号无法通过 Spotify Connect 输出音频，无法使用本功能。
+> - **使用你自己在 Spotify Developer Dashboard 注册的应用（Client ID）。** 本项目**不内置任何共享凭据**，也不会替你代管账号。
+> - Spotify 官方并未开放第三方播放的公开授权，本功能处于 **Spotify 服务条款的灰色地带**，是否使用请自行评估，**风险自负**。
+> - 该音源**默认关闭**（`spotify.enabled = false`），需要手动开启并完成授权。
+> - 这**不是** YouTube 那样的「免登录回退音源」，而是**真正的 Spotify 音频**，必须有 Premium 才能出声；音频链路依赖第三方开源解码器（librespot / go-librespot），本项目仅在本地作为独立子进程调用，**播放效果不做保证**，也未在本仓库端到端测试。
+
+### 工作原理（简述）
+
+1. `librespot`（Rust）或 `go-librespot`（Linux）作为**独立子进程**登录 Spotify Connect 并解码音频，输出原始 **PCM**。
+2. 本项目用内置 **ffmpeg** 把 PCM 重采样到 **48kHz**。
+3. 重采样后的音频接入**现有的 Opus 编码 / 发送管线**（与其它音源共用），推送到 TeamSpeak。
+4. 歌名、歌手、封面等**元数据来自 Spotify Web API**。
+
+### 平台矩阵
+
+后端由配置项 `spotify.backend` 决定，可选 `auto`（默认）/ `go-librespot` / `librespot`：
+
+| 平台 | 默认后端（`auto`） | 说明 |
+|------|------|------|
+| Windows | `librespot`（Rust） | 不支持 go-librespot（FIFO 仅限 POSIX，且官方无 Windows 资产） |
+| Linux / Docker | `go-librespot`（可回退 `librespot`） | `auto` 优先 go-librespot，未检测到时自动改用 librespot |
+| macOS | `librespot`（Rust） | 与 Windows 同，仅支持 Rust 版 |
+
+> `auto` 会按平台与二进制可用性自动选择：优先 `go-librespot`（若可用），否则 `librespot`。若显式指定 `go-librespot` 或 `librespot` 但对应二进制不存在，则该音源保持不可用。
+
+### 获取二进制
+
+程序会先在项目根目录的 `bin/` 中查找，找不到再回退到系统 `PATH`。`bin/` 已被 `.gitignore` 忽略，不影响代码更新。
+
+**Rust librespot（librespot-org，全平台）** — 官方**没有预编译发布包**，需自行获取（任选其一）：
+
+```bash
+# 方式 1：用 Cargo 编译安装（需要 Rust 工具链）
+cargo install librespot
+
+# 方式 2（Windows）：scoop / choco
+scoop install librespot        # 或：choco install librespot
+
+# 方式 3：把可执行文件放到项目 bin/ 目录
+#   Windows:      bin/librespot.exe
+#   Linux/macOS:  bin/librespot
+```
+
+或直接把 `librespot` 加入系统 `PATH`。
+
+**go-librespot（仅 Linux）** — 官方仅提供 **Linux** 预编译资产：
+
+```bash
+# 从 Release 页下载对应架构的二进制：
+#   https://github.com/devgianlu/go-librespot/releases
+# 放到项目 bin/ 目录（或加入系统 PATH）：
+#   bin/go-librespot
+```
+
+> **Windows 不支持 go-librespot**：它依赖 POSIX FIFO（`mkfifo`），官方也只发布 Linux 资产。Windows / macOS 请使用 Rust `librespot`。
+
+### 注册 Spotify 开发者应用 + 回调地址
+
+1. 打开 [Spotify Developer Dashboard](https://developer.spotify.com/dashboard)，新建一个应用，记下 **Client ID**。
+2. 在应用设置里添加 **Redirect URI（回调地址）**，精确填写：
+
+   ```
+   http://127.0.0.1:<webPort>/api/spotify/callback
+   ```
+
+   其中 `<webPort>` 与本项目设置里的 Web 端口一致（默认 `3000`，即 `http://127.0.0.1:3000/api/spotify/callback`）。回调路径必须精确为 `/api/spotify/callback`。
+3. 本项目使用 **Authorization Code + PKCE** 流程，**不需要 Client Secret**（配置里的 `clientSecret` 可留空）。
+4. 授权时请求的权限范围（scope）：
+
+   ```
+   streaming user-read-playback-state user-modify-playback-state user-read-currently-playing playlist-read-private
+   ```
+
+### 启用步骤
+
+1. 进入**设置页**的「连接 Spotify」卡片。
+2. 填入 **Client ID**、选择**后端**（`auto` / `go-librespot` / `librespot`）、打开**开关**。
+3. 点击**保存**。
+4. 点击「**连接 Spotify**」，在弹出的 Spotify 页面完成 **OAuth 授权**。
+
+> 仅当 **`enabled = true`** + **已完成 OAuth 授权** + **检测到可用的后端二进制** 三者同时满足时，Spotify 音源才可播放。任一条件不满足，该音源保持**不可用**（点播会被跳过，播放队列照常前进，不影响其它音源）。
+
+对应的配置块（`data/config.json`）：
+
+```json
+{
+  "spotify": {
+    "enabled": false,
+    "backend": "auto",
+    "clientId": "",
+    "clientSecret": "",
+    "deviceName": "TSMusicBot",
+    "bitrate": 320
+  }
+}
+```
+
+OAuth 相关端点：`/api/spotify/login`、`/api/spotify/callback`、`/api/spotify/status`。
+
+### 许可与来源
+
+- **go-librespot** 采用 **GPL-3.0** 许可。本项目**仅将其作为独立子进程调用**（mere aggregation / 独立聚合），**不链接、不打包**其代码，因此**不影响本项目自身的 MIT 许可**。
+  - 源码与许可：<https://github.com/devgianlu/go-librespot>（GPL-3.0；如需其对应源码请前往该仓库获取 —— source offer）。
+- **Rust librespot** 采用 **MIT** 许可：<https://github.com/librespot-org/librespot>。
+
+### 故障排查
+
+| 现象 | 处理 |
+|------|------|
+| 提示「未检测到 librespot / go-librespot」 | 检查项目 `bin/` 目录里是否放了可执行文件，或该命令是否在系统 `PATH` 中；Rust 版可用 `cargo install librespot` 安装 |
+| 提示「未授权 / 需要连接 Spotify」 | 在设置页「连接 Spotify」卡片点击「连接 Spotify」完成 OAuth 授权 |
+| Windows 上无法使用 go-librespot | 属预期行为（FIFO 仅限 POSIX，官方无 Windows 资产）；请把 `backend` 设为 `librespot` 或 `auto` |
+| 完全没有声音 | 确认账号为 **Spotify Premium**；免费账号无法通过 Spotify Connect 输出音频 |
+
+### 已知限制
+
+- 在多租户/共享主机上，librespot 通过命令行参数接收访问令牌，同机其他本地进程理论上可读取（令牌约 1 小时有效，需本地访问权限）。
+- Spotify 连续播放（gapless spotify→spotify）时，网页进度条的"已播放时间"可能不准确（以后端上报的播放进度为准）。
+- 运行多个启用 Spotify 的 bot 时，若两个 bot 的 id 端口哈希发生冲突（同一 % 1000 桶），第二个 go-librespot 边车会因端口占用而启动失败、该 bot 的 Spotify 不可用（后续将改为按需分配空闲端口）。
+- **一个 Spotify Premium 账号只支持「一路」正在播放的音频流**。因此若要同时运行**多个**启用 Spotify 的 bot 并让它们各自独立播放，必须为**每个 bot 配置独立的 Spotify 账号**。在 Rust（librespot）后端上，本机制已把播放控制（暂停/继续/跳转）限定到 bot 自己的设备，并在读取播放状态时忽略其它设备的状态，以避免多 bot 之间互相抢占、来回抖动（cross-control/thrash）；但受 Spotify 平台限制，**共用同一账号无法实现多路同时播放**（第二个 bot 开始播放会夺走该账号唯一的活跃会话）。go-librespot 后端为每个边车独立的本地 REST API，不受此账号级抢占影响。
+
 ## 配置文件
 
 配置文件位于 **`data/config.json`**（与数据库、Cookie、日志同在持久化的 `data/` 目录，Docker 部署对应挂载卷），首次运行时自动生成，可手动编辑：

docs/superpowers/plans/2026-07-03-spotify-stage4-polish-docs.md

@@ -0,0 +1,520 @@
+# Spotify Source — Stage 4 (Polish, Config UI, Docs) Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Complete the Spotify source's last mile — make it user-configurable and user-authorizable from the web UI, document it (with the required safety warnings), and land the deferred OAuth-robustness hardening.
+
+**Architecture:** The Stage-3 OAuth endpoints (`/api/spotify/{login,callback,status}`) and the single shared `SpotifyOAuth` already exist but are unreachable from the UI. Stage 4 adds: (1) a `spotify` block to the `/api/bot/settings` config API (secret masked); (2) a resolved-backend indicator on `/status`; (3) OAuth refresh/verifier hardening; (4) the approved Settings "Connect Spotify" card (spec §8); (5) the README Spotify section (spec §11/§12). Deferred (documented, NOT built this stage): runtime binary auto-download, connect play-not-reflected diagnostics, extra watchdog.
+
+**Tech Stack:** TypeScript (ESM, `.js` specifiers), Express, Vitest (root config also runs `web/src/**/*.test.ts` — stores/composables only; there is NO `.vue` component-test harness), Vue 3 + `<script setup>` + Pinia, `vue-tsc` (web build gate).
+
+## Global Constraints
+
+- **Safe-by-default (spec §2/§7):** Spotify stays inert unless `enabled` AND authorized AND a resolvable binary. Nothing in this stage may change behavior when `spotify.enabled === false`.
+- **Never expose secrets (spec §2/§7):** `clientSecret` is the operator's own value. The config GET API MUST NOT return the raw `clientSecret` — return only a boolean `hasClientSecret`. POST overwrites `clientSecret` only when a non-empty string is supplied (blank/omitted = unchanged). No bundled credentials, no shared Developer app.
+- **Permissions:** config writes gated by `requirePermission("bot.manage")`; reads by `requireNotGuest`; the OAuth login card is additionally shown only to `can('platform.auth')` / admins — mirror the existing Settings gating.
+- **Validation mirrors `src/data/config.ts`:** `backend` ∈ `{"auto","go-librespot","librespot"}`; `bitrate` ∈ `{96,160,320}`; `deviceName` non-empty trimmed string (else keep prior); strings coerced/guarded. Reuse the exact value sets.
+- **Language:** README additions are in Chinese (repo is Chinese-only), matching existing heading style (`## 功能特性` etc.).
+- **Licensing (spec §9):** go-librespot is GPL-3.0 — the README must include its license note + a source offer; the bot ships NO binary (source-only Rust librespot; Linux-only go-librespot assets).
+- **Honesty:** live audio / Connect control / real OAuth round-trip remain NOT verifiable here (need Premium + real binaries + a real account). "Done" = unit-tested (mocked process/HTTP/FS), `tsc --noEmit` clean, `cd web && npm run build` clean, full suite green, reviewed. Never claim audio works.
+- **Branch:** all work on `feat/spotify-audio`. Do NOT create per-task branches.
+- **Full suite command:** `npx vitest run --no-file-parallelism` (avoids the users.test.ts bcrypt LOAD flake). Typecheck: `npx tsc --noEmit`. Web build: `cd web && npm run build`.
+
+---
+
+### Task 1: `/api/bot/settings` reads & writes the `spotify` block (secret masked)
+
+**Files:**
+- Modify: `src/web/api/bot.ts` (GET `/settings` response + POST `/settings` destructure/validate/echo)
+- Test: `src/web/api/bot.test.ts`
+
+**Interfaces:**
+- Consumes: `config.spotify: SpotifyConfig` (`{ enabled, backend, clientId, clientSecret, deviceName, bitrate }`), `saveConfig(configPath, config)`, `requirePermission("bot.manage")`, `requireNotGuest` — all already imported/used in this file.
+- Produces (new response shape on GET+POST, both add a `spotify` key):
+  ```ts
+  // masked view — NEVER includes clientSecret
+  spotify: {
+    enabled: boolean;
+    backend: "auto" | "go-librespot" | "librespot";
+    clientId: string;
+    deviceName: string;
+    bitrate: number;
+    hasClientSecret: boolean;   // whether a non-empty secret is stored
+  }
+  ```
+
+- [ ] **Step 1.1: Write failing tests.** Add to `src/web/api/bot.test.ts` (mirror the existing settings tests — reuse their app/harness + auth stubs). Cover:
+  1. GET `/api/bot/settings` includes a `spotify` object with `enabled/backend/clientId/deviceName/bitrate/hasClientSecret` and does NOT include a `clientSecret` key. With a stored secret, `hasClientSecret === true`; with `clientSecret: ""`, `false`.
+  2. POST `/api/bot/settings` with `{ spotify: { enabled: true, backend: "librespot", clientId: "cid", deviceName: "Dev", bitrate: 160 } }` updates all those fields and echoes the masked view; `saveConfig` called.
+  3. POST with `{ spotify: { backend: "bogus" } }` leaves `backend` unchanged (invalid rejected, not 400 for the whole request — partial-merge semantics like the other fields). Same for `bitrate: 999`.
+  4. POST with `{ spotify: { clientSecret: "newsecret" } }` sets the secret (assert via `hasClientSecret === true` in the echo AND that `config.spotify.clientSecret === "newsecret"`). POST with `{ spotify: { clientSecret: "" } }` does NOT overwrite an existing secret.
+  5. POST `/api/bot/settings` spotify write requires `bot.manage` (a member without it → 403; reuse the existing 403 test pattern).
+  6. An existing settings POST that omits `spotify` still works and does not touch `config.spotify` (no regression).
+
+- [ ] **Step 1.2: Run the tests — expect failure** (`npx vitest run src/web/api/bot.test.ts`): the `spotify` key is absent from responses.
+
+- [ ] **Step 1.3: Extend GET `/settings`.** Add the masked spotify view to the response object (both the GET at ~line 36 and the POST echo at ~line 103 — extract a local helper to avoid duplication):
+  ```ts
+  // near the top of createBotRouter, after other helpers:
+  const maskedSpotify = () => ({
+    enabled: config.spotify.enabled,
+    backend: config.spotify.backend,
+    clientId: config.spotify.clientId,
+    deviceName: config.spotify.deviceName,
+    bitrate: config.spotify.bitrate,
+    hasClientSecret: config.spotify.clientSecret.length > 0,
+  });
+  ```
+  Add `spotify: maskedSpotify(),` to BOTH the GET response and the POST echo object.
+
+- [ ] **Step 1.4: Handle `spotify` in POST `/settings`.** Add `spotify` to the destructure and a partial-merge block (mirror config.ts validation). Place before `saveConfig(...)`:
+  ```ts
+  const VALID_BACKENDS = ["auto", "go-librespot", "librespot"] as const;
+  const VALID_BITRATES = [96, 160, 320];
+  const sp = req.body?.spotify;
+  if (sp && typeof sp === "object") {
+    const t = config.spotify;
+    if (typeof sp.enabled === "boolean") t.enabled = sp.enabled;
+    if (typeof sp.backend === "string" && (VALID_BACKENDS as readonly string[]).includes(sp.backend)) {
+      t.backend = sp.backend as SpotifyConfig["backend"];
+    }
+    if (typeof sp.clientId === "string") t.clientId = sp.clientId;
+    // Secret is write-only + set-on-non-empty so a blank field never wipes it.
+    if (typeof sp.clientSecret === "string" && sp.clientSecret.length > 0) {
+      t.clientSecret = sp.clientSecret;
+    }
+    if (typeof sp.deviceName === "string" && sp.deviceName.trim().length > 0) {
+      t.deviceName = sp.deviceName.trim();
+    }
+    if (typeof sp.bitrate === "number" && VALID_BITRATES.includes(sp.bitrate)) {
+      t.bitrate = sp.bitrate;
+    }
+  }
+  ```
+  Add the type import if not present: `import type { SpotifyConfig } from "../../data/config.js";`.
+
+- [ ] **Step 1.5: Run tests — expect pass** (`npx vitest run src/web/api/bot.test.ts`). Then `npx tsc --noEmit` clean.
+
+- [ ] **Step 1.6: Commit.**
+  ```bash
+  git add src/web/api/bot.ts src/web/api/bot.test.ts
+  git commit -m "feat(spotify): expose spotify config on /api/bot/settings (secret masked) [S4.1]"
+  ```
+
+---
+
+### Task 2: `/api/spotify/status` reports the RESOLVED backend + binary availability (D11)
+
+**Files:**
+- Create: `src/music/spotify/backend-select.ts` (pure resolver) + `src/music/spotify/backend-select.test.ts`
+- Modify: `src/music/spotify/controller.ts` (delegate `chooseBackend` to the resolver)
+- Modify: `src/web/api/spotify.ts` (`/status` shape + `getBackendInfo` type)
+- Modify: `src/web/server.ts` (`getBackendInfo` computes the resolved kind)
+- Modify: `src/web/api/spotify.test.ts` (update the `/status` shape assertion)
+
+**Interfaces:**
+- Produces:
+  ```ts
+  // backend-select.ts
+  export type SpotifyBackendKind = "go-librespot" | "librespot";
+  export function resolveSpotifyBackendKind(
+    backend: "auto" | "go-librespot" | "librespot",
+    goPresent: boolean,
+    rustPresent: boolean,
+  ): SpotifyBackendKind | null;
+  // spotify.ts getBackendInfo now returns:
+  { backend: string; deviceName: string; binaryAvailable: boolean }
+  // /status response now:
+  { authorized: boolean; backend: string; deviceName: string; binaryAvailable: boolean }
+  ```
+- Note: `SpotifyBackendKind` currently lives in `controller.ts`. Move the canonical definition to `backend-select.ts` and re-export it from `controller.ts` (`export type { SpotifyBackendKind } from "./backend-select.js";`) so existing importers are unaffected.
+
+- [ ] **Step 2.1: Write failing resolver tests** `src/music/spotify/backend-select.test.ts` — the same 8-case matrix S3.5 used, but against the pure function:
+  ```ts
+  import { describe, it, expect } from "vitest";
+  import { resolveSpotifyBackendKind as pick } from "./backend-select.js";
+  describe("resolveSpotifyBackendKind", () => {
+    it("auto: go present -> go-librespot", () => expect(pick("auto", true, true)).toBe("go-librespot"));
+    it("auto: go absent, rust present -> librespot", () => expect(pick("auto", false, true)).toBe("librespot"));
+    it("auto: neither -> null", () => expect(pick("auto", false, false)).toBeNull());
+    it("go-librespot: present -> go-librespot", () => expect(pick("go-librespot", true, true)).toBe("go-librespot"));
+    it("go-librespot: absent -> null even if rust present", () => expect(pick("go-librespot", false, true)).toBeNull());
+    it("librespot: present -> librespot", () => expect(pick("librespot", true, true)).toBe("librespot"));
+    it("librespot: absent -> null even if go present", () => expect(pick("librespot", true, false)).toBeNull());
+    it("auto default fallthrough matches auto", () => expect(pick("auto", true, false)).toBe("go-librespot"));
+  });
+  ```
+
+- [ ] **Step 2.2: Run — expect failure** (module missing).
+
+- [ ] **Step 2.3: Create the resolver** `src/music/spotify/backend-select.ts`:
+  ```ts
+  /** Which concrete backend runs for a given config + host binary availability. */
+  export type SpotifyBackendKind = "go-librespot" | "librespot";
+
+  /**
+   * Pure backend selection shared by SpotifyController.chooseBackend() (per-bot)
+   * and the web /status endpoint (process-wide). Booleans in, no IO — the caller
+   * supplies platform+binary presence.
+   */
+  export function resolveSpotifyBackendKind(
+    backend: "auto" | "go-librespot" | "librespot",
+    goPresent: boolean,
+    rustPresent: boolean,
+  ): SpotifyBackendKind | null {
+    switch (backend) {
+      case "go-librespot":
+        return goPresent ? "go-librespot" : null;
+      case "librespot":
+        return rustPresent ? "librespot" : null;
+      case "auto":
+      default:
+        if (goPresent) return "go-librespot";
+        if (rustPresent) return "librespot";
+        return null;
+    }
+  }
+  ```
+
+- [ ] **Step 2.4: Run resolver tests — expect pass.**
+
+- [ ] **Step 2.5: Delegate from the controller.** In `controller.ts`, import the type+resolver **with a local binding** (a bare `export … from` re-export does NOT create a local name, and `SpotifyBackendKind` is still referenced locally at `chooseBackend()`'s return type and `buildBackend(kind: SpotifyBackendKind)` → would fail TS2304). Use:
+  ```ts
+  import { resolveSpotifyBackendKind, type SpotifyBackendKind } from "./backend-select.js";
+  export type { SpotifyBackendKind };   // keep the name exported for existing importers
+  // ...
+  chooseBackend(): SpotifyBackendKind | null {
+    return resolveSpotifyBackendKind(this.config.backend, this.goPresent(), this.rustPresent());
+  }
+  ```
+  Remove the old inline `switch` body and the standalone `export type SpotifyBackendKind = "go-librespot" | "librespot";` line. Keep `goPresent()/rustPresent()` as-is. Run `npx vitest run src/music/spotify/controller.test.ts` — the S3.5 matrix + auth-gate specs MUST still pass unchanged.
+
+- [ ] **Step 2.6: Update `/status`** in `src/web/api/spotify.ts` — extend the `getBackendInfo` type and the response:
+  ```ts
+  getBackendInfo: () => { backend: string; deviceName: string; binaryAvailable: boolean };
+  // ...
+  router.get("/status", requireNotGuest, (_req, res) => {
+    const info = opts.getBackendInfo();
+    res.json({
+      authorized: oauth.isAuthorized(),
+      backend: info.backend,
+      deviceName: info.deviceName,
+      binaryAvailable: info.binaryAvailable,
+    });
+  });
+  ```
+
+- [ ] **Step 2.7: Compute the resolved kind in `server.ts`.** Replace the `getBackendInfo` closure (currently returns raw `config.spotify.backend`) with a resolver call using live probes. Add imports `import { resolveSpotifyBackendKind } from "../music/spotify/backend-select.js";` and `import { isGoLibrespotSupported, findGoLibrespot, isRustLibrespotSupported, findLibrespot } from "../music/spotify/binary.js";` and `import { existsSync } from "node:fs";` (verify existsSync isn't already imported):
+  ```ts
+  getBackendInfo: () => {
+    const goPresent = isGoLibrespotSupported() && existsSync(findGoLibrespot());
+    const rustPresent = isRustLibrespotSupported() && existsSync(findLibrespot());
+    const resolved = resolveSpotifyBackendKind(options.config.spotify.backend, goPresent, rustPresent);
+    return {
+      backend: resolved ?? "none",
+      deviceName: options.config.spotify.deviceName,
+      binaryAvailable: resolved !== null,
+    };
+  },
+  ```
+
+- [ ] **Step 2.8: Update the `/status` test** in `src/web/api/spotify.test.ts` — the existing `toEqual({ authorized, backend, deviceName })` must become `toEqual({ authorized, backend, deviceName, binaryAvailable })`; extend the fake `getBackendInfo` in that test to return `binaryAvailable`. This is THIS task's contract change; update only the status test.
+
+- [ ] **Step 2.9: Verify.** `npx vitest run src/music/spotify/backend-select.test.ts src/music/spotify/controller.test.ts src/web/api/spotify.test.ts` all pass; `npx tsc --noEmit` clean.
+
+- [ ] **Step 2.10: Commit.**
+  ```bash
+  git add src/music/spotify/backend-select.ts src/music/spotify/backend-select.test.ts src/music/spotify/controller.ts src/web/api/spotify.ts src/web/server.ts src/web/api/spotify.test.ts
+  git commit -m "feat(spotify): report resolved backend + binaryAvailable on /status; share backend resolver [S4.2]"
+  ```
+
+---
+
+### Task 3: OAuth robustness — in-flight refresh cache + verifier TTL/cap (D9)
+
+**Files:**
+- Modify: `src/music/spotify/spotify-oauth.ts`
+- Test: `src/music/spotify/spotify-oauth.test.ts`
+
+**Interfaces:**
+- Consumes: existing `SpotifyOAuthOptions.deps?: { http?: AxiosInstance }`. Extend deps with an optional clock for testability: `deps?: { http?: AxiosInstance; now?: () => number }`.
+- Produces: no public API change. Internals: `refreshInFlight: Promise<string|null> | null`; `pendingVerifiers: Map<string, { verifier: string; expiresAt: number }>`.
+
+- [ ] **Step 3.1: Write failing tests.** Add to `src/music/spotify/spotify-oauth.test.ts`:
+  1. **Concurrent refresh collapses to one POST.** Build with a fake `http` whose `post("/api/token")` returns a promise you resolve manually (a `Deferred`) or counts calls, and a MUTABLE fake store (`save()` persists, `load()` returns the last saved value) seeded with an expired token. Fire two `getAccessToken()` calls before the POST resolves; assert `http.post` called exactly ONCE and both awaited results equal the new access token.
+  2. **In-flight clears after settle.** Using the same mutable store + a mutable `now` (`let t=…; now=()=>t`), after test 1 resolves, advance `t` past the newly-saved `expiresAt` and call `getAccessToken()` again → a NEW POST fires (count → 2). (Requires `toTokens` on `this.now()` per Step 3.3.)
+  3. **Verifier TTL.** With injected mutable `now`, `buildAuthorizeUrl()` at t=0 (capture its `state`), advance `now` to TTL+1, then `handleCallback(code, state)` → returns false (expired) and the entry is gone.
+  4. **Verifier cap.** Call `buildAuthorizeUrl()` `VERIFIER_MAX + 1` times (capturing the FIRST `state`); assert **behaviorally** that `handleCallback(code, <first state>)` now returns false (evicted as oldest). Prefer this behavioral assertion over inspecting the private `pendingVerifiers` map (no `as any` cast). Use the injected `now` for all timing.
+
+- [ ] **Step 3.2: Run — expect failure.**
+
+- [ ] **Step 3.3: Add the clock + in-flight refresh.** In the constructor: `this.now = o.deps?.now ?? (() => Date.now());` (add `private now: () => number;`). Rewrite `getAccessToken()` + add the in-flight field:
+  ```ts
+  private refreshInFlight: Promise<string | null> | null = null;
+
+  async getAccessToken(): Promise<string | null> {
+    if (!this.clientId) return null;
+    const tokens = this.store.load();
+    if (!tokens?.refreshToken) return null;
+    if (tokens.accessToken && this.now() < tokens.expiresAt) return tokens.accessToken;
+    // Collapse concurrent refreshes: rotation makes a second in-flight refresh
+    // use a refresh token the first one already invalidated.
+    if (this.refreshInFlight) return this.refreshInFlight;
+    this.refreshInFlight = this.refresh(tokens).finally(() => {
+      this.refreshInFlight = null;
+    });
+    return this.refreshInFlight;
+  }
+  ```
+  Replace the `Date.now()` in `getAccessToken` (spotify-oauth.ts:188) with `this.now()`. **Also change `toTokens` (spotify-oauth.ts:~221) to compute `expiresAt` from `this.now()` instead of `Date.now()`** — this is REQUIRED for testability: if `toTokens` kept real `Date.now()` while `getAccessToken` used an injected fixed clock, a freshly-refreshed token's `expiresAt` would sit far in the injected past/future and the "subsequent call → new POST" test would be non-deterministic. With both on `this.now()`, the test drives a mutable `now` (e.g. `let t = 0; const now = () => t;`) and advances it past the new `expiresAt` to force the second refresh.
+
+- [ ] **Step 3.4: Add verifier TTL + cap.** Change the map type and the two touch points:
+  ```ts
+  private pendingVerifiers = new Map<string, { verifier: string; expiresAt: number }>();
+  private static readonly VERIFIER_TTL_MS = 10 * 60 * 1000;
+  private static readonly VERIFIER_MAX = 32;
+
+  private evictStaleVerifiers(): void {
+    const t = this.now();
+    for (const [state, e] of this.pendingVerifiers) {
+      if (e.expiresAt < t) this.pendingVerifiers.delete(state);
+    }
+    // Bound memory even if all are unexpired: drop oldest (insertion order).
+    while (this.pendingVerifiers.size >= SpotifyOAuth.VERIFIER_MAX) {
+      const oldest = this.pendingVerifiers.keys().next().value;
+      if (oldest === undefined) break;
+      this.pendingVerifiers.delete(oldest);
+    }
+  }
+  ```
+  In `buildAuthorizeUrl()`: call `this.evictStaleVerifiers();` before the set, then `this.pendingVerifiers.set(state, { verifier, expiresAt: this.now() + SpotifyOAuth.VERIFIER_TTL_MS });`.
+  In `handleCallback()`: replace the `get`:
+  ```ts
+  const entry = this.pendingVerifiers.get(state);
+  if (!entry || entry.expiresAt < this.now()) {
+    this.pendingVerifiers.delete(state);
+    return false;
+  }
+  const verifier = entry.verifier;
+  ```
+  (Keep the existing `finally { this.pendingVerifiers.delete(state); }`.)
+
+- [ ] **Step 3.5: Run tests — expect pass.** Then `npx vitest run src/music/spotify/spotify-oauth.test.ts` (all, incl. prior S3.2 tests) + `npx tsc --noEmit` clean.
+
+- [ ] **Step 3.6: Commit.**
+  ```bash
+  git add src/music/spotify/spotify-oauth.ts src/music/spotify/spotify-oauth.test.ts
+  git commit -m "fix(spotify): collapse concurrent OAuth refresh + TTL/cap PKCE verifiers [S4.3]"
+  ```
+
+---
+
+### Task 4: Settings "Connect Spotify" card (spec §8)
+
+**Files:**
+- Create: `web/src/composables/useSpotifySettings.ts` (pure, testable logic) + `web/src/composables/useSpotifySettings.test.ts`
+- Modify: `web/src/views/Settings.vue` (add the card + wiring)
+
+**Interfaces (composable — the unit-tested surface):**
+```ts
+export interface SpotifyConfigForm {
+  enabled: boolean;
+  backend: "auto" | "go-librespot" | "librespot";
+  clientId: string;
+  clientSecret: string;   // blank means "unchanged"
+  deviceName: string;
+  bitrate: number;
+}
+export interface SpotifyStatus { authorized: boolean; backend: string; deviceName: string; binaryAvailable: boolean; }
+export const SPOTIFY_DISCLAIMER: string;                    // Chinese risk copy
+export function buildSpotifyPayload(f: SpotifyConfigForm): { spotify: Record<string, unknown> };  // omits clientSecret when blank
+export function parseSpotifyRedirect(search: string): "success" | "error" | null;  // from ?spotify=...
+export function statusSummary(s: SpotifyStatus | null, enabled: boolean): { label: string; tone: "ok" | "warn" | "off" };
+```
+
+- [ ] **Step 4.1: Write failing composable tests** `web/src/composables/useSpotifySettings.test.ts` (root vitest picks it up; import from `./useSpotifySettings.js` per the repo's ESM `.js` convention):
+  - `buildSpotifyPayload` includes `clientSecret` only when non-blank; always includes enabled/backend/clientId/deviceName/bitrate under a `spotify` key.
+  - `parseSpotifyRedirect("?spotify=success")==="success"`, `"?spotify=error"==="error"`, `"?x=1"===null`.
+  - `statusSummary(null, false)` → tone `"off"`; `statusSummary({authorized:false,binaryAvailable:false,...}, true)` → tone `"warn"`; `statusSummary({authorized:true,binaryAvailable:true,...}, true)` → tone `"ok"`.
+
+- [ ] **Step 4.2: Run — expect failure** (module missing).
+
+- [ ] **Step 4.3: Implement the composable** `web/src/composables/useSpotifySettings.ts`:
+  ```ts
+  export interface SpotifyConfigForm { enabled: boolean; backend: "auto" | "go-librespot" | "librespot"; clientId: string; clientSecret: string; deviceName: string; bitrate: number; }
+  export interface SpotifyStatus { authorized: boolean; backend: string; deviceName: string; binaryAvailable: boolean; }
+
+  // 实验性 · 灰色地带 · 需要 Premium · 使用你自己的开发者应用凭据
+  export const SPOTIFY_DISCLAIMER =
+    "实验性功能：通过 librespot 播放 Spotify 需要 Spotify Premium 账号，并使用你自己注册的 Spotify 开发者应用凭据。" +
+    "该方式处于 Spotify 服务条款的灰色地带，风险自负；默认关闭，不会内置任何共享凭据。";
+
+  export function buildSpotifyPayload(f: SpotifyConfigForm): { spotify: Record<string, unknown> } {
+    const spotify: Record<string, unknown> = {
+      enabled: f.enabled,
+      backend: f.backend,
+      clientId: f.clientId,
+      deviceName: f.deviceName,
+      bitrate: f.bitrate,
+    };
+    if (f.clientSecret && f.clientSecret.length > 0) spotify.clientSecret = f.clientSecret;
+    return { spotify };
+  }
+
+  export function parseSpotifyRedirect(search: string): "success" | "error" | null {
+    const v = new URLSearchParams(search).get("spotify");
+    return v === "success" || v === "error" ? v : null;
+  }
+
+  export function statusSummary(s: SpotifyStatus | null, enabled: boolean): { label: string; tone: "ok" | "warn" | "off" } {
+    if (!enabled) return { label: "已关闭", tone: "off" };
+    if (!s) return { label: "未知", tone: "warn" };
+    if (!s.binaryAvailable) return { label: "未检测到 librespot 可执行文件", tone: "warn" };
+    if (!s.authorized) return { label: "未授权（点击“连接 Spotify”登录）", tone: "warn" };
+    return { label: `已就绪 · 后端 ${s.backend}`, tone: "ok" };
+  }
+  ```
+
+- [ ] **Step 4.4: Run composable tests — expect pass.**
+
+- [ ] **Step 4.5: Add the card to `Settings.vue`.** Insert a new `.account-card`-style section in the settings surface, gated `v-if="can('platform.auth')"` (mirror the platform-login section). Follow the EXISTING patterns in this file:
+  - typed input + Save → mirror the idle-timeout input/saver (`Settings.vue` idle-timeout block + `saveIdleTimeout()` → `POST /api/bot/settings`);
+  - select group → mirror the quality button-group for `backend` and `bitrate`;
+  - toggle → mirror `localAudioEnabled` for `enabled`.
+  **Fields to render (all six):** the `enabled` toggle, `backend` group, `bitrate` group, a `clientId` text input, a `deviceName` text input, and a **Client Secret** field — a write-only password input (`type="password"`, `autocomplete="off"`), left BLANK on load with a "已设置 / 未设置" hint from `hasClientSecret`; a blank secret on Save means "unchanged" (never wipes). The Secret is §8-mandated — do not omit it.
+  Wiring (use `axios`, matching the file's other calls):
+  - **Load** on mount: `GET /api/bot/settings` → populate the form from `res.data.spotify` (leave `clientSecret` blank; show “已设置/未设置” from `hasClientSecret`); `GET /api/spotify/status` → status indicator via `statusSummary`.
+  - **Save**: `POST /api/bot/settings` with `buildSpotifyPayload(form)`; on success re-load status.
+  - **Connect**: `const { data } = await axios.get('/api/spotify/login'); window.location.href = data.url;` (guard errors → show message; a 403 means missing `platform.auth`).
+  - **Redirect handling**: on mount, `parseSpotifyRedirect(window.location.search)`; if `success`/`error`, show a toast/message and strip the param (e.g. `history.replaceState`). Reuse the file's existing notification/toast mechanism if present; otherwise a simple reactive message line.
+  - **Disclaimer**: render `SPOTIFY_DISCLAIMER` prominently in the card.
+  Keep the `<script setup>` logic thin — delegate payload/summary/redirect parsing to the composable (already tested). Do not add a `.vue` test (no harness).
+  - **Keep the two "spotify auth" concepts distinct:** the card's status comes ONLY from `/api/spotify/status` (playback OAuth `authorized`). Do NOT wire it to the player store's `authStatus.spotify`, which reflects `/api/auth/status?platform=spotify` (metadata Web-API `loggedIn`) — a different thing. Leave the player store untouched.
+  - **`vue-tsc` typing:** annotate the form as `reactive<SpotifyConfigForm>({...})` and the status as `ref<SpotifyStatus | null>(null)`; otherwise a button-group assignment (`form.backend = 'librespot'`) widens `backend` to `string` and the `null` init breaks the union passed into `buildSpotifyPayload`/`statusSummary` under `vue-tsc --noEmit`.
+  - **Hard order dependency:** `binaryAvailable` on `/api/spotify/status` exists only after Task 2 — execute this task AFTER Task 2.
+
+- [ ] **Step 4.6: Build gate.** `cd web && npm run build` → `vue-tsc --noEmit` clean + vite build succeeds. Then from root `npx vitest run web/src/composables/useSpotifySettings.test.ts` green.
+
+- [ ] **Step 4.7: Commit.**
+  ```bash
+  git add web/src/composables/useSpotifySettings.ts web/src/composables/useSpotifySettings.test.ts web/src/views/Settings.vue
+  git commit -m "feat(spotify): Connect-Spotify settings card (config + OAuth login + status) [S4.4]"
+  ```
+
+**Notes:** NOT e2e-verifiable (needs a real account/binary). Verified = composable unit tests + `vue-tsc` typecheck + build. The card only exposes config + a login trigger; it never displays the stored `clientSecret` (GET returns `hasClientSecret`, not the value).
+
+---
+
+### Task 5: README Spotify section (Chinese) — spec §9/§11/§12
+
+**Files:**
+- Modify: `README.md` (add a `## Spotify 音源（实验性）` section; add "Spotify" to the feature bullet if appropriate)
+
+**Content (required — write real prose, not placeholders):**
+- [ ] **Step 5.1:** Add a top-level section `## Spotify 音源（实验性）` covering, in Chinese:
+  1. **醒目警告框:** 实验性；需要 **Spotify Premium**；使用**你自己注册的 Spotify 开发者应用**（不内置任何共享凭据）；处于 Spotify 服务条款灰色地带，风险自负；**默认关闭**。
+  2. **工作原理（简述）:** 通过 librespot（Rust）/ go-librespot（Linux）作为独立进程解码 → PCM → ffmpeg 重采样到 48k → 走现有 Opus 发送管线。元数据来自 Spotify Web API。
+  3. **平台矩阵:** Windows → `librespot`(Rust)；Linux/Docker → `go-librespot`（可回退 `librespot`）；`auto` 自动选择（表格）。
+  4. **获取二进制:** Rust librespot 无预编译包 → `cargo install librespot` 或 scoop/choco，或将可执行文件放入项目 `bin/`（`bin/librespot.exe` / `bin/librespot`）。go-librespot 仅提供 Linux 资产（`github.com/devgianlu/go-librespot/releases`），放入 `bin/go-librespot` 或加入 PATH。
+  5. **注册开发者应用 + 回调:** 在 Spotify Developer Dashboard 建应用，取 Client ID，回调地址填 `http://127.0.0.1:<webPort>/api/spotify/callback`（与设置里的 Web 端口一致）；PKCE 不需要 Client Secret。
+  6. **启用步骤:** 设置页「连接 Spotify」卡片 → 填 Client ID、选后端、开启开关 → 保存 → 点「连接 Spotify」完成 OAuth 授权。
+  7. **许可与来源:** go-librespot 为 **GPL-3.0**，作为独立子进程调用（mere aggregation，不影响本项目许可）；附上其源码地址与许可说明（source offer）。
+  8. **故障排查:** 「未检测到 librespot」→ 检查 `bin/` 或 PATH；「未授权」→ 完成 OAuth；Windows 不支持 go-librespot（FIFO 仅限 POSIX）。
+- [ ] **Step 5.2:** Sanity-check the doc renders (headings consistent with existing `##` style; links valid). No code test.
+- [ ] **Step 5.3: Commit.**
+  ```bash
+  git add README.md
+  git commit -m "docs(spotify): README Spotify source section — setup, binaries, warnings, license [S4.5]"
+  ```
+
+---
+
+### Task 6: Rust Connect command retry/backoff (recovery/watchdog — spec §4.3/§13)
+
+**Rationale:** Spec §4.3 mandates "a watchdog + retry/backoff for device-visibility latency and command 202/404 flakiness"; §13 lists Rust Connect as the **top risk** (mitigation: retry/backoff + degrade-to-skipped). Stage 3 already landed the device-visibility watchdog (`waitForDevice()` bounded poll in `rust-librespot.ts`) and per-track device-absence → skip (`findDeviceByName`→null → `playTrack` false → BotInstance skips). The remaining, un-built piece is transient-failure retry/backoff on the Connect **mutating commands** — this task adds it while preserving C3.6 (never reject up the queue path; swallow on exhaustion).
+
+**Files:**
+- Modify: `src/music/spotify/connect-api.ts` (retry/backoff around the mutating PUTs; optional injected `sleep`/`logger`)
+- Modify: `src/music/spotify/controller.ts` (pass `this.logger` into the default `SpotifyConnectApi`)
+- Test: `src/music/spotify/connect-api.test.ts`
+
+**Interfaces:**
+- Consumes: existing `constructor(getToken: () => Promise<string|null>, deps?: { http?: AxiosInstance })`. **Extend deps** to `{ http?: AxiosInstance; sleep?: (ms: number) => Promise<void>; logger?: import("pino").Logger }` (all optional → source-compatible).
+- No public method signature change: `transfer/play/pause/resume/seek` stay `Promise<void>` and still swallow on final failure.
+
+- [ ] **Step 6.1: Write failing tests** in `src/music/spotify/connect-api.test.ts` (reuse the existing `getToken`/`http` injection pattern; inject a no-op `sleep: async () => {}` so no real timers run):
+  1. `play()` retries a transient 404 then succeeds: `http.put` rejects once with `{ response: { status: 404 } }` then resolves → assert `http.put` called TWICE, no throw.
+  2. `play()` exhausts on persistent 500: `http.put` always rejects `{ response: { status: 500 } }` → assert exactly `MAX_ATTEMPTS` calls, no throw (swallowed), and `logger.warn` called once (logger provided).
+  3. Non-transient (403) is NOT retried: reject `{ response: { status: 403 } }` → exactly ONE call, no throw.
+  4. `429` honors a **capped** `Retry-After`: reject once `{ response: { status: 429, headers: { "retry-after": "1" } } }` then resolve → `sleep` called with a bounded delay (≤ the cap) and 2 calls total.
+  5. `transfer()` uses the same retry path (one representative non-`play` mutator) — 404-then-success → 2 calls.
+
+- [ ] **Step 6.2: Confirm Red** (`npx vitest run src/music/spotify/connect-api.test.ts`).
+
+- [ ] **Step 6.3: Implement retry/backoff.** Add module constants + a private helper and route each mutating PUT through it:
+  ```ts
+  const TRANSIENT = new Set([404, 429, 500, 502, 503]);
+  const MAX_ATTEMPTS = 3;
+  const BASE_DELAY_MS = 150;
+  const MAX_DELAY_MS = 2_000;
+  // ...
+  private async mutateWithRetry(put: () => Promise<unknown>): Promise<void> {
+    for (let attempt = 1; attempt <= MAX_ATTEMPTS; attempt++) {
+      try {
+        await put();
+        return;
+      } catch (err: any) {
+        const status = err?.response?.status;
+        if (!TRANSIENT.has(status) || attempt === MAX_ATTEMPTS) {
+          // C3.6: never reject up the queue path — swallow, but surface once.
+          this.logger?.warn({ status }, "Spotify Connect command failed (exhausted/non-retryable)");
+          return;
+        }
+        let delay = Math.min(BASE_DELAY_MS * 2 ** (attempt - 1), MAX_DELAY_MS);
+        if (status === 429) {
+          const ra = Number(err?.response?.headers?.["retry-after"]);
+          if (Number.isFinite(ra) && ra > 0) delay = Math.min(ra * 1000, MAX_DELAY_MS);
+        }
+        await this.sleep(delay);
+      }
+    }
+  }
+  ```
+  In the constructor: `this.sleep = deps?.sleep ?? ((ms) => new Promise((r) => setTimeout(r, ms)));` and `this.logger = deps?.logger;`. Rewrite `transfer/play/pause/resume/seek` so their body becomes: guard `authHeaders()` as today (unauth → `return;`, no retry), then `await this.mutateWithRetry(() => this.http.put(<url>, <body>, { headers, ... }));`. Keep the exact URLs/bodies/params from the current methods.
+
+- [ ] **Step 6.4: Thread the logger from the controller.** In `controller.ts` where the default connect is built:
+  ```ts
+  this.connect = o.connect ?? new SpotifyConnectApi(() => this.oauth.getAccessToken(), { logger: this.logger });
+  ```
+  (`Logger` is already imported in controller.ts.) Injected-connect tests are unaffected.
+
+- [ ] **Step 6.5: Update any conflicting existing connect test.** The S3.3 (C3.6) tests assert mutating calls swallow errors. If any asserts a single `http.put` call on a *transient*-status rejection, it now expects `MAX_ATTEMPTS` — update ONLY that count (this task's contract change). Do NOT weaken the swallow/no-throw guarantees or the non-transient single-call behavior.
+
+- [ ] **Step 6.6: Verify.** `npx vitest run src/music/spotify/connect-api.test.ts src/music/spotify/controller.test.ts` all pass; `npx tsc --noEmit` clean.
+
+- [ ] **Step 6.7: Commit.**
+  ```bash
+  git add src/music/spotify/connect-api.ts src/music/spotify/connect-api.test.ts src/music/spotify/controller.ts
+  git commit -m "feat(spotify): retry/backoff on Connect commands (device-latency/flakiness watchdog) [S4.6]"
+  ```
+
+**Notes:** Bounded retry adds latency only on the (off-audio-path) control commands and only on transient failure; the injected `sleep` makes tests instant. Not e2e-verifiable (no real account). The `202`-accepted-but-not-effective case from §4.3 is NOT handled here (it needs post-command state verification, which the existing poll-based track-end already tolerates) — noted as a follow-up.
+
+---
+
+### Task 7: Stage 4 verification + whole-branch final review + finish
+
+**Files:** none (verification + review + branch finish).
+
+- [ ] **Step 7.1: Full verification.** `npx vitest run --no-file-parallelism` (all green), `npx tsc --noEmit` (clean), `cd web && npm run build` (clean). Record counts.
+- [ ] **Step 7.2: Whole-branch adversarial review.** Review the ENTIRE `feat/spotify-audio` branch (`git merge-base main HEAD`..HEAD) — Stages 2+3+4 — with a fan-out of finders across dimensions (lifecycle/teardown correctness, OAuth/token security + no-secret-leak, backend selection + gating, async races, error handling, test hygiene) and adversarial verification of each finding. Dispatch ONE fix subagent with the consolidated Critical/Important findings; roll up Minors.
+- [ ] **Step 7.3: Address findings**, re-verify, then use superpowers:finishing-a-development-branch to complete the branch (tests green → present options → execute choice).
+
+---
+
+## Self-Review (author)
+
+- **Spec coverage:** §8 Settings card → Task 4; §9 binaries/GPL note + §11 README → Task 5; §12 "docs, README" → Task 5; §12/§4.3/§13 "recovery/watchdog" (Connect retry/backoff) → Task 6 (device-visibility watchdog + per-track skip already landed in Stage 3); D9 → Task 3; D11 → Task 2; config editability (prereq for the card) → Task 1.
+- **Deferred (documented, NOT built this stage), with rationale:** runtime binary auto-download (spec §9 calls it "optional"; README documents manual install + Docker instead — avoids a tar.gz/checksum/platform-detection downloader on the critical path); the §4.3 `202`-accepted-but-not-effective verification (needs post-command playback-state confirmation the poll-based track-end already tolerates); D10's separate play-not-reflected diagnostic beyond the retry-exhaustion `logger.warn` Task 6 adds. These are listed in the ledger as follow-ups.
+- **Placeholder scan:** backend tasks (1–3) carry complete code; frontend/docs (4–5) carry the tested composable in full + explicit wiring contracts + named existing patterns to mirror (Settings.vue has no component-test harness, so exact `.vue` template text is intentionally pattern-referenced, not dictated line-by-line).
+- **Type consistency:** `SpotifyBackendKind` canonicalized in `backend-select.ts`, re-exported from `controller.ts`; `getBackendInfo` return type updated in both `spotify.ts` and its `server.ts` supplier and the `/status` test; `buildSpotifyPayload`/`SpotifyStatus` shapes match Task 1's masked view (`hasClientSecret`) and Task 2's `/status` (`binaryAvailable`).

src/audio/player.test.ts

@@ -1,8 +1,10 @@
-import { describe, it, expect } from "vitest";
+import { describe, it, expect, vi } from "vitest";
 import { mkdtempSync, writeFileSync, existsSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir, shouldEndOnStall, volumeToFactor } from "./player.js";
+import { Readable } from "node:stream";
+import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir, shouldEndOnStall, volumeToFactor, AudioPlayer } from "./player.js";
+import type { Logger } from "../logger.js";
 
 function getHeadersArg(args: string[]): string {
   const idx = args.indexOf("-headers");
@@ -200,3 +202,387 @@ describe("shouldEndOnStall (#89 mid-track stall watchdog)", () => {
     expect(shouldEndOnStall(10, false, MAX_EMPTY, MAX_STALL)).toBe(false);
   });
 });
+
+// Minimal stub: AudioPlayer only calls debug/info/warn/error; child() returns self.
+const silentLogger = {
+  debug() {},
+  info() {},
+  warn() {},
+  error() {},
+  fatal() {},
+  trace() {},
+  child() {
+    return silentLogger;
+  },
+} as unknown as Logger;
+
+// A readable we fully control: no underlying source; we push PCM manually and
+// keep it open (never push(null)) to model the long-lived go-librespot sidecar.
+function openPcmReadable(): Readable {
+  return new Readable({ read() {} });
+}
+
+const wait = (ms: number): Promise<void> => new Promise<void>((r) => setTimeout(r, ms));
+const FRAME_BYTES = 3840; // PCM_FRAME_BYTES: 960 samples * 2ch * 2 bytes @48k s16le
+
+describe("AudioPlayer external-PCM mode (playPcmStream)", () => {
+  it("emits Opus 'frame' events from the external PCM stream without spawning ffmpeg", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 10)); // ~10 frames of PCM
+
+    await wait(150); // ~7 frame ticks at 20ms
+
+    expect(player.getState()).toBe("playing");
+    expect(frames.length).toBeGreaterThan(0);
+    expect(Buffer.isBuffer(frames[0])).toBe(true);
+    player.stop();
+  });
+
+  it("does NOT emit 'trackEnd' on underrun while external (stream stays open)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    let ended = 0;
+    const frames: Buffer[] = [];
+    player.on("trackEnd", () => ended++);
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 2)); // only 2 frames, then underrun
+
+    await wait(200); // long after those 2 frames have drained
+
+    // In the url path, ffmpeg===null + empty buffer would fire trackEnd; here it must not.
+    expect(ended).toBe(0);
+    // Silence frames keep the 20ms timeline alive -> more than the 2 fed frames emitted.
+    expect(frames.length).toBeGreaterThan(2);
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  // CORRECTION C2 (c): stop() DETACHES the shared readable — it must NOT be destroyed
+  // (destroying the sidecar's long-lived ffmpeg stdout would kill it for every future
+  // track). The sessionId bump + listener removal fence stale PCM out of pcmBuffer.
+  it("stop() detaches external mode without destroying the readable, and fences via sessionId", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    expect(stream.listenerCount("data")).toBe(1);
+    stream.push(Buffer.alloc(FRAME_BYTES * 5));
+    await wait(80);
+
+    player.stop();
+    expect(player.getState()).toBe("idle");
+    // C2: the shared sidecar stream must NOT be destroyed by teardown.
+    expect(stream.destroyed).toBe(false);
+    // Player's listeners are removed on detach (data/end/error).
+    expect(stream.listenerCount("data")).toBe(0);
+    expect(stream.listenerCount("end")).toBe(0);
+    expect(stream.listenerCount("error")).toBe(0);
+
+    const countAtStop = frames.length;
+    // sessionId fence + detached listeners: PCM pushed after stop must not
+    // resurrect the timeline or re-feed pcmBuffer.
+    stream.push(Buffer.alloc(FRAME_BYTES * 5));
+    await wait(80);
+    expect(frames.length).toBe(countAtStop);
+  });
+
+  // CORRECTION C2 (a): a gapless track change is driven by the sidecar pushing LATER
+  // PCM over the SAME already-attached stream. The player must NOT detach/re-attach
+  // (no second playPcmStream) — one persistent data listener serves every track.
+  it("(C2-a) feeds a later chunk over the SAME single attachment — gapless track change, no re-attach", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    expect(stream.listenerCount("data")).toBe(1); // attached exactly once
+
+    stream.push(Buffer.alloc(FRAME_BYTES * 4)); // "track 1" PCM
+    await wait(120);
+    const afterFirst = frames.length;
+    expect(afterFirst).toBeGreaterThan(0);
+
+    stream.push(Buffer.alloc(FRAME_BYTES * 4)); // sidecar seamlessly rolls into "track 2"
+    await wait(120);
+    expect(frames.length).toBeGreaterThan(afterFirst);
+
+    // Still exactly ONE listener — no detach/re-attach across the handoff.
+    expect(stream.listenerCount("data")).toBe(1);
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  // CORRECTION C2 (b): a second playPcmStream detaches the first (NOT destroyed, and it
+  // stops feeding pcmBuffer) and attaches the second.
+  it("(C2-b) a second playPcmStream detaches the first (not destroyed, stops feeding) and attaches the second", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    const first = openPcmReadable();
+    player.playPcmStream(first, {});
+    first.push(Buffer.alloc(FRAME_BYTES * 4));
+    await wait(120);
+    expect(frames.length).toBeGreaterThan(0);
+    expect(first.listenerCount("data")).toBe(1);
+
+    const second = openPcmReadable();
+    player.playPcmStream(second, {}); // fences + detaches `first`, attaches `second`
+
+    // C2: `first` is DETACHED, not destroyed.
+    expect(first.destroyed).toBe(false);
+    // `first` no longer feeds pcmBuffer — its data listener was removed.
+    expect(first.listenerCount("data")).toBe(0);
+    // `second` is now the attached source.
+    expect(second.listenerCount("data")).toBe(1);
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  it("fires onExternalEnd when the readable ends (drives controller-based advance)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    let endedCb = 0;
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, { onExternalEnd: () => endedCb++ });
+    stream.push(Buffer.alloc(FRAME_BYTES));
+    await wait(40);
+    stream.push(null); // end-of-stream
+    await wait(40);
+
+    expect(endedCb).toBe(1);
+    player.stop();
+  });
+
+  it("seek() is a local no-op in external mode (never respawns ffmpeg on a spotify sentinel)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 3));
+    await wait(40);
+
+    expect(() => player.seek(30)).not.toThrow();
+    // Still external, still playing — no url-ffmpeg respawn, state unchanged.
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  it("isExternalActive() is false initially, true after playPcmStream, false after stop()", () => {
+    const player = new AudioPlayer(silentLogger);
+    // Idle: never attached.
+    expect(player.isExternalActive()).toBe(false);
+
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    // Attached to the external sidecar stream.
+    expect(player.isExternalActive()).toBe(true);
+
+    player.stop();
+    // Detached again — the orchestrator uses this to know it must re-attach.
+    expect(player.isExternalActive()).toBe(false);
+  });
+
+  it("pause()/resume() still gate local emission in external mode (unchanged semantics)", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const stream = openPcmReadable();
+    player.playPcmStream(stream, {});
+    stream.push(Buffer.alloc(FRAME_BYTES * 3));
+    await wait(40);
+
+    player.pause();
+    expect(player.getState()).toBe("paused");
+    player.resume();
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+
+  // CORRECTION C1 (whole-branch): mixed queue [spotifyA, neteaseB, spotifyC].
+  // Advancing A -> B (a NON-spotify track) calls stop(), whose detachExternalStream()
+  // PAUSES the backend's long-lived SHARED readable (state.flowing = false). When the
+  // LATER spotify track C reuses the SAME backend, the orchestrator re-attaches that
+  // SAME readable via playPcmStream(). Node's Readable.on('data') only auto-resumes
+  // when flowing !== false, so without an explicit resume() the shared stream stays
+  // paused, onData never fires, pcmBuffer stays empty, and C plays only silence frames.
+  // Regression: after re-attach the shared stream MUST be flowing again and real PCM
+  // MUST reach the player.
+  it("(C1) resumes a re-attached, previously-paused SHARED stream so a later Spotify track isn't silent", async () => {
+    const player = new AudioPlayer(silentLogger);
+    const frames: Buffer[] = [];
+    player.on("frame", (f) => frames.push(f));
+
+    // The backend's long-lived, SHARED readable, reused across every track.
+    const shared = openPcmReadable();
+
+    // --- Spotify track A: first attach (auto-resumes, flowing was null !== false).
+    player.playPcmStream(shared, {});
+    shared.push(Buffer.alloc(FRAME_BYTES * 4));
+    await wait(120);
+    expect(frames.length).toBeGreaterThan(0);
+
+    // --- Advance to a NON-spotify track (neteaseB): play(url) begins with stop(),
+    // which detaches AND pauses the shared stream (state.flowing = false).
+    player.stop();
+    expect(shared.isPaused()).toBe(true); // shared stream is now paused
+    expect(player.getState()).toBe("idle");
+
+    // --- Spotify track C reuses the SAME backend: isExternalActive() is false so the
+    // orchestrator re-attaches the SAME (paused) shared readable.
+    expect(player.isExternalActive()).toBe(false);
+
+    // Spy on the shared stream to observe whether real PCM actually flows to the
+    // player. Adding a 'data' listener while flowing===false does NOT resume it
+    // (Node semantics), so this spy cannot mask the bug — pre-fix it stays at 0.
+    let spyBytes = 0;
+    shared.on("data", (c: Buffer) => {
+      spyBytes += c.length;
+    });
+
+    player.playPcmStream(shared, {}); // re-attach the SAME shared readable
+
+    // The re-attached stream must be flowing again, or track C is silent.
+    expect(shared.isPaused()).toBe(false);
+
+    shared.push(Buffer.alloc(FRAME_BYTES * 4)); // "track C" PCM
+    await wait(120);
+
+    // onData must have run (real PCM reached the player), not just silence frames.
+    expect(spyBytes).toBeGreaterThan(0);
+    expect(player.getState()).toBe("playing");
+    player.stop();
+  });
+});
+
+// R3-4: the 20ms frame loop keeps running while paused (so a live-but-silent
+// stream can refill on resume). But the stall/EOF end-detection branches MUST
+// only run while state==="playing" — otherwise pausing a stalled or
+// unknown-duration stream still accumulates emptyFrameAttempts and auto-emits
+// trackEnd (~5s later), making the controller skip the paused track.
+//
+// These tests drive the real url-path frame loop (this.ffmpeg !== null, NOT
+// external mode), which cannot be exercised via playPcmStream (that sets
+// externalMode and suppresses both branches). We inject a fake live ffmpeg +
+// an empty pcmBuffer (a stream that stays alive but never yields a full PCM
+// frame) and run the actual startFrameLoop() under fake timers. `performance`
+// is faked in lockstep with the timer clock so each tick advances a real 20ms,
+// letting us cheaply cross MAX_EMPTY_ATTEMPTS (250 ticks ≈ 5s) deterministically.
+describe("AudioPlayer stall/EOF end-detection is gated on playing state (R3-4)", () => {
+  // Fake `performance` in lockstep with the timer clock so each advanced 20ms is
+  // a real frame tick (the loop computes its delay from performance.now()).
+  const FAKE_TIMER_OPTS: Parameters<typeof vi.useFakeTimers>[0] = {
+    toFake: ["setTimeout", "clearTimeout", "setInterval", "clearInterval", "Date", "performance"],
+  };
+
+  // A player primed as "playing" with a LIVE ffmpeg that never produces a full
+  // PCM frame (unknown duration -> isNearEnd forced true). startFrameLoop() runs
+  // the genuine loop; no real process is spawned (fake ffmpeg has no pid, so the
+  // end path never touches forceCleanup/process.kill).
+  function makeStalledPlaying(): AudioPlayer {
+    const player = new AudioPlayer(silentLogger);
+    const p = player as unknown as {
+      ffmpeg: unknown;
+      currentSongDuration: number;
+      pcmBuffer: Buffer;
+      emptyFrameAttempts: number;
+      framesPlayed: number;
+      state: string;
+      startFrameLoop(): void;
+    };
+    p.ffmpeg = { pid: undefined }; // live ffmpeg, but delivers no PCM
+    p.currentSongDuration = 0; // unknown duration -> isNearEnd === true
+    p.pcmBuffer = Buffer.alloc(0); // always < one PCM frame
+    p.emptyFrameAttempts = 0;
+    p.framesPlayed = 0;
+    p.state = "playing";
+    p.startFrameLoop();
+    return player;
+  }
+
+  it("does NOT emit trackEnd (and stays paused) when a stalled unknown-duration stream is paused past the stall threshold", () => {
+    vi.useFakeTimers(FAKE_TIMER_OPTS);
+    try {
+      const player = makeStalledPlaying();
+      let ended = 0;
+      player.on("trackEnd", () => ended++);
+
+      player.pause();
+      expect(player.getState()).toBe("paused");
+
+      // Advance well past MAX_EMPTY_ATTEMPTS (250 ticks ≈ 5s): ~300 ticks.
+      vi.advanceTimersByTime(20 * 300);
+
+      expect(ended).toBe(0);
+      expect(player.getState()).toBe("paused");
+      player.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("STILL emits trackEnd when the SAME stalled unknown-duration stream is left playing (dead-stream recovery #89 preserved)", () => {
+    vi.useFakeTimers(FAKE_TIMER_OPTS);
+    try {
+      const player = makeStalledPlaying(); // stays "playing"
+      let ended = 0;
+      player.on("trackEnd", () => ended++);
+
+      vi.advanceTimersByTime(20 * 300); // cross the 250-tick stall threshold
+
+      expect(ended).toBe(1);
+      expect(player.getState()).toBe("idle");
+      player.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("does not spuriously end after a brief pause+resume on a healthy stream", () => {
+    vi.useFakeTimers(FAKE_TIMER_OPTS);
+    try {
+      const player = new AudioPlayer(silentLogger);
+      const p = player as unknown as {
+        ffmpeg: unknown;
+        currentSongDuration: number;
+        pcmBuffer: Buffer;
+        emptyFrameAttempts: number;
+        framesPlayed: number;
+        state: string;
+        startFrameLoop(): void;
+      };
+      // Healthy: a live ffmpeg with a large buffered runway that never drains
+      // empty across the ticks below, so no underrun is ever seen.
+      p.ffmpeg = { pid: undefined };
+      p.currentSongDuration = 0;
+      p.pcmBuffer = Buffer.alloc(FRAME_BYTES * 400);
+      p.emptyFrameAttempts = 0;
+      p.framesPlayed = 0;
+      p.state = "playing";
+      p.startFrameLoop();
+
+      let ended = 0;
+      player.on("trackEnd", () => ended++);
+
+      vi.advanceTimersByTime(20 * 5); // play a few frames
+      player.pause();
+      vi.advanceTimersByTime(20 * 100); // brief pause (buffer NOT drained while paused)
+      player.resume();
+      expect(player.getState()).toBe("playing");
+      vi.advanceTimersByTime(20 * 100); // resume; still plenty of runway
+
+      expect(ended).toBe(0);
+      expect(player.getState()).toBe("playing");
+      player.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+});

src/audio/player.ts

@@ -5,6 +5,7 @@ import { accessSync, chmodSync, constants, mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { createOpusEncoder, PCM_FRAME_BYTES, type Encoder } from "./encoder.js";
+import type { Readable } from "node:stream";
 import type { Logger } from "../logger.js";
 
 const require = createRequire(import.meta.url);
@@ -47,7 +48,7 @@ const resolvedFfmpeg: string = (() => {
   return "ffmpeg";
 })();
 
-function getFfmpegCommand(): string {
+export function getFfmpegCommand(): string {
   return resolvedFfmpeg;
 }
 
@@ -187,6 +188,22 @@ export class AudioPlayer extends EventEmitter {
   private static readonly MAX_STALL_ATTEMPTS = 3000;
   private currentSongDuration = 0; // 当前歌曲总时长（秒）
 
+  // --- External PCM mode (Stage 2: go-librespot Spotify sidecar) ---
+  // When true, PCM arrives from a long-lived external Readable instead of a
+  // per-URL ffmpeg: this.ffmpeg stays null, and the underrun-driven trackEnd
+  // branches are suppressed (advance is driven by the controller, not EOF).
+  //
+  // CORRECTION C2: externalStream is the backend's LONG-LIVED, SHARED ffmpeg
+  // stdout (one stream reused across every track). Teardown must DETACH (remove
+  // the listeners we added + pause), never destroy it. We keep references to the
+  // exact handler functions so detach can removeListener precisely.
+  private externalMode = false;
+  private externalStream: Readable | null = null;
+  private onExternalEnd: (() => void) | null = null;
+  private externalDataHandler: ((chunk: Buffer) => void) | null = null;
+  private externalEndHandler: (() => void) | null = null;
+  private externalErrorHandler: ((err: Error) => void) | null = null;
+
   constructor(logger: Logger) {
     super();
     this.encoder = createOpusEncoder();
@@ -390,6 +407,117 @@ export class AudioPlayer extends EventEmitter {
     this.startFrameLoop();
   }
 
+  /**
+   * External-PCM mode (Stage 2 go-librespot Spotify sidecar).
+   *
+   * Feeds an already-normalized 48kHz/s16le/stereo PCM Readable (the
+   * go-librespot FIFO -> ffmpeg output) straight into the existing pcmBuffer +
+   * 20ms frame loop + Opus encoder + "frame" emission, WITHOUT spawning a
+   * per-URL ffmpeg. The url play() path is left completely untouched.
+   *
+   * Track advance is NOT driven by buffer underrun here (the sidecar stream is
+   * continuous and never EOFs per song); the caller drives advance via the
+   * SpotifyController "trackEnded" WebSocket event. onExternalEnd fires only if
+   * the underlying readable itself ends or errors.
+   *
+   * CORRECTION C2: the readable is the backend's long-lived, SHARED ffmpeg
+   * stdout reused across every track — a gapless track change is just LATER PCM
+   * on this SAME already-attached stream (no re-attach). Teardown DETACHES
+   * (removes our listeners + pauses); it never destroys the shared stream.
+   */
+  playPcmStream(readable: Readable, opts: { onExternalEnd?: () => void } = {}): void {
+    // 1. Fence current playback: stop() bumps sessionId, clears pcmBuffer, kills
+    //    any ffmpeg, and DETACHES (never destroys) any prior external stream.
+    this.stop();
+
+    const currentSessionId = this.sessionId;
+    this.externalMode = true;
+    this.externalStream = readable;
+    this.onExternalEnd = opts.onExternalEnd ?? null;
+    // Leave this.ffmpeg = null; clear currentUrl so seek() cannot respawn ffmpeg.
+    this.currentUrl = "";
+    this.seekOffset = 0;
+    this.framesPlayed = 0;
+    this.healthyFrames = 0;
+    this.ffmpegPaused = false;
+    this.spawnFailed = false;
+    this.emptyFrameAttempts = 0;
+    this.currentSongDuration = 0;
+
+    // Same ingestion + high-water backpressure as the ffmpeg.stdout handler,
+    // but pausing the Readable instead of ffmpeg.stdout. sessionId-guarded so
+    // stale sidecar PCM can't leak into a new track after stop()/skip. Handler
+    // refs are stored so detach can remove exactly these listeners (C2).
+    const onData = (chunk: Buffer): void => {
+      if (this.sessionId !== currentSessionId) return;
+      this.pcmBuffer = Buffer.concat([this.pcmBuffer, chunk]);
+      if (
+        this.pcmBuffer.length > AudioPlayer.BUFFER_HIGH_WATER &&
+        !this.ffmpegPaused &&
+        this.externalStream === readable
+      ) {
+        readable.pause();
+        this.ffmpegPaused = true;
+      }
+    };
+    const onEnd = (): void => {
+      if (this.sessionId !== currentSessionId) return;
+      this.onExternalEnd?.();
+    };
+    const onError = (err: Error): void => {
+      if (this.sessionId !== currentSessionId) return;
+      this.logger.warn({ err }, "External PCM stream error");
+      this.onExternalEnd?.();
+    };
+
+    this.externalDataHandler = onData;
+    this.externalEndHandler = onEnd;
+    this.externalErrorHandler = onError;
+
+    readable.on("data", onData);
+    readable.on("end", onEnd);
+    readable.on("error", onError);
+
+    // CORRECTION C1: explicitly resume a re-attached, previously-paused Readable.
+    // The backend's SHARED stdout is reused across every track; a prior non-spotify
+    // advance ran stop() -> detachExternalStream() which pause()d it (state.flowing =
+    // false). Node's Readable.on('data') only auto-resumes when flowing !== false, so
+    // re-attaching a paused stream would leave it stuck: onData never fires, pcmBuffer
+    // stays empty, and a later Spotify track plays only silence. resume() is safe/
+    // idempotent on a first attach (never-paused/already-flowing) stream.
+    readable.resume();
+
+    this.state = "playing";
+    this.startFrameLoop();
+  }
+
+  /**
+   * CORRECTION C2: DETACH, never destroy. The external readable is the backend's
+   * long-lived, SHARED ffmpeg stdout reused across every track; destroying it
+   * would kill the sidecar pipe for all future tracks. Remove only the listeners
+   * WE added and pause the flow so stale PCM stops landing in pcmBuffer, then
+   * clear the external-mode state.
+   */
+  private detachExternalStream(): void {
+    const stream = this.externalStream;
+    if (stream) {
+      if (this.externalDataHandler) stream.off("data", this.externalDataHandler);
+      if (this.externalEndHandler) stream.off("end", this.externalEndHandler);
+      if (this.externalErrorHandler) stream.off("error", this.externalErrorHandler);
+      try {
+        stream.pause();
+      } catch {
+        /* best-effort: never destroy the shared sidecar stream */
+      }
+    }
+    this.externalDataHandler = null;
+    this.externalEndHandler = null;
+    this.externalErrorHandler = null;
+    this.externalStream = null;
+    this.externalMode = false;
+    this.onExternalEnd = null;
+  }
+
   stop(): void {
     // 3. 递增 ID 是最有效的逻辑“隔离墙”
     this.sessionId++; 
@@ -419,6 +547,11 @@ export class AudioPlayer extends EventEmitter {
       this.currentTempDir = null;
     }
 
+    // CORRECTION C2: tear down external mode by DETACHING (remove our listeners +
+    // pause) — never destroy the shared, long-lived sidecar stream. The
+    // sessionId++ above already fences the external data/end/error handlers.
+    this.detachExternalStream();
+
     this.ffmpegPaused = false;
     this.spawnFailed = false;
     this.state = "idle";
@@ -480,7 +613,17 @@ export class AudioPlayer extends EventEmitter {
         ? (this.currentSongDuration - elapsed) <= 5 // 距离结尾不足5秒
         : true; // 未知时长时保守处理
       
-      if (this.ffmpeg !== null && this.pcmBuffer.length < PCM_FRAME_BYTES) {
+      // External mode: the sidecar PCM stream is continuous and never EOFs per
+      // song; a transient underrun must NOT end the track (advance is driven by
+      // the controller). Skip BOTH drain/stall branches while externalMode.
+      //
+      // R3-4: gate BOTH end-detection branches on state==="playing". While paused
+      // the loop still ticks (so a resumed stream can refill), but it must NOT
+      // accumulate stall attempts or emit trackEnd — otherwise pausing a stalled/
+      // unknown-duration stream would auto-advance ~5s later. Because the if is
+      // now false while paused, the else resets emptyFrameAttempts to 0, so a
+      // resumed healthy stream starts fresh and never ends instantly.
+      if (this.state === "playing" && !this.externalMode && this.ffmpeg !== null && this.pcmBuffer.length < PCM_FRAME_BYTES) {
         this.emptyFrameAttempts++;
         
         // End the track when FFmpeg has gone silent: quickly if we're near the
@@ -505,20 +648,20 @@ export class AudioPlayer extends EventEmitter {
             nearEnd: isNearEnd,
           }, "FFmpeg stopped outputting data, ending track");
           this.frameLoopRunning = false;
-          if (this.state !== "idle") {
-            this.state = "idle";
-            // 清理FFmpeg进程
-            if (this.ffmpeg) {
-              const procToKill = this.ffmpeg;
-              const pidToKill = procToKill.pid;
-              this.ffmpeg = null;
-              if (pidToKill) {
-                this.forceCleanup(procToKill, pidToKill);
-              }
+          // The outer gate guarantees state==="playing" here, so no !=="idle"
+          // guard is needed: end the track directly.
+          this.state = "idle";
+          // 清理FFmpeg进程
+          if (this.ffmpeg) {
+            const procToKill = this.ffmpeg;
+            const pidToKill = procToKill.pid;
+            this.ffmpeg = null;
+            if (pidToKill) {
+              this.forceCleanup(procToKill, pidToKill);
             }
-            this.consecutiveFailures = 0;
-            this.emit("trackEnd");
           }
+          this.consecutiveFailures = 0;
+          this.emit("trackEnd");
           return;
         }
       } else {
@@ -526,14 +669,15 @@ export class AudioPlayer extends EventEmitter {
         this.emptyFrameAttempts = 0;
       }
 
-      if (!this.ffmpeg && this.pcmBuffer.length < PCM_FRAME_BYTES) {
+      // R3-4: likewise gated on state==="playing" — a drained/EOF'd stream must
+      // not emit trackEnd while paused; end-detection resumes on resume().
+      if (this.state === "playing" && !this.externalMode && !this.ffmpeg && this.pcmBuffer.length < PCM_FRAME_BYTES) {
         this.frameLoopRunning = false;
-        if (this.state !== "idle") {
-          this.state = "idle";
-          if (!this.spawnFailed) {
-            this.consecutiveFailures = 0;
-            this.emit("trackEnd");
-          }
+        // Outer gate guarantees state==="playing"; end directly (no !=="idle" guard).
+        this.state = "idle";
+        if (!this.spawnFailed) {
+          this.consecutiveFailures = 0;
+          this.emit("trackEnd");
         }
         return;
       }
@@ -542,13 +686,24 @@ export class AudioPlayer extends EventEmitter {
   }
 
   private sendNextFrame(): void {
-    if (this.pcmBuffer.length < PCM_FRAME_BYTES) return;
+    if (this.pcmBuffer.length < PCM_FRAME_BYTES) {
+      // External mode: the sidecar PCM stream is long-lived and must NOT end on
+      // a transient underrun. Emit an encoded silence frame so the 20ms voice
+      // timeline stays continuous instead of returning (which would desync TS).
+      if (this.externalMode) this.emitSilenceFrame();
+      return;
+    }
     const pcmFrame = this.pcmBuffer.subarray(0, PCM_FRAME_BYTES);
     this.pcmBuffer = this.pcmBuffer.subarray(PCM_FRAME_BYTES);
 
-    if (this.ffmpegPaused && this.pcmBuffer.length < AudioPlayer.BUFFER_LOW_WATER && this.ffmpeg?.stdout) {
-      this.ffmpeg.stdout.resume();
-      this.ffmpegPaused = false;
+    if (this.ffmpegPaused && this.pcmBuffer.length < AudioPlayer.BUFFER_LOW_WATER) {
+      if (this.externalMode && this.externalStream) {
+        this.externalStream.resume();
+        this.ffmpegPaused = false;
+      } else if (this.ffmpeg?.stdout) {
+        this.ffmpeg.stdout.resume();
+        this.ffmpegPaused = false;
+      }
     }
 
     try {
@@ -566,6 +721,16 @@ export class AudioPlayer extends EventEmitter {
     }
   }
 
+  private emitSilenceFrame(): void {
+    try {
+      const opusFrame = this.encoder.encode(Buffer.alloc(PCM_FRAME_BYTES));
+      this.emit("frame", opusFrame);
+      this.framesPlayed++;
+    } catch (err) {
+      this.emit("error", err as Error);
+    }
+  }
+
   private applyVolume(pcm: Buffer): Buffer {
     const factor = volumeToFactor(this.volume);
     // factor === 1 only at volume 100; skip the per-sample loop at full loudness.
@@ -578,8 +743,16 @@ export class AudioPlayer extends EventEmitter {
     return out;
   }
 
+  // NOTE: in external (Spotify sidecar) mode getElapsed() is frame-count based
+  // (framesPlayed includes silence frames emitted on underrun) and therefore
+  // only APPROXIMATE — the authoritative position is the controller's live
+  // status.track.position. This approximation is acceptable for Spotify.
   getElapsed(): number { return this.seekOffset + (this.framesPlayed * FRAME_DURATION_MS) / 1000; }
-  seek(seconds: number): void { 
+  seek(seconds: number): void {
+    // External (Spotify sidecar) mode: local seek is a no-op. Respawning ffmpeg
+    // on the spotify: sentinel would collide with the continuous PCM source;
+    // transport is delegated to the SpotifyController by the caller (Task 7).
+    if (this.externalMode) return;
     if (this.currentUrl && Number.isFinite(seconds) && seconds >= 0) {
       this.play(this.currentUrl, seconds, this.currentSongDuration);
     }
@@ -590,4 +763,8 @@ export class AudioPlayer extends EventEmitter {
   setVolume(vol: number): void { this.volume = Math.max(0, Math.min(100, vol)); }
   getVolume(): number { return this.volume; }
   getState(): PlayerState { return this.state; }
+  // True only while attached to an external (Spotify sidecar) PCM stream. Used
+  // by the orchestrator to decide whether to re-attach: stop() detaches (sets
+  // externalMode=false) so this is false after any player.stop().
+  isExternalActive(): boolean { return this.externalMode; }
 }

src/bot/instance.test.ts

@@ -1,6 +1,13 @@
 import { describe, it, expect, vi } from "vitest";
-import { BotInstance, COMMAND_DENIED_MESSAGE } from "./instance.js";
+import { BotInstance, COMMAND_DENIED_MESSAGE, spotifyPortsForBotId } from "./instance.js";
+import type { BotInstanceOptions } from "./instance.js";
 import type { TS3TextMessage } from "../ts-protocol/client.js";
+import type { SpotifyController } from "../music/spotify/controller.js";
+import type { SpotifyOAuth } from "../music/spotify/spotify-oauth.js";
+import type { MusicProvider } from "../music/provider.js";
+import type { BotDatabase } from "../data/database.js";
+import type { AvatarStore } from "../data/avatars.js";
+import type { BotConfig } from "../data/config.js";
 
 // Constructing a real BotInstance is heavy (spawns a TS3Client, AudioPlayer,
 // reads avatars, etc.), and runExclusive only touches a single private field
@@ -218,3 +225,689 @@ describe("BotInstance.getProviderFor — spotify routing", () => {
     expect(BotInstance.prototype.getProviderFor.call(ctx, "spotify" as any)).toBe(spotify);
   });
 });
+
+// --- Spotify orchestration (Task 7 + Correction C4) ------------------------
+// These drive the REAL prototype methods on a hand-built ctx (the file's
+// established `.call(ctx)` style) and assert the routing DECISIONS. Live audio
+// is not testable here. C4 supersedes the brief where they conflict: switching
+// a URL track -> spotify does NOT call player.stop() (playPcmStream fences the
+// prior ffmpeg internally), and a spotify -> spotify handoff does NOT re-attach
+// the persistent PCM stream (playPcmStream is called ONCE across both tracks).
+
+const resolveAndPlay = BotInstance.prototype.resolveAndPlay as (
+  this: unknown,
+  song: any,
+) => Promise<boolean>;
+const setupPlayerEvents = (BotInstance.prototype as any).setupPlayerEvents as (
+  this: unknown,
+) => void;
+const cmdPause = (BotInstance.prototype as any).cmdPause as (this: unknown) => string;
+const cmdResume = (BotInstance.prototype as any).cmdResume as (this: unknown) => string;
+const cmdStop = (BotInstance.prototype as any).cmdStop as (this: unknown) => string;
+const handleOccupancy = (BotInstance.prototype as any).handleOccupancy as (
+  this: unknown,
+  userCount: number,
+) => void;
+const seek = (BotInstance.prototype as any).seek as (this: unknown, seconds: number) => void;
+const playNext = (BotInstance.prototype as any).playNext as (
+  this: unknown,
+  maxRetries?: number,
+) => Promise<boolean>;
+const cmdRemove = (BotInstance.prototype as any).cmdRemove as (
+  this: unknown,
+  cmd: any,
+) => Promise<string> | string;
+
+function makeController() {
+  return {
+    ensureStarted: vi.fn(async () => true),
+    playTrack: vi.fn(async () => true),
+    getPcmStream: vi.fn(() => ({ kind: "pcm" } as any)),
+    pause: vi.fn(async () => {}),
+    resume: vi.fn(async () => {}),
+    seek: vi.fn(async () => {}),
+    stop: vi.fn(() => {}),
+    on: vi.fn(),
+  };
+}
+function makePlayer() {
+  // `externalActive` mirrors the real AudioPlayer: playPcmStream attaches the
+  // external stream (true), and both stop() and play() detach it (false). The
+  // re-attach guard reads isExternalActive(), so this must track that state.
+  // `state` mirrors the real player's PlayerState transitions so tests can
+  // observe paused→playing (R3-3): playPcmStream/play → "playing", stop →
+  // "idle", pause → "paused" (only from playing), resume → "playing" (only
+  // from paused). Existing tests never read state, so tracking it is inert
+  // for them.
+  let externalActive = false;
+  let state: "idle" | "playing" | "paused" = "idle";
+  return {
+    play: vi.fn((..._args: any[]) => { externalActive = false; state = "playing"; }),
+    stop: vi.fn(() => { externalActive = false; state = "idle"; }),
+    playPcmStream: vi.fn((..._args: any[]) => { externalActive = true; state = "playing"; }),
+    pause: vi.fn(() => { if (state === "playing") state = "paused"; }),
+    resume: vi.fn(() => { if (state === "paused") state = "playing"; }),
+    seek: vi.fn(),
+    isExternalActive: vi.fn(() => externalActive),
+    getState: vi.fn(() => state),
+  };
+}
+function makeResolveCtx(opts: {
+  controller: ReturnType<typeof makeController>;
+  player: ReturnType<typeof makePlayer>;
+  url: string;
+  currentSourceIsSpotify?: boolean;
+}) {
+  return {
+    connected: true,
+    config: {},
+    id: "bot1",
+    voteSkipUsers: new Set<string>(),
+    autoPaused: false,
+    currentSourceIsSpotify: opts.currentSourceIsSpotify ?? false,
+    effectiveDuration: undefined,
+    logger: { info: vi.fn(), warn: vi.fn(), error: vi.fn(), debug: vi.fn() },
+    tsClient: { sendTextMessage: vi.fn(async () => {}) },
+    database: { addPlayHistory: vi.fn() },
+    spotifyController: opts.controller,
+    player: opts.player,
+    getProviderFor: vi.fn(() => ({ getSongUrl: async () => ({ url: opts.url }) })),
+    syncProfileToSong: vi.fn(async () => {}),
+    emit: vi.fn(),
+  } as any;
+}
+function spotifySong() {
+  return {
+    id: "abc",
+    name: "Song",
+    artist: "Artist",
+    album: "Album",
+    platform: "spotify",
+    coverUrl: "c",
+    duration: 200,
+    url: "",
+  };
+}
+
+describe("BotInstance.resolveAndPlay — Spotify routing (C4)", () => {
+  it("routes a spotify song to controller.playTrack + player.playPcmStream, not player.play", async () => {
+    const controller = makeController();
+    const player = makePlayer();
+    const ctx = makeResolveCtx({ controller, player, url: "spotify:track:abc" });
+
+    const ok = await resolveAndPlay.call(ctx, spotifySong());
+
+    expect(ok).toBe(true);
+    expect(controller.ensureStarted).toHaveBeenCalledTimes(1);
+    expect(controller.playTrack).toHaveBeenCalledWith("spotify:track:abc");
+    expect(player.playPcmStream).toHaveBeenCalledTimes(1);
+    expect(player.playPcmStream.mock.calls[0][0]).toEqual({ kind: "pcm" });
+    expect(player.play).not.toHaveBeenCalled();
+    // C4: playPcmStream fences the prior url-ffmpeg internally — no player.stop().
+    expect(player.stop).not.toHaveBeenCalled();
+    expect(ctx.currentSourceIsSpotify).toBe(true);
+    expect(ctx.database.addPlayHistory).toHaveBeenCalledTimes(1);
+    expect(ctx.emit).toHaveBeenCalledWith("stateChange");
+  });
+
+  it("returns false + sends the Stage-1 fallback when the backend is unavailable", async () => {
+    const controller = makeController();
+    controller.ensureStarted = vi.fn(async () => false);
+    const player = makePlayer();
+    const ctx = makeResolveCtx({ controller, player, url: "spotify:track:abc" });
+
+    const ok = await resolveAndPlay.call(ctx, spotifySong());
+
+    expect(ok).toBe(false);
+    expect(ctx.tsClient.sendTextMessage).toHaveBeenCalledTimes(1);
+    expect(controller.playTrack).not.toHaveBeenCalled();
+    expect(player.playPcmStream).not.toHaveBeenCalled();
+    expect(player.play).not.toHaveBeenCalled();
+  });
+
+  it("attaches the PCM stream once (no player.stop) when switching URL -> spotify", async () => {
+    const controller = makeController();
+    const player = makePlayer();
+    const ctx = makeResolveCtx({
+      controller, player, url: "spotify:track:abc", currentSourceIsSpotify: false,
+    });
+
+    await resolveAndPlay.call(ctx, spotifySong());
+
+    // C4: NO player.stop() on the URL -> spotify transition.
+    expect(player.stop).not.toHaveBeenCalled();
+    expect(player.playPcmStream).toHaveBeenCalledTimes(1);
+  });
+
+  it("does NOT re-attach the stream on a spotify -> spotify handoff (playPcmStream called once across two tracks)", async () => {
+    const controller = makeController();
+    const player = makePlayer();
+    const ctx = makeResolveCtx({
+      controller, player, url: "spotify:track:abc", currentSourceIsSpotify: false,
+    });
+
+    // First spotify track: coming from a URL/idle source -> attach.
+    await resolveAndPlay.call(ctx, spotifySong());
+    expect(ctx.currentSourceIsSpotify).toBe(true);
+    // Second spotify track: go-librespot changes tracks into the SAME FIFO.
+    await resolveAndPlay.call(ctx, spotifySong());
+
+    expect(player.playPcmStream).toHaveBeenCalledTimes(1); // NOT re-attached
+    expect(controller.playTrack).toHaveBeenCalledTimes(2); // both tracks played
+    expect(player.stop).not.toHaveBeenCalled();
+  });
+
+  it("RE-attaches on a spotify -> (command player.stop) -> spotify sequence (does not stay silent)", async () => {
+    // Regression: command paths (cmdPlay/cmdPlaylist/cmdAlbum/cmdFm) call
+    // player.stop() — which DETACHES the external stream — WITHOUT clearing the
+    // currentSourceIsSpotify flag. Gating re-attach on the stale flag skipped
+    // playPcmStream, silencing the next spotify track. We now gate on the
+    // player's actual external state, so the re-attach happens.
+    const controller = makeController();
+    const player = makePlayer();
+    const ctx = makeResolveCtx({
+      controller, player, url: "spotify:track:abc", currentSourceIsSpotify: false,
+    });
+
+    // First spotify track attaches the persistent PCM stream.
+    await resolveAndPlay.call(ctx, spotifySong());
+    expect(player.playPcmStream).toHaveBeenCalledTimes(1);
+    expect(player.isExternalActive()).toBe(true);
+
+    // A command path stops the player (detaches the stream) but leaves the
+    // spotify flag stale-true — exactly the state that used to cause silence.
+    player.stop();
+    expect(player.isExternalActive()).toBe(false);
+    expect(ctx.currentSourceIsSpotify).toBe(true); // flag NOT cleared by stop()
+
+    // Next spotify track MUST re-attach (gate on player external state, not flag).
+    await resolveAndPlay.call(ctx, spotifySong());
+    expect(player.playPcmStream).toHaveBeenCalledTimes(2);
+    expect(player.isExternalActive()).toBe(true);
+  });
+
+  it("returns false + sends the fallback when playTrack resolves false (dead/failed sidecar)", async () => {
+    const controller = makeController();
+    controller.playTrack = vi.fn(async () => false);
+    const player = makePlayer();
+    const ctx = makeResolveCtx({ controller, player, url: "spotify:track:abc" });
+
+    const ok = await resolveAndPlay.call(ctx, spotifySong());
+
+    expect(ok).toBe(false);
+    expect(controller.playTrack).toHaveBeenCalledTimes(1);
+    // Same Stage-1 fallback message as the backend-unavailable path.
+    expect(ctx.tsClient.sendTextMessage).toHaveBeenCalledTimes(1);
+    // Never attach the player to a dead stream.
+    expect(player.playPcmStream).not.toHaveBeenCalled();
+    expect(ctx.currentSourceIsSpotify).toBe(false);
+  });
+
+  it("recovers on mid-session sidecar death: onExternalEnd stops controller+player and clears the flag", async () => {
+    const controller = makeController();
+    const player = makePlayer();
+    const ctx = makeResolveCtx({ controller, player, url: "spotify:track:abc" });
+
+    await resolveAndPlay.call(ctx, spotifySong());
+    expect(ctx.currentSourceIsSpotify).toBe(true);
+    expect(player.playPcmStream).toHaveBeenCalledTimes(1);
+
+    // The sidecar PCM stream EOFs mid-session → fire the wired onExternalEnd.
+    const opts = player.playPcmStream.mock.calls[0][1] as { onExternalEnd?: () => void };
+    expect(typeof opts.onExternalEnd).toBe("function");
+    opts.onExternalEnd!();
+
+    // Recovery: controller torn down (next track rebuilds), player stopped
+    // (drops external mode so the next track re-attaches), flag cleared.
+    expect(controller.stop).toHaveBeenCalledTimes(1);
+    expect(player.stop).toHaveBeenCalledTimes(1);
+    expect(ctx.currentSourceIsSpotify).toBe(false);
+  });
+
+  it("pauses the sidecar and clears the flag when switching to a non-spotify track", async () => {
+    const controller = makeController();
+    const player = makePlayer();
+    const song = { ...spotifySong(), platform: "netease" };
+    const ctx = makeResolveCtx({
+      controller, player, url: "http://cdn/x.mp3", currentSourceIsSpotify: true,
+    });
+
+    const ok = await resolveAndPlay.call(ctx, song);
+
+    expect(ok).toBe(true);
+    expect(controller.pause).toHaveBeenCalledTimes(1);
+    expect(ctx.currentSourceIsSpotify).toBe(false);
+    expect(player.play).toHaveBeenCalledWith("http://cdn/x.mp3", 0, 200);
+    expect(player.playPcmStream).not.toHaveBeenCalled();
+  });
+
+  // R3-3: spotify A playing → pause → skip to spotify B. The persistent PCM
+  // stream stays attached through the pause, so the re-attach gate skips
+  // playPcmStream (which is what sets state='playing'). Without an explicit
+  // resume the player would sit 'paused' and emit silence while the sidecar
+  // decodes B. The reuse path must force the player back to PLAYING.
+  it("resumes the player when skipping from a PAUSED spotify track to another spotify track (R3-3)", async () => {
+    const controller = makeController();
+    const player = makePlayer();
+    const ctx = makeResolveCtx({
+      controller, player, url: "spotify:track:abc", currentSourceIsSpotify: false,
+    });
+
+    // Spotify A starts → PCM stream attached, player playing.
+    await resolveAndPlay.call(ctx, spotifySong());
+    expect(player.getState()).toBe("playing");
+    expect(player.isExternalActive()).toBe(true);
+
+    // User pauses: player paused; the external stream stays attached.
+    player.pause();
+    expect(player.getState()).toBe("paused");
+    expect(player.isExternalActive()).toBe(true);
+
+    // Skip to spotify B via the external-stream-reuse path (playPcmStream skipped).
+    await resolveAndPlay.call(ctx, spotifySong());
+
+    // Player must be PLAYING again (frames would emit), not stuck paused.
+    expect(player.getState()).toBe("playing");
+    // Reuse path — the persistent stream was NOT re-attached.
+    expect(player.playPcmStream).toHaveBeenCalledTimes(1);
+    expect(player.resume).toHaveBeenCalled();
+  });
+
+  // The normal (non-paused) spotify→spotify handoff must be unaffected: resume
+  // on an already-playing player is a no-op, so state stays 'playing'.
+  it("keeps a non-paused spotify→spotify handoff playing (R3-3 no-regression)", async () => {
+    const controller = makeController();
+    const player = makePlayer();
+    const ctx = makeResolveCtx({
+      controller, player, url: "spotify:track:abc", currentSourceIsSpotify: false,
+    });
+
+    await resolveAndPlay.call(ctx, spotifySong());
+    await resolveAndPlay.call(ctx, spotifySong());
+
+    expect(player.getState()).toBe("playing");
+    expect(player.playPcmStream).toHaveBeenCalledTimes(1);
+  });
+});
+
+describe("BotInstance.setupPlayerEvents — controller trackEnded wiring", () => {
+  function makeEventCtx(currentPlatform: string) {
+    return {
+      spotifyController: { on: vi.fn() },
+      player: { on: vi.fn() },
+      queue: { current: vi.fn(() => ({ platform: currentPlatform })) },
+      logger: { debug: vi.fn(), error: vi.fn() },
+      playNext: vi.fn(async () => true),
+    } as any;
+  }
+  function trackEndedHandler(ctx: any) {
+    const call = ctx.spotifyController.on.mock.calls.find(
+      (c: any[]) => c[0] === "trackEnded",
+    );
+    expect(call).toBeDefined();
+    return call[1] as (e: any) => void;
+  }
+
+  it("advances via playNext when the current song is spotify", () => {
+    const ctx = makeEventCtx("spotify");
+    setupPlayerEvents.call(ctx);
+    trackEndedHandler(ctx)({ uri: "spotify:track:x", reason: "ended" });
+    expect(ctx.playNext).toHaveBeenCalledTimes(1);
+  });
+
+  it("ignores controller trackEnded when the current song is not spotify", () => {
+    const ctx = makeEventCtx("netease");
+    setupPlayerEvents.call(ctx);
+    trackEndedHandler(ctx)({ uri: "spotify:track:x", reason: "ended" });
+    expect(ctx.playNext).not.toHaveBeenCalled();
+  });
+});
+
+describe("BotInstance transport delegation — spotify current song", () => {
+  function makeCmdCtx(currentPlatform: string) {
+    return {
+      player: { pause: vi.fn(), resume: vi.fn(), stop: vi.fn() },
+      spotifyController: {
+        pause: vi.fn(async () => {}),
+        resume: vi.fn(async () => {}),
+        stop: vi.fn(() => {}),
+      },
+      queue: { current: vi.fn(() => ({ platform: currentPlatform })), clear: vi.fn() },
+      logger: { warn: vi.fn() },
+      emit: vi.fn(),
+      autoPaused: true,
+      currentSourceIsSpotify: true,
+      sweepLocalAudio: vi.fn(),
+      disableFmMode: vi.fn(),
+      profileManager: { onSongChange: vi.fn(async () => {}) },
+    } as any;
+  }
+
+  it("cmdPause delegates to controller.pause when current is spotify", () => {
+    const ctx = makeCmdCtx("spotify");
+    cmdPause.call(ctx);
+    expect(ctx.player.pause).toHaveBeenCalled();
+    expect(ctx.spotifyController.pause).toHaveBeenCalledTimes(1);
+  });
+
+  it("cmdResume delegates to controller.resume when current is spotify", () => {
+    const ctx = makeCmdCtx("spotify");
+    cmdResume.call(ctx);
+    expect(ctx.player.resume).toHaveBeenCalled();
+    expect(ctx.spotifyController.resume).toHaveBeenCalledTimes(1);
+  });
+
+  it("cmdStop stops the sidecar + player and clears the spotify flag", () => {
+    const ctx = makeCmdCtx("spotify");
+    cmdStop.call(ctx);
+    expect(ctx.spotifyController.stop).toHaveBeenCalledTimes(1);
+    expect(ctx.player.stop).toHaveBeenCalledTimes(1);
+    expect(ctx.queue.clear).toHaveBeenCalledTimes(1);
+    expect(ctx.currentSourceIsSpotify).toBe(false);
+  });
+
+  it("does NOT touch the controller when current is not spotify", () => {
+    const ctx = makeCmdCtx("netease");
+    cmdPause.call(ctx);
+    cmdResume.call(ctx);
+    expect(ctx.spotifyController.pause).not.toHaveBeenCalled();
+    expect(ctx.spotifyController.resume).not.toHaveBeenCalled();
+  });
+});
+
+// --- R3-2: removing the currently-playing Spotify track ---------------------
+// queue.remove() of the current index only decrements currentIndex; it never
+// stops the player or sidecar. A spotify track has NO player self-EOF advance
+// path, so leaving the sidecar running while queue.current() is no longer that
+// track wedges the bot in silence. cmdRemove must reconcile: stop the sidecar
+// and advance (or stop cleanly) — but ONLY for a current spotify track.
+describe("BotInstance.cmdRemove — spotify current-track reconciliation (R3-2)", () => {
+  function makeRemoveCtx(opts: {
+    currentIndex: number;
+    currentSourceIsSpotify: boolean;
+    removed: any;
+  }) {
+    return {
+      currentSourceIsSpotify: opts.currentSourceIsSpotify,
+      queue: {
+        getCurrentIndex: vi.fn(() => opts.currentIndex),
+        remove: vi.fn(() => opts.removed),
+      },
+      spotifyController: makeController(),
+      player: makePlayer(),
+      playNext: vi.fn(async () => true),
+      sweepLocalAudio: vi.fn(),
+      emit: vi.fn(),
+      logger: { warn: vi.fn(), info: vi.fn(), error: vi.fn(), debug: vi.fn() },
+    } as any;
+  }
+
+  it("stops the sidecar and advances when removing the CURRENT spotify track", async () => {
+    const ctx = makeRemoveCtx({
+      currentIndex: 0,
+      currentSourceIsSpotify: true,
+      removed: { name: "A", platform: "spotify" },
+    });
+
+    const reply = await cmdRemove.call(ctx, { args: "1" }); // index 0 == current
+
+    expect(reply).toContain("Removed: A");
+    expect(ctx.spotifyController.stop).toHaveBeenCalledTimes(1);
+    expect(ctx.currentSourceIsSpotify).toBe(false);
+    expect(ctx.player.stop).toHaveBeenCalledTimes(1);
+    // Advance to whatever is now current (or stop cleanly if empty).
+    expect(ctx.playNext).toHaveBeenCalledTimes(1);
+  });
+
+  it("does NOT stop the sidecar when removing a NON-current track", async () => {
+    const ctx = makeRemoveCtx({
+      currentIndex: 0,
+      currentSourceIsSpotify: true,
+      removed: { name: "B", platform: "spotify" },
+    });
+
+    await cmdRemove.call(ctx, { args: "2" }); // index 1 != current (0)
+
+    expect(ctx.spotifyController.stop).not.toHaveBeenCalled();
+    expect(ctx.player.stop).not.toHaveBeenCalled();
+    expect(ctx.playNext).not.toHaveBeenCalled();
+    expect(ctx.currentSourceIsSpotify).toBe(true);
+  });
+
+  it("does NOT stop the sidecar when the current track is NOT spotify (URL self-heals)", async () => {
+    const ctx = makeRemoveCtx({
+      currentIndex: 0,
+      currentSourceIsSpotify: false, // current is a URL track
+      removed: { name: "A", platform: "netease" },
+    });
+
+    await cmdRemove.call(ctx, { args: "1" }); // index 0 == current
+
+    expect(ctx.spotifyController.stop).not.toHaveBeenCalled();
+    expect(ctx.player.stop).not.toHaveBeenCalled();
+    expect(ctx.playNext).not.toHaveBeenCalled();
+  });
+
+  it("returns 'Invalid position' without touching the sidecar on a bad index", async () => {
+    const ctx = makeRemoveCtx({
+      currentIndex: 0,
+      currentSourceIsSpotify: true,
+      removed: null, // queue.remove() rejects the index
+    });
+
+    const reply = await cmdRemove.call(ctx, { args: "9" });
+
+    expect(reply).toBe("Invalid position");
+    expect(ctx.spotifyController.stop).not.toHaveBeenCalled();
+    expect(ctx.playNext).not.toHaveBeenCalled();
+  });
+});
+
+// --- R3-6: queue exhausts on a spotify track --------------------------------
+// playNext's exhausted (non-FM) branch only called player.stop(); it left the
+// sidecar decoding and currentSourceIsSpotify stale — diverging from cmdStop.
+describe("BotInstance.playNext — spotify teardown on queue exhaust (R3-6)", () => {
+  function makeExhaustCtx(opts: { currentSourceIsSpotify: boolean }) {
+    return {
+      connected: true,
+      isAdvancing: false,
+      isFmMode: false,
+      currentSourceIsSpotify: opts.currentSourceIsSpotify,
+      voteSkipUsers: new Set<string>(),
+      queue: { next: vi.fn(() => null), unplayedCount: vi.fn(() => 0) },
+      player: makePlayer(),
+      spotifyController: makeController(),
+      profileManager: { onSongChange: vi.fn(async () => {}) },
+      resolveAndPlay: vi.fn(async () => true),
+      sweepLocalAudio: vi.fn(),
+      emit: vi.fn(),
+      logger: { warn: vi.fn(), info: vi.fn(), error: vi.fn(), debug: vi.fn() },
+    } as any;
+  }
+
+  it("stops the sidecar and clears the flag when a spotify queue exhausts", async () => {
+    const ctx = makeExhaustCtx({ currentSourceIsSpotify: true });
+
+    const started = await playNext.call(ctx);
+
+    expect(started).toBe(false);
+    expect(ctx.spotifyController.stop).toHaveBeenCalledTimes(1);
+    expect(ctx.currentSourceIsSpotify).toBe(false);
+    expect(ctx.player.stop).toHaveBeenCalledTimes(1);
+  });
+
+  it("does NOT stop the sidecar when a NON-spotify queue exhausts", async () => {
+    const ctx = makeExhaustCtx({ currentSourceIsSpotify: false });
+
+    const started = await playNext.call(ctx);
+
+    expect(started).toBe(false);
+    expect(ctx.spotifyController.stop).not.toHaveBeenCalled();
+    expect(ctx.player.stop).toHaveBeenCalledTimes(1);
+  });
+});
+
+describe("BotInstance.handleOccupancy — spotify auto-pause delegation (C4)", () => {
+  function makeOccupancyCtx(currentPlatform: string, state: string) {
+    return {
+      player: { getState: () => state, pause: vi.fn(), resume: vi.fn() },
+      spotifyController: { pause: vi.fn(async () => {}), resume: vi.fn(async () => {}) },
+      queue: { current: vi.fn(() => ({ platform: currentPlatform })) },
+      config: { autoPauseOnEmpty: true },
+      autoPaused: false,
+      logger: { warn: vi.fn() },
+      emit: vi.fn(),
+      _scheduleIdleCheck: vi.fn(),
+      _cancelIdleTimer: vi.fn(),
+    } as any;
+  }
+
+  it("delegates pause to the controller when auto-pausing a spotify track (empty channel)", () => {
+    const ctx = makeOccupancyCtx("spotify", "playing");
+    handleOccupancy.call(ctx, 0);
+    expect(ctx.player.pause).toHaveBeenCalledTimes(1);
+    expect(ctx.spotifyController.pause).toHaveBeenCalledTimes(1);
+    expect(ctx.autoPaused).toBe(true);
+  });
+
+  it("delegates resume to the controller when a listener returns to a spotify track", () => {
+    const ctx = makeOccupancyCtx("spotify", "paused");
+    ctx.autoPaused = true;
+    handleOccupancy.call(ctx, 1);
+    expect(ctx.player.resume).toHaveBeenCalledTimes(1);
+    expect(ctx.spotifyController.resume).toHaveBeenCalledTimes(1);
+    expect(ctx.autoPaused).toBe(false);
+  });
+
+  it("does NOT touch the controller when auto-pausing a non-spotify track", () => {
+    const ctx = makeOccupancyCtx("netease", "playing");
+    handleOccupancy.call(ctx, 0);
+    expect(ctx.player.pause).toHaveBeenCalledTimes(1);
+    expect(ctx.spotifyController.pause).not.toHaveBeenCalled();
+  });
+});
+
+describe("BotInstance.seek — spotify routing (C4)", () => {
+  function makeSeekCtx(currentPlatform: string) {
+    return {
+      queue: { current: vi.fn(() => ({ platform: currentPlatform })) },
+      spotifyController: { seek: vi.fn(async () => {}) },
+      player: { seek: vi.fn() },
+      logger: { warn: vi.fn() },
+    } as any;
+  }
+
+  it("routes seek to the controller for a spotify track, converting seconds -> ms", () => {
+    const ctx = makeSeekCtx("spotify");
+    seek.call(ctx, 30); // 30 seconds
+    // SpotifyController.seek is millisecond-based: 30s -> 30000ms (not 30).
+    expect(ctx.spotifyController.seek).toHaveBeenCalledWith(30000);
+    expect(ctx.player.seek).not.toHaveBeenCalled();
+  });
+
+  it("routes seek to the player (seconds-based) for a non-spotify track", () => {
+    const ctx = makeSeekCtx("netease");
+    seek.call(ctx, 30);
+    expect(ctx.player.seek).toHaveBeenCalledWith(30);
+    expect(ctx.spotifyController.seek).not.toHaveBeenCalled();
+  });
+});
+
+// --- Spotify OAuth threading (Task 6, C3.1) --------------------------------
+// The process-wide shared SpotifyOAuth must reach the SpotifyController via the
+// controller factory. We drive the REAL BotInstance constructor with a fake
+// controller factory that captures its param object, so the thread is observed
+// end-to-end (options.spotifyOAuth -> buildController({ oauth })).
+describe("BotInstance — spotifyOAuth threading to the controller factory (C3.1)", () => {
+  function makeInstanceOptions(over: Partial<BotInstanceOptions> = {}): {
+    options: BotInstanceOptions;
+    captured: { param?: { oauth?: SpotifyOAuth; instanceId?: string } };
+  } {
+    const captured: { param?: { oauth?: SpotifyOAuth; instanceId?: string } } = {};
+    const provider = { platform: "netease" } as unknown as MusicProvider;
+    const logger: any = {
+      info() {}, warn() {}, error() {}, debug() {},
+      child() { return logger; },
+    };
+    const database = {
+      getProfileConfig: () => ({}),
+      getCustomAvatarPath: () => null,
+    } as unknown as BotDatabase;
+    const options: BotInstanceOptions = {
+      id: "bot-oauth-test",
+      name: "OAuthBot",
+      tsOptions: { host: "localhost", port: 9987, queryPort: 10011, nickname: "OAuthBot" } as any,
+      neteaseProvider: provider,
+      qqProvider: provider,
+      bilibiliProvider: provider,
+      youtubeProvider: provider,
+      database,
+      config: { spotify: {} } as unknown as BotConfig,
+      logger,
+      avatarStore: { read: () => null } as unknown as AvatarStore,
+      spotifyControllerFactory: (o) => {
+        captured.param = o;
+        // Only `on` is touched during construction (setupPlayerEvents wires
+        // the "trackEnded" listener); return a minimal fake controller.
+        return { on: () => {} } as unknown as SpotifyController;
+      },
+      ...over,
+    };
+    return { options, captured };
+  }
+
+  it("forwards the injected spotifyOAuth to the controller factory as `oauth`", () => {
+    const sentinel = {} as unknown as SpotifyOAuth;
+    const { options, captured } = makeInstanceOptions({ spotifyOAuth: sentinel });
+    // eslint-disable-next-line no-new
+    new BotInstance(options);
+    expect(captured.param).toBeDefined();
+    expect(captured.param?.oauth).toBe(sentinel);
+  });
+
+  it("leaves the factory `oauth` undefined when no spotifyOAuth is supplied (behavior-unchanged)", () => {
+    const { options, captured } = makeInstanceOptions();
+    // eslint-disable-next-line no-new
+    new BotInstance(options);
+    expect(captured.param).toBeDefined();
+    expect(captured.param?.oauth).toBeUndefined();
+  });
+
+  // R2-5: the bot id must reach the controller as `instanceId` so the backend
+  // derives a UNIQUE Spotify Connect device name (<base>-<id>). Without it two
+  // bots share the process-global deviceName and Connect commands misroute.
+  it("forwards the bot id to the controller factory as `instanceId` (corner-case R2-5)", () => {
+    const { options, captured } = makeInstanceOptions({ id: "bot-xyz" });
+    // eslint-disable-next-line no-new
+    new BotInstance(options);
+    expect(captured.param).toBeDefined();
+    expect(captured.param?.instanceId).toBe("bot-xyz");
+  });
+});
+
+describe("spotifyPortsForBotId — per-bot go-librespot ports (Fix 3)", () => {
+  it("yields the SAME ports for the same bot id (stable across restarts)", () => {
+    const a = spotifyPortsForBotId("bot-alpha");
+    const b = spotifyPortsForBotId("bot-alpha");
+    expect(a).toEqual(b);
+  });
+
+  it("yields DIFFERENT ports for different bot ids", () => {
+    const a = spotifyPortsForBotId("bot-alpha");
+    const b = spotifyPortsForBotId("bot-beta");
+    expect(a.apiPort).not.toBe(b.apiPort);
+    expect(a.callbackPort).not.toBe(b.callbackPort);
+  });
+
+  it("keeps apiPort and callbackPort in disjoint ranges", () => {
+    for (const id of ["bot-alpha", "bot-beta", "x", "a-very-long-bot-identifier-123"]) {
+      const { apiPort, callbackPort } = spotifyPortsForBotId(id);
+      expect(apiPort).toBeGreaterThanOrEqual(3700);
+      expect(apiPort).toBeLessThan(4700);
+      expect(callbackPort).toBeGreaterThanOrEqual(8700);
+      expect(callbackPort).toBeLessThan(9700);
+      // Same offset within each range → the two never collide with each other.
+      expect(callbackPort - apiPort).toBe(5000);
+    }
+  });
+});

src/bot/instance.ts

@@ -15,7 +15,7 @@ import {
 import { parseSongRef, parseSelectionIndex } from "./song-ref.js";
 import type { Logger } from "../logger.js";
 import type { BotDatabase, ProfileConfig } from "../data/database.js";
-import type { BotConfig } from "../data/config.js";
+import type { BotConfig, SpotifyConfig } from "../data/config.js";
 import { BotProfileManager } from "./profile.js";
 import type { AvatarStore } from "../data/avatars.js";
 import {
@@ -24,10 +24,50 @@ import {
   shouldResumeOnReturn,
 } from "./auto-pause.js";
 import { isSpotifyUri } from "../music/spotify/webapi.js";
+import path from "node:path";
+import { SpotifyController } from "../music/spotify/controller.js";
+import type { SpotifyTrackEndedEvent } from "../music/spotify/backend.js";
+import type { SpotifyOAuth } from "../music/spotify/spotify-oauth.js";
 
 /** Reply sent when a non-admin invokes an admin-only chat command. */
 export const COMMAND_DENIED_MESSAGE = "⛔ 需要管理员权限（该命令仅限管理员服务器组）";
 
+/** Fallback message when Spotify audio can't be served (backend unavailable
+ *  OR a per-track playTrack failure against a dead/failed sidecar). */
+const SPOTIFY_UNAVAILABLE_MESSAGE =
+  "⚠️ Spotify 播放尚未启用（需要 librespot 音频后端，将在后续版本支持）。";
+
+/** FNV-1a deterministic string hash (unsigned 32-bit). Stable across restarts
+ *  and processes, unlike a random/insertion-order value — used to derive
+ *  per-bot go-librespot ports. */
+function stableHash(s: string): number {
+  let h = 2166136261;
+  for (let i = 0; i < s.length; i++) {
+    h ^= s.charCodeAt(i);
+    h = Math.imul(h, 16777619);
+  }
+  return h >>> 0;
+}
+
+/**
+ * STABLE per-bot go-librespot ports derived from the bot id, kept fixed across
+ * restarts. The two ranges (37xx / 87xx) are disjoint so a single bot's API and
+ * callback ports never clash with each other.
+ *
+ * NOTE: the hash (`% 1000`) only REDUCES collisions between bots — it does NOT
+ * guarantee uniqueness. Two Spotify-enabled bots whose ids collide mod 1000
+ * (birthday-bound: likely well below 1000 bots) get IDENTICAL ports; the second
+ * bot's go-librespot sidecar then fails to bind 127.0.0.1:<apiPort>, start()
+ * throws, and that bot's Spotify stays permanently unavailable. Proper fix (out
+ * of scope here): assign each Spotify-enabled bot a unique free port —
+ * manager-coordinated allocation or bind-retry on EADDRINUSE — instead of a
+ * pure hash.
+ */
+export function spotifyPortsForBotId(id: string): { apiPort: number; callbackPort: number } {
+  const off = stableHash(id) % 1000;
+  return { apiPort: 3700 + off, callbackPort: 8700 + off };
+}
+
 export interface BotInstanceOptions {
   id: string;
   name: string;
@@ -43,6 +83,22 @@ export interface BotInstanceOptions {
   config: BotConfig;
   logger: Logger;
   avatarStore: AvatarStore;
+  /** Base dir (under DATA_DIR) for per-bot go-librespot work/config trees. */
+  spotifyDataDir?: string;
+  /** Process-wide shared Spotify OAuth (single account); injected into the
+   *  SpotifyController so web-login authorization is visible to playback (C3.1). */
+  spotifyOAuth?: SpotifyOAuth;
+  /** Test seam: build a fake controller instead of a real go-librespot one. */
+  spotifyControllerFactory?: (o: {
+    config: SpotifyConfig;
+    workDir: string;
+    configDir: string;
+    logger: Logger;
+    instanceId: string;
+    apiPort: number;
+    callbackPort: number;
+    oauth?: SpotifyOAuth;
+  }) => SpotifyController;
 }
 
 export interface BotStatus {
@@ -66,6 +122,7 @@ export class BotInstance extends EventEmitter {
 
   private tsClient: TS3Client;
   private player: AudioPlayer;
+  private spotifyController: SpotifyController;
   private queue: PlayQueue;
   private neteaseProvider: MusicProvider;
   private qqProvider: MusicProvider;
@@ -85,6 +142,9 @@ export class BotInstance extends EventEmitter {
   private idleTimer: ReturnType<typeof setTimeout> | null = null;
   private channelUserCount = 0;
   private autoPaused = false;
+  /** True while the audible track is served by the Spotify sidecar (external
+   *  PCM mode) — drives fence/handoff decisions in resolveAndPlay + cmdStop. */
+  private currentSourceIsSpotify = false;
   private profileManager: BotProfileManager;
   private isFmMode = false;
   private fmProvider: MusicProvider | null = null;
@@ -114,6 +174,35 @@ export class BotInstance extends EventEmitter {
     this.player = new AudioPlayer(this.logger);
     this.queue = new PlayQueue();
 
+    // One long-lived Spotify sidecar controller per bot. Construction is
+    // cheap and side-effect-free — nothing spawns until ensureStarted().
+    const spotifyBase =
+      options.spotifyDataDir ?? path.join(process.cwd(), "data", "spotify");
+    const spotifyWorkDir = path.join(spotifyBase, this.id, "work");
+    const spotifyConfigDir = path.join(spotifyBase, this.id, "config");
+    // Stable per-bot ports for the go-librespot control API / OAuth callback.
+    // These only reduce (not eliminate) cross-bot collisions: two bot ids that
+    // hash to the same % 1000 bucket share ports and the second sidecar fails to
+    // bind — see spotifyPortsForBotId() for the recommended per-bot free-port fix.
+    const { apiPort: spotifyApiPort, callbackPort: spotifyCallbackPort } =
+      spotifyPortsForBotId(this.id);
+    const buildController =
+      options.spotifyControllerFactory ??
+      ((o) => new SpotifyController({ ...o }));
+    this.spotifyController = buildController({
+      config: this.config.spotify,
+      workDir: spotifyWorkDir,
+      configDir: spotifyConfigDir,
+      logger: this.logger,
+      // Per-bot id → unique Spotify Connect device name ("<base>-<id>"), so two
+      // bots under the one shared account never register the same name and
+      // misroute Connect commands (corner-case R2-5).
+      instanceId: this.id,
+      apiPort: spotifyApiPort,
+      callbackPort: spotifyCallbackPort,
+      oauth: options.spotifyOAuth,
+    });
+
     const profileConfig = this.database.getProfileConfig(this.id);
     this.profileManager = new BotProfileManager(
       this.tsClient,
@@ -155,6 +244,19 @@ export class BotInstance extends EventEmitter {
         this.logger.error({ err: err2 }, "playNext failed after player error");
       });
     });
+
+    // Spotify advances exclusively via the sidecar's WebSocket "trackEnded"
+    // (the continuous go-librespot→ffmpeg pipe never EOFs per track, so the
+    // player's own underrun "trackEnd" is suppressed in external mode). Guard
+    // on the current song being spotify so a stray event can't double-advance
+    // a URL track; playNext()'s isAdvancing guard covers any residual race.
+    this.spotifyController.on("trackEnded", (_e: SpotifyTrackEndedEvent) => {
+      if (this.queue.current()?.platform !== "spotify") return;
+      this.logger.debug("Spotify track ended, advancing queue");
+      this.playNext().catch((err) => {
+        this.logger.error({ err }, "playNext failed after spotify trackEnded");
+      });
+    });
   }
 
   isLocalAudioEnabled(): boolean {
@@ -205,6 +307,8 @@ export class BotInstance extends EventEmitter {
       // this.connected was never flipped to true. Previously this handler
       // short-circuited on !this.connected, leaving player stuck as "playing".
       this.connected = false;
+      this.spotifyController.stop();
+      this.currentSourceIsSpotify = false;
       this.player.stop();
       this.queue.clear();
       this.sweepLocalAudio("disconnected");
@@ -288,6 +392,8 @@ export class BotInstance extends EventEmitter {
 
   disconnect(): void {
     this._cancelIdleTimer();
+    this.spotifyController.stop();
+    this.currentSourceIsSpotify = false;
     this.player.stop();
     this.queue.clear();
     this.sweepLocalAudio("disconnected");
@@ -310,6 +416,10 @@ export class BotInstance extends EventEmitter {
     this.config.autoPauseOnEmpty = enabled;
     if (!enabled && this.autoPaused && this.player.getState() === "paused") {
       this.player.resume();
+      if (this.queue.current()?.platform === "spotify") {
+        this.spotifyController.resume().catch((err) =>
+          this.logger.warn({ err }, "Spotify resume failed (auto-pause disabled)"));
+      }
       this.autoPaused = false;
       this.emit("stateChange");
     }
@@ -343,10 +453,21 @@ export class BotInstance extends EventEmitter {
     );
     if (action === "pause") {
       this.player.pause();
+      // Occupancy paths drive player.pause()/resume() DIRECTLY (bypassing the
+      // cmd handlers), so they must ALSO stop/resume the sidecar — else it
+      // keeps decoding into an empty channel.
+      if (this.queue.current()?.platform === "spotify") {
+        this.spotifyController.pause().catch((err) =>
+          this.logger.warn({ err }, "Spotify pause failed (occupancy)"));
+      }
       this.autoPaused = true;
       this.emit("stateChange");
     } else if (action === "resume") {
       this.player.resume();
+      if (this.queue.current()?.platform === "spotify") {
+        this.spotifyController.resume().catch((err) =>
+          this.logger.warn({ err }, "Spotify resume failed (occupancy)"));
+      }
       this.autoPaused = false;
       this.emit("stateChange");
     }
@@ -587,16 +708,94 @@ export class BotInstance extends EventEmitter {
         );
         return false;
       }
-      // Stage 1: Spotify metadata works but audio is not wired yet. getSongUrl
-      // returns a `spotify:` sentinel — never hand it to ffmpeg. Tell the user
-      // and skip so the queue keeps moving. `sendTextMessage` is the same
-      // channel-message helper the command handlers use elsewhere in this file.
+      // Stage 2: a `spotify:` sentinel URI means the go-librespot sidecar
+      // serves the audio, NOT ffmpeg. Start the per-bot sidecar on demand; if
+      // it can't run (disabled / non-Linux / binary missing) keep the Stage-1
+      // fallback message + skip so the queue keeps moving.
       if (isSpotifyUri(result.url)) {
-        this.logger.info({ songId: song.id, name: song.name }, "Spotify playback not enabled yet — skipping");
-        await this.tsClient.sendTextMessage(
-          "⚠️ Spotify 播放尚未启用（需要 librespot 音频后端，将在后续版本支持）。"
-        );
-        return false;
+        const ready = await this.spotifyController.ensureStarted();
+        if (!ready) {
+          this.logger.info({ songId: song.id, name: song.name }, "Spotify backend unavailable — skipping");
+          await this.tsClient.sendTextMessage(SPOTIFY_UNAVAILABLE_MESSAGE);
+          return false;
+        }
+        // `spotify:track:<id>` is the URI. go-librespot decodes into a SINGLE
+        // continuous FIFO/PCM stream, so per-track playback is just a REST
+        // playTrack — the stream keeps flowing. A false result means the
+        // sidecar failed the play (dead/errored backend): never attach the
+        // player to a dead stream — send the same fallback and skip.
+        const played = await this.spotifyController.playTrack(result.url);
+        if (!played) {
+          this.logger.info({ songId: song.id, name: song.name }, "Spotify playTrack failed — skipping");
+          await this.tsClient.sendTextMessage(SPOTIFY_UNAVAILABLE_MESSAGE);
+          return false;
+        }
+        // Only ATTACH the persistent PCM stream when the player is NOT already
+        // attached to it. Gate on the player's ACTUAL external state, not the
+        // currentSourceIsSpotify flag: command paths (cmdPlay/cmdPlaylist/…)
+        // call player.stop() (which detaches the external stream) WITHOUT
+        // clearing the flag, so a stale-true flag would skip the re-attach and
+        // silence playback. playPcmStream internally fences the prior url-ffmpeg
+        // (so NO player.stop() here). On the gapless auto-advance path the
+        // player is still attached (isExternalActive() === true) so we do NOT
+        // re-attach — the sidecar rolls the SAME FIFO into the next track.
+        // KNOWN LIMITATION: on a gapless spotify->spotify advance the player's
+        // frame counter is not reset, so player.getElapsed() over-reads for the
+        // 2nd+ consecutive Spotify track. Cosmetic only — the authoritative
+        // elapsed shown to users is status.track.position from the backend poll.
+        if (!this.player.isExternalActive()) {
+          this.player.playPcmStream(this.spotifyController.getPcmStream(), {
+            // The sidecar PCM pipe is long-lived; per-track end arrives via the
+            // controller "trackEnded" WS event, not stream EOF. A real EOF here
+            // means the sidecar died mid-session — RECOVER instead of emitting
+            // silence forever (which would also leave the player stuck in
+            // externalMode, so the re-attach gate skips every future track).
+            // Tear the controller down (next ensureStarted() rebuilds a fresh
+            // backend), stop the player (drop external mode so it re-attaches),
+            // and clear the flag so a non-spotify track isn't mis-handled.
+            onExternalEnd: () => {
+              this.logger.warn("Spotify PCM stream ended unexpectedly — recovering");
+              this.spotifyController.stop();
+              this.player.stop();
+              this.currentSourceIsSpotify = false;
+            },
+          });
+        } else {
+          // External-stream-reuse path: the persistent PCM stream is still
+          // attached (e.g. we arrived here via pause → skip-within-spotify,
+          // where the stream stays attached through the pause). playPcmStream —
+          // which is what puts the player into the 'playing' state — is skipped,
+          // so without this the player would stay 'paused' and emit silence
+          // while the sidecar decodes the new track (corner-case R3-3). resume()
+          // is a no-op on an already-playing player, so the normal (non-paused)
+          // spotify→spotify handoff is unaffected.
+          this.player.resume();
+        }
+        this.currentSourceIsSpotify = true;
+        song.url = result.url;
+        // No trial clip for Spotify — full-track duration only (the near-end
+        // stall watchdog is disabled for the external stream anyway).
+        this.effectiveDuration = song.duration;
+        this.autoPaused = false;
+        this.database.addPlayHistory({
+          botId: this.id,
+          songId: song.id,
+          songName: song.name,
+          artist: song.artist,
+          album: song.album,
+          platform: song.platform,
+          coverUrl: song.coverUrl,
+        });
+        await this.syncProfileToSong(song);
+        this.emit("stateChange");
+        return true;
+      }
+      // Non-Spotify track: if we were on Spotify, pause the sidecar so it stops
+      // decoding ahead before the URL ffmpeg reclaims the PCM buffer.
+      if (this.currentSourceIsSpotify) {
+        this.spotifyController.pause().catch((err) =>
+          this.logger.warn({ err }, "Failed to pause Spotify sidecar on source switch"));
+        this.currentSourceIsSpotify = false;
       }
       song.url = result.url;
       // 试听片段用试听时长（让 player nearEnd 正确触发自动切歌）；完整曲回退 song.duration
@@ -767,6 +966,10 @@ export class BotInstance extends EventEmitter {
 
   private cmdPause(): string {
     this.player.pause();
+    if (this.queue.current()?.platform === "spotify") {
+      this.spotifyController.pause().catch((err) =>
+        this.logger.warn({ err }, "Spotify pause failed"));
+    }
     // User-initiated pause — clear auto-pause so occupancy won't auto-resume it.
     this.autoPaused = false;
     this.emit("stateChange");
@@ -775,6 +978,10 @@ export class BotInstance extends EventEmitter {
 
   private cmdResume(): string {
     this.player.resume();
+    if (this.queue.current()?.platform === "spotify") {
+      this.spotifyController.resume().catch((err) =>
+        this.logger.warn({ err }, "Spotify resume failed"));
+    }
     // User-initiated resume — drop any auto-pause flag.
     this.autoPaused = false;
     this.emit("stateChange");
@@ -782,6 +989,12 @@ export class BotInstance extends EventEmitter {
   }
 
   private cmdStop(): string {
+    // Read the current song BEFORE queue.clear() so we can tell whether the
+    // sidecar needs stopping.
+    if (this.queue.current()?.platform === "spotify") {
+      this.spotifyController.stop();
+    }
+    this.currentSourceIsSpotify = false;
     this.player.stop();
     this.autoPaused = false;
     this.queue.clear();
@@ -842,6 +1055,8 @@ export class BotInstance extends EventEmitter {
   }
 
   private cmdClear(): string {
+    this.spotifyController.stop();
+    this.currentSourceIsSpotify = false;
     this.player.stop();
     this.queue.clear();
     this.sweepLocalAudio("queue_cleared");
@@ -853,11 +1068,31 @@ export class BotInstance extends EventEmitter {
     return "Queue cleared";
   }
 
-  private cmdRemove(cmd: ParsedCommand): string {
+  private async cmdRemove(cmd: ParsedCommand): Promise<string> {
     const index = parseInt(cmd.args, 10) - 1;
     if (isNaN(index) || index < 0) return "Usage: !remove <number>";
+    // Capture BEFORE the splice whether we're removing the currently-playing
+    // Spotify track: queue.remove() decrements currentIndex, so getCurrentIndex()
+    // is only meaningful pre-remove.
+    const removingCurrentSpotify =
+      index === this.queue.getCurrentIndex() && this.currentSourceIsSpotify;
     const removed = this.queue.remove(index);
     if (!removed) return "Invalid position";
+    // Corner-case R3-2: removing the track the Spotify sidecar is decoding
+    // right now leaves it running while queue.current() is no longer that
+    // track. Since a spotify track has NO player self-EOF advance path, the
+    // controller "trackEnded" handler would return early (current is no longer
+    // spotify) and the bot would wedge in silence. Reconcile like cmdStop/skip:
+    // tear the sidecar down, then advance to whatever is now current — or stop
+    // cleanly if the queue is now empty (playNext's exhausted branch stops the
+    // player). Non-current or non-spotify removals are untouched (a URL current
+    // track self-heals via its own EOF).
+    if (removingCurrentSpotify) {
+      this.spotifyController.stop();
+      this.currentSourceIsSpotify = false;
+      this.player.stop();
+      await this.playNext();
+    }
     // Sweep after the entry is gone — the file is deleted only if no other
     // queue position (or bot) still references this upload.
     this.sweepLocalAudio("removed_from_queue");
@@ -1176,6 +1411,16 @@ export class BotInstance extends EventEmitter {
             this.profileManager.onSongChange(null).catch(() => {});
           }
         } else {
+          // Queue exhausted on a non-FM source (skip-past-end or natural
+          // last-track end via trackEnded→playNext). If the ending track was
+          // served by the Spotify sidecar, tear it down like cmdStop —
+          // otherwise the go-librespot Connect device stays active with the
+          // track loaded (decoding into a detached/backpressured stream) and
+          // currentSourceIsSpotify stays stale (corner-case R3-6).
+          if (this.currentSourceIsSpotify) {
+            this.spotifyController.stop();
+            this.currentSourceIsSpotify = false;
+          }
           this.player.stop();
           this.profileManager.onSongChange(null).catch(() => {});
         }
@@ -1231,6 +1476,29 @@ export class BotInstance extends EventEmitter {
     return this.player;
   }
 
+  /** The per-bot Spotify sidecar controller. Exposed like getPlayer()/
+   *  getQueueManager() so the shared, process-wide OAuth threaded in at
+   *  construction (C3.1) is observable to callers/tests via getOAuth(). */
+  getSpotifyController(): SpotifyController {
+    return this.spotifyController;
+  }
+
+  /**
+   * Route a seek to the Spotify sidecar for a spotify track (its PCM stream is
+   * external — AudioPlayer.seek would respawn ffmpeg on the `spotify:` sentinel
+   * and collide with the running stream), otherwise to the URL player.
+   */
+  seek(seconds: number): void {
+    if (this.queue.current()?.platform === "spotify") {
+      // The web route + AudioPlayer.seek are seconds-based, but
+      // SpotifyController.seek expects milliseconds — convert here.
+      this.spotifyController.seek(seconds * 1000).catch((err) =>
+        this.logger.warn({ err }, "Spotify seek failed"));
+      return;
+    }
+    this.player.seek(seconds);
+  }
+
   getQueueManager(): PlayQueue {
     return this.queue;
   }

src/bot/manager.test.ts

@@ -9,6 +9,7 @@ import { getDefaultConfig, loadConfig, saveConfig, type BotConfig } from "../dat
 import type { Logger } from "../logger.js";
 import type { MusicProvider } from "../music/provider.js";
 import type { AvatarStore } from "../data/avatars.js";
+import type { SpotifyOAuth } from "../music/spotify/spotify-oauth.js";
 
 // removeBot only calls logger.info; provide the full shape it could touch.
 const stubLogger = {
@@ -85,3 +86,68 @@ describe("BotManager.removeBot — guest scope pruning", () => {
     expect(loadConfig(configPath).guestMode.bots).toBe("all");
   });
 });
+
+// --- Spotify OAuth threading (Task 6, C3.1) --------------------------------
+// The single process-wide SpotifyOAuth built in index.ts must reach every bot's
+// SpotifyController: index -> BotManager (trailing positional arg) -> BotInstance
+// -> controller. createBot() builds a REAL (side-effect-free) SpotifyController,
+// so we assert the shared instance surfaces via the controller's getOAuth().
+describe("BotManager — spotifyOAuth threading to bot controllers (C3.1)", () => {
+  const dirs: string[] = [];
+  let db: BotDatabase | undefined;
+
+  afterEach(() => {
+    try {
+      db?.close();
+    } catch {
+      /* ignore */
+    }
+    db = undefined;
+    for (const d of dirs) {
+      rmSync(d, { recursive: true, force: true });
+    }
+    dirs.length = 0;
+  });
+
+  it("forwards its shared SpotifyOAuth into a created bot's controller", async () => {
+    const dir = mkdtempSync(join(tmpdir(), "tsmusicbot-oauth-thread-"));
+    dirs.push(dir);
+    const configPath = join(dir, "config.json");
+    const config = getDefaultConfig();
+    saveConfig(configPath, config);
+    db = createDatabase(":memory:");
+    const permissions = createPermissionStore(db.db);
+    const provider = {} as unknown as MusicProvider;
+    const sentinel = {} as unknown as SpotifyOAuth;
+
+    const manager = new BotManager(
+      provider,
+      provider,
+      provider,
+      db,
+      config,
+      stubLogger,
+      {} as unknown as AvatarStore,
+      permissions,
+      configPath,
+      undefined, // localProvider
+      undefined, // kugouProvider
+      undefined, // spotifyProvider
+      join(dir, "spotify"), // spotifyDataDir
+      sentinel, // spotifyOAuth (the single shared instance)
+    );
+
+    const bot = await manager.createBot({
+      name: "b1",
+      serverAddress: "localhost",
+      serverPort: 9987,
+      nickname: "b1",
+    });
+
+    // Full chain observed: the manager's single shared instance is the exact
+    // one the per-bot controller now owns (getOAuth() returns it unchanged).
+    expect(bot.getSpotifyController().getOAuth()).toBe(sentinel);
+
+    bot.disconnect();
+  });
+});

src/bot/manager.ts

@@ -1,5 +1,6 @@
 import crypto from "node:crypto";
 import { EventEmitter } from "node:events";
+import path from "node:path";
 import {
   BotInstance,
   type BotInstanceOptions,
@@ -13,6 +14,7 @@ import type { Logger } from "../logger.js";
 import type { ServerProtocol } from "../ts-protocol/client.js";
 import type { AvatarStore } from "../data/avatars.js";
 import type { PermissionStore } from "../data/permissions.js";
+import type { SpotifyOAuth } from "../music/spotify/spotify-oauth.js";
 
 /**
  * Run bot.connect() with a hard deadline. If the handshake hangs (e.g. the
@@ -77,6 +79,8 @@ export class BotManager extends EventEmitter {
   private localProvider: MusicProvider;
   private kugouProvider: MusicProvider;
   private spotifyProvider: MusicProvider;
+  private spotifyDataDir: string;
+  private readonly spotifyOAuth?: SpotifyOAuth;
   private database: BotDatabase;
   private config: BotConfig;
   private logger: Logger;
@@ -96,7 +100,9 @@ export class BotManager extends EventEmitter {
     configPath: string,
     localProvider?: MusicProvider,
     kugouProvider?: MusicProvider,
-    spotifyProvider?: MusicProvider
+    spotifyProvider?: MusicProvider,
+    spotifyDataDir?: string,
+    spotifyOAuth?: SpotifyOAuth
   ) {
     super();
     this.neteaseProvider = neteaseProvider;
@@ -106,6 +112,8 @@ export class BotManager extends EventEmitter {
     this.localProvider = localProvider ?? neteaseProvider;
     this.kugouProvider = kugouProvider ?? neteaseProvider;
     this.spotifyProvider = spotifyProvider ?? neteaseProvider;
+    this.spotifyDataDir = spotifyDataDir ?? path.join(process.cwd(), "data", "spotify");
+    this.spotifyOAuth = spotifyOAuth;
     // Let the local provider see which uploads are still referenced by any
     // bot's queue, so it never deletes a file another queue/bot still needs.
     const referenceable = this.localProvider as Partial<{
@@ -149,6 +157,8 @@ export class BotManager extends EventEmitter {
       config: this.config,
       logger: this.logger,
       avatarStore: this.avatarStore,
+      spotifyDataDir: this.spotifyDataDir,
+      spotifyOAuth: this.spotifyOAuth,
     });
 
     this.bots.set(id, bot);
@@ -290,6 +300,8 @@ export class BotManager extends EventEmitter {
         config: this.config,
         logger: this.logger,
         avatarStore: this.avatarStore,
+        spotifyDataDir: this.spotifyDataDir,
+        spotifyOAuth: this.spotifyOAuth,
       });
       this.bots.set(id, bot);
       this.emit("botInstance", bot);
@@ -345,6 +357,8 @@ export class BotManager extends EventEmitter {
         config: this.config,
         logger: this.logger,
         avatarStore: this.avatarStore,
+        spotifyDataDir: this.spotifyDataDir,
+        spotifyOAuth: this.spotifyOAuth,
       });
 
       this.bots.set(saved.id, bot);

src/data/config.test.ts

@@ -1,9 +1,32 @@
-import { describe, it, expect, afterEach } from "vitest";
+import { describe, it, expect, afterEach, beforeEach, vi } from "vitest";
 import { join } from "node:path";
-import { mkdtempSync, rmSync, writeFileSync, existsSync, readFileSync } from "node:fs";
+import {
+  mkdtempSync,
+  rmSync,
+  writeFileSync,
+  existsSync,
+  readFileSync,
+  readdirSync,
+  renameSync,
+} from "node:fs";
 import { tmpdir } from "node:os";
 import { getDefaultConfig, loadConfig, saveConfig, migrateLegacyConfig } from "./config.js";
 
+// Wrap the fs functions config.ts uses in call-through spies so the atomic-write
+// and transient-read-error paths can be observed/forced. Everything else (mkdtemp,
+// rmSync, existsSync, …) is the real implementation via `...actual`, so all other
+// tests keep their real filesystem behavior. `vi.spyOn` can't be used here because
+// the node:fs ESM namespace is non-configurable in this setup.
+vi.mock("node:fs", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:fs")>();
+  return {
+    ...actual,
+    readFileSync: vi.fn(actual.readFileSync),
+    writeFileSync: vi.fn(actual.writeFileSync),
+    renameSync: vi.fn(actual.renameSync),
+  };
+});
+
 describe("config", () => {
   const dirs: string[] = [];
 
@@ -263,3 +286,168 @@ describe("spotify config", () => {
     });
   });
 });
+
+// --- R2-1: saveConfig must write atomically (temp file + rename), never truncate ---
+
+describe("saveConfig atomic write", () => {
+  const dirs: string[] = [];
+  function makeTmpDir(): string {
+    const dir = mkdtempSync(join(tmpdir(), "tsmb-atomic-"));
+    dirs.push(dir);
+    return dir;
+  }
+  beforeEach(() => {
+    vi.clearAllMocks(); // reset call history, keep the call-through implementations
+  });
+  afterEach(() => {
+    for (const d of dirs) rmSync(d, { recursive: true, force: true });
+    dirs.length = 0;
+  });
+
+  it("round-trips (save then load equals) and leaves NO .tmp file behind", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    const config = { ...getDefaultConfig(), webPort: 4567, adminPassword: "pw" };
+
+    saveConfig(path, config);
+
+    expect(loadConfig(path)).toEqual(config);
+    // No temp remnants in the target directory.
+    expect(readdirSync(dir).filter((f) => f.includes(".tmp"))).toEqual([]);
+  });
+
+  it("writes via a same-dir temp file then renameSync onto the final path", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+
+    saveConfig(path, getDefaultConfig());
+
+    expect(vi.mocked(renameSync)).toHaveBeenCalled();
+    const [from, to] = vi.mocked(renameSync).mock.calls[0] as [string, string];
+    expect(to).toBe(path); // renamed ONTO the real path
+    expect(String(from)).not.toBe(path); // ...from a distinct temp file
+    expect(join(String(from), "..")).toBe(join(path, "..")); // ...in the SAME directory
+  });
+
+  it("does not corrupt a pre-existing valid config when saving over it", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    saveConfig(path, { ...getDefaultConfig(), adminPassword: "first", webPort: 1234 });
+
+    // Overwrite with a different, fully-formed config.
+    saveConfig(path, { ...getDefaultConfig(), adminPassword: "second", webPort: 9999 });
+
+    const loaded = loadConfig(path);
+    expect(loaded.adminPassword).toBe("second");
+    expect(loaded.webPort).toBe(9999);
+    // The on-disk file is a single complete JSON document (no partial/truncated write).
+    expect(() => JSON.parse(readFileSync(path, "utf-8"))).not.toThrow();
+    expect(readdirSync(dir).filter((f) => f.includes(".tmp"))).toEqual([]);
+  });
+
+  it("cleans up the temp file (no .tmp remnant) when the rename fails", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    vi.mocked(renameSync).mockImplementationOnce(() => {
+      throw new Error("rename boom");
+    });
+
+    expect(() => saveConfig(path, getDefaultConfig())).toThrow(/rename boom/);
+    // The failed write left no temp file lying around.
+    expect(readdirSync(dir).filter((f) => f.includes(".tmp"))).toEqual([]);
+  });
+});
+
+// --- R2-2: loadConfig must not treat a transient/corrupt read as "missing" ---
+
+describe("loadConfig error handling", () => {
+  const dirs: string[] = [];
+  function makeTmpDir(): string {
+    const dir = mkdtempSync(join(tmpdir(), "tsmb-load-"));
+    dirs.push(dir);
+    return dir;
+  }
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+  afterEach(() => {
+    for (const d of dirs) rmSync(d, { recursive: true, force: true });
+    dirs.length = 0;
+  });
+
+  it("(a) ENOENT (missing file) returns defaults — unchanged first-run behavior", () => {
+    const dir = makeTmpDir();
+    expect(loadConfig(join(dir, "config.json"))).toEqual(getDefaultConfig());
+  });
+
+  it("(b) a non-ENOENT read error (EBUSY) rethrows instead of returning defaults", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    // A REAL config exists on disk; a transient lock must NOT collapse to defaults
+    // (the caller would otherwise overwrite this real config with defaults).
+    saveConfig(path, { ...getDefaultConfig(), adminPassword: "keep-me" });
+    vi.mocked(readFileSync).mockImplementationOnce(() => {
+      const err = new Error("EBUSY: resource busy or locked") as NodeJS.ErrnoException;
+      err.code = "EBUSY";
+      throw err;
+    });
+
+    expect(() => loadConfig(path)).toThrow(/EBUSY/);
+    // The on-disk config is untouched and still readable once the lock clears.
+    expect(loadConfig(path).adminPassword).toBe("keep-me");
+  });
+
+  it("(c) corrupt JSON returns defaults AND backs up the original to *.corrupt-*", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    const garbage = "{ not: valid json, ";
+    writeFileSync(path, garbage, "utf-8");
+
+    const loaded = loadConfig(path);
+
+    expect(loaded).toEqual(getDefaultConfig());
+    const backups = readdirSync(dir).filter((f) => f.includes(".corrupt-"));
+    expect(backups.length).toBeGreaterThan(0);
+    // The corrupt original is preserved verbatim (recoverable, never deleted).
+    expect(readFileSync(join(dir, backups[0]), "utf-8")).toBe(garbage);
+  });
+
+  // (d)/(e) Valid JSON that is NOT a non-null object (null / [] / 42 / "str") passes
+  // JSON.parse but would throw a raw TypeError in the per-field sanitize block
+  // (property access on a non-object), bypassing the corrupt-backup path. It must be
+  // treated EXACTLY like corrupt JSON: back up to *.corrupt-* (original preserved),
+  // return defaults — NOT a thrown TypeError, and NOT a silent defaults-with-no-backup.
+  it("(d) a `null` config is treated as corrupt: defaults + *.corrupt-* backup (original preserved)", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    writeFileSync(path, "null", "utf-8");
+
+    let loaded: ReturnType<typeof getDefaultConfig>;
+    expect(() => {
+      loaded = loadConfig(path);
+    }).not.toThrow();
+
+    expect(loaded!).toEqual(getDefaultConfig());
+    const backups = readdirSync(dir).filter((f) => f.includes(".corrupt-"));
+    expect(backups.length).toBeGreaterThan(0);
+    expect(readFileSync(join(dir, backups[0]), "utf-8")).toBe("null");
+  });
+
+  it("(e) a non-object config (`[]` / `42`) is backed up + defaults, not a thrown TypeError", () => {
+    for (const content of ["[]", "42"]) {
+      const dir = makeTmpDir();
+      const path = join(dir, "config.json");
+      writeFileSync(path, content, "utf-8");
+
+      let loaded: ReturnType<typeof getDefaultConfig>;
+      expect(() => {
+        loaded = loadConfig(path);
+      }).not.toThrow();
+
+      expect(loaded!).toEqual(getDefaultConfig());
+      const backups = readdirSync(dir).filter((f) => f.includes(".corrupt-"));
+      expect(backups.length).toBeGreaterThan(0);
+      expect(readFileSync(join(dir, backups[0]), "utf-8")).toBe(content);
+    }
+  });
+});

src/data/config.ts

@@ -1,4 +1,12 @@
-import { readFileSync, writeFileSync, mkdirSync, existsSync, copyFileSync, rmSync } from "node:fs";
+import {
+  readFileSync,
+  writeFileSync,
+  mkdirSync,
+  existsSync,
+  copyFileSync,
+  rmSync,
+  renameSync,
+} from "node:fs";
 import { dirname } from "node:path";
 import type { BotAccess, GuestPermissions } from "./permissions.js";
 import { GUEST_PERMISSION_FLAGS } from "./permissions.js";
@@ -90,12 +98,69 @@ export function getDefaultConfig(): BotConfig {
   };
 }
 
+/**
+ * Move an unusable config aside to a timestamped `*.corrupt-*` backup so the data
+ * stays recoverable (it is NEVER deleted), for both the corrupt-JSON case and the
+ * parses-but-not-an-object case. Prefer an atomic same-dir rename; if that fails,
+ * copy instead. If it can't be preserved at all, rethrow rather than let the caller
+ * overwrite unrecoverable data.
+ */
+function backupCorruptConfig(path: string): void {
+  const backup = `${path}.corrupt-${Date.now()}`;
+  try {
+    renameSync(path, backup);
+  } catch {
+    try {
+      copyFileSync(path, backup);
+    } catch (backupErr) {
+      throw backupErr;
+    }
+  }
+}
+
 export function loadConfig(path: string): BotConfig {
   const defaults = getDefaultConfig();
-  try {
-    const raw = readFileSync(path, "utf-8");
-    const partial = JSON.parse(raw) as Partial<BotConfig>;
 
+  // Distinguish the three failure modes so a *real* on-disk config is NEVER
+  // silently replaced with defaults (the caller saveConfig()s right after load,
+  // which would otherwise erase spotify creds / adminPassword / adminGroups /
+  // guestMode permanently):
+  //   (a) file ABSENT (ENOENT) — normal first run → defaults.
+  //   (b) any OTHER read error (EBUSY/EACCES/EPERM/EISDIR/…) on an existing file —
+  //       rethrow (fail-fast at boot). A loud crash beats silent credential loss.
+  //   (c) file readable but JSON.parse fails (corrupt) — back the file up first
+  //       (never delete it), THEN return defaults so boot can proceed.
+  let raw: string;
+  try {
+    raw = readFileSync(path, "utf-8");
+  } catch (err) {
+    if ((err as NodeJS.ErrnoException).code === "ENOENT") {
+      return defaults; // (a) missing file — first run
+    }
+    throw err; // (b) transient/permission error on an existing file — do not clobber it
+  }
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw);
+  } catch {
+    // (c) Corrupt content: move the unreadable file aside to a timestamped backup
+    // so the data stays recoverable, then fall back to defaults.
+    backupCorruptConfig(path);
+    return defaults;
+  }
+
+  // (d) Parses cleanly but is NOT a non-null object (e.g. `null`, `42`, `"str"`,
+  // `[]`). The per-field sanitize below assumes an object and would throw a raw
+  // TypeError (or silently spread junk), bypassing the corrupt-backup path. Treat
+  // it EXACTLY like corrupt JSON: back it up (never delete), then return defaults.
+  if (typeof parsed !== "object" || parsed === null || Array.isArray(parsed)) {
+    backupCorruptConfig(path);
+    return defaults;
+  }
+  const partial = parsed as Partial<BotConfig>;
+
+  {
     // Normalize/sanitize guestMode on load. The WRITE path (POST /api/bot/settings)
     // sanitizes too, but a hand-edited/legacy/corrupt config.json reaches the gate
     // directly — so coerce it here as well, mirroring that write-path logic.
@@ -163,14 +228,33 @@ export function loadConfig(path: string): BotConfig {
       guestMode: gm,
       spotify,
     };
-  } catch {
-    return defaults;
   }
 }
 
 export function saveConfig(path: string, config: BotConfig): void {
   mkdirSync(dirname(path), { recursive: true });
-  writeFileSync(path, JSON.stringify(config, null, 2), "utf-8");
+  const json = JSON.stringify(config, null, 2);
+
+  // Atomic write: serialize to a sibling temp file in the SAME directory, then
+  // rename it onto the final path. rename is an atomic replace on POSIX and modern
+  // Windows, so a crash / power loss / ENOSPC mid-write can never leave config.json
+  // truncated — a reader always sees either the previous file or the fully-written
+  // new one, never a partial. The temp lives in the same dir so the rename stays on
+  // one filesystem (a cross-device rename would fail); pid + timestamp keep
+  // concurrent writers from colliding on the temp name.
+  const tmp = `${path}.${process.pid}.${Date.now()}.tmp`;
+  try {
+    writeFileSync(tmp, json, "utf-8");
+    renameSync(tmp, path);
+  } catch (err) {
+    // Never leave a partial temp file behind on failure.
+    try {
+      rmSync(tmp, { force: true });
+    } catch {
+      /* best-effort cleanup */
+    }
+    throw err;
+  }
 }
 
 /**

src/index.ts

@@ -10,6 +10,7 @@ import { BiliBiliProvider } from "./music/bilibili.js";
 import { LocalMusicProvider } from "./music/local.js";
 import { KugouProvider } from "./music/kugou.js";
 import { SpotifyProvider } from "./music/spotify/provider.js";
+import { SpotifyOAuth, createFileOAuthTokenStore } from "./music/spotify/spotify-oauth.js";
 import { createCookieStore } from "./music/auth.js";
 import { createAvatarStore } from "./data/avatars.js";
 import { createPermissionStore } from "./data/permissions.js";
@@ -29,6 +30,7 @@ const LOG_DIR = path.join(DATA_DIR, "logs");
 const COOKIE_DIR = path.join(DATA_DIR, "cookies");
 const AVATAR_DIR = path.join(DATA_DIR, "avatars");
 const LOCAL_AUDIO_DIR = path.join(DATA_DIR, "local-audio");
+const SPOTIFY_DATA_DIR = path.join(DATA_DIR, "spotify");
 const STATIC_DIR = path.join(ROOT_DIR, "web", "dist");
 
 async function main() {
@@ -82,6 +84,22 @@ async function main() {
 
   const permissions = createPermissionStore(db.db);
 
+  // Single process-wide Spotify authorization (one Premium account for Stage 3).
+  // Threaded into BOTH the web OAuth router and every bot's SpotifyController so
+  // a web login immediately authorizes playback (C3.1). Own-app clientId => the
+  // redirect points at this bot's web callback; empty clientId leaves OAuth
+  // disabled (isAuthorized() stays false and the Rust backend never starts).
+  const spotifyOAuthClientId = config.spotify.clientId.trim();
+  const spotifyOAuth = new SpotifyOAuth({
+    clientId: spotifyOAuthClientId || undefined,
+    redirectUri: spotifyOAuthClientId
+      ? `http://127.0.0.1:${config.webPort}/api/spotify/callback`
+      : undefined,
+    store: createFileOAuthTokenStore(
+      path.join(SPOTIFY_DATA_DIR, "oauth", "oauth-tokens.json"),
+    ),
+  });
+
   const botManager = new BotManager(
     neteaseProvider,
     qqProvider,
@@ -94,7 +112,9 @@ async function main() {
     CONFIG_PATH,
     localProvider,
     kugouProvider,
-    spotifyProvider
+    spotifyProvider,
+    SPOTIFY_DATA_DIR,
+    spotifyOAuth
   );
   await botManager.loadSavedBots();
 
@@ -114,6 +134,7 @@ async function main() {
     logger,
     cookieStore,
     staticDir: STATIC_DIR,
+    spotifyOAuth,
   });
   await webServer.start();
 

src/music/spotify/backend-select.test.ts

@@ -0,0 +1,12 @@
+import { describe, it, expect } from "vitest";
+import { resolveSpotifyBackendKind as pick } from "./backend-select.js";
+describe("resolveSpotifyBackendKind", () => {
+  it("auto: go present -> go-librespot", () => expect(pick("auto", true, true)).toBe("go-librespot"));
+  it("auto: go absent, rust present -> librespot", () => expect(pick("auto", false, true)).toBe("librespot"));
+  it("auto: neither -> null", () => expect(pick("auto", false, false)).toBeNull());
+  it("go-librespot: present -> go-librespot", () => expect(pick("go-librespot", true, true)).toBe("go-librespot"));
+  it("go-librespot: absent -> null even if rust present", () => expect(pick("go-librespot", false, true)).toBeNull());
+  it("librespot: present -> librespot", () => expect(pick("librespot", true, true)).toBe("librespot"));
+  it("librespot: absent -> null even if go present", () => expect(pick("librespot", true, false)).toBeNull());
+  it("auto default fallthrough matches auto", () => expect(pick("auto", true, false)).toBe("go-librespot"));
+});

src/music/spotify/backend-select.ts

@@ -0,0 +1,25 @@
+/** Which concrete backend runs for a given config + host binary availability. */
+export type SpotifyBackendKind = "go-librespot" | "librespot";
+
+/**
+ * Pure backend selection shared by SpotifyController.chooseBackend() (per-bot)
+ * and the web /status endpoint (process-wide). Booleans in, no IO — the caller
+ * supplies platform+binary presence.
+ */
+export function resolveSpotifyBackendKind(
+  backend: "auto" | "go-librespot" | "librespot",
+  goPresent: boolean,
+  rustPresent: boolean,
+): SpotifyBackendKind | null {
+  switch (backend) {
+    case "go-librespot":
+      return goPresent ? "go-librespot" : null;
+    case "librespot":
+      return rustPresent ? "librespot" : null;
+    case "auto":
+    default:
+      if (goPresent) return "go-librespot";
+      if (rustPresent) return "librespot";
+      return null;
+  }
+}

src/music/spotify/backend.ts

@@ -0,0 +1,41 @@
+// src/music/spotify/backend.ts
+// Type contract for the Spotify audio backend (go-librespot sidecar).
+// Interface-only: this module intentionally contains NO runtime code so it
+// can be imported for types by go-librespot.ts and controller.ts without
+// pulling in child_process/ws/ffmpeg at type-check time.
+
+/** Emitted when the currently playing Spotify track finishes or is stopped. */
+export interface SpotifyTrackEndedEvent {
+  uri: string;
+  reason: "ended" | "stopped" | "error";
+}
+
+/** Now-playing metadata surfaced from the go-librespot "metadata" event. */
+export interface SpotifyNowPlaying {
+  uri: string;
+  name: string;
+  artist: string;
+  album: string;
+  coverUrl: string;
+  durationMs: number;
+}
+
+/**
+ * Long-lived Spotify audio source: owns the go-librespot sidecar + FIFO->ffmpeg
+ * PCM pipe and exposes transport control plus a continuous 48kHz s16le stereo
+ * PCM stream to feed AudioPlayer.playPcmStream().
+ */
+export interface SpotifyAudioBackend {
+  start(): Promise<void>;
+  stop(): void;
+  isReady(): boolean;
+  playTrack(uri: string): Promise<void>;
+  pause(): Promise<void>;
+  resume(): Promise<void>;
+  seek(ms: number): Promise<void>;
+  getPcmStream(): import("node:stream").Readable;
+  getPositionMs(): number;
+  on(event: "trackEnded", cb: (e: SpotifyTrackEndedEvent) => void): void;
+  on(event: "metadata", cb: (m: SpotifyNowPlaying) => void): void;
+  on(event: "ready" | "error", cb: (arg?: unknown) => void): void;
+}

src/music/spotify/binary.test.ts

@@ -0,0 +1,363 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { join } from "node:path";
+import { mkdtempSync, writeFileSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import {
+  isGoLibrespotSupported,
+  pickGoLibrespotPath,
+  findGoLibrespot,
+  checkGoLibrespotAvailable,
+  resetGoLibrespotBinaryCache,
+  __setGoLibrespotVersionProbe,
+  isRustLibrespotSupported,
+  pickLibrespotPath,
+  findLibrespot,
+  checkLibrespotAvailable,
+  resetLibrespotBinaryCache,
+  __setLibrespotVersionProbe,
+  resolveExecutable,
+  isLibrespotPresent,
+  isGoLibrespotPresent,
+} from "./binary.js";
+
+const origPlatform = process.platform;
+function setPlatform(p: NodeJS.Platform): void {
+  Object.defineProperty(process, "platform", { value: p, configurable: true });
+}
+
+// Snapshot the PATH env so PATH-resolution tests can point it at temp dirs and
+// restore afterwards. PATHEXT is optional (undefined on posix); track presence.
+const origPath = process.env.PATH;
+const origPathExt = process.env.PATHEXT;
+const tmpDirs: string[] = [];
+function mkTmpDir(prefix: string): string {
+  const dir = mkdtempSync(join(tmpdir(), prefix));
+  tmpDirs.push(dir);
+  return dir;
+}
+
+afterEach(() => {
+  setPlatform(origPlatform);
+  __setGoLibrespotVersionProbe(null);
+  resetGoLibrespotBinaryCache();
+  __setLibrespotVersionProbe(null);
+  resetLibrespotBinaryCache();
+  process.env.PATH = origPath;
+  if (origPathExt === undefined) delete process.env.PATHEXT;
+  else process.env.PATHEXT = origPathExt;
+  for (const dir of tmpDirs.splice(0)) {
+    try {
+      rmSync(dir, { recursive: true, force: true });
+    } catch {
+      /* ignore */
+    }
+  }
+});
+
+describe("isGoLibrespotSupported", () => {
+  it("is true only on linux", () => {
+    setPlatform("linux");
+    expect(isGoLibrespotSupported()).toBe(true);
+    setPlatform("win32");
+    expect(isGoLibrespotSupported()).toBe(false);
+    setPlatform("darwin");
+    expect(isGoLibrespotSupported()).toBe(false);
+  });
+});
+
+describe("pickGoLibrespotPath (bin/ then PATH ordering)", () => {
+  const binPath = join("some", "root", "bin", "go-librespot");
+
+  it("prefers the bin/ path when the file exists", () => {
+    expect(
+      pickGoLibrespotPath([binPath, "go-librespot"], (p) => p === binPath),
+    ).toBe(binPath);
+  });
+
+  it("falls through to the bare PATH name when the bin/ file is missing", () => {
+    expect(pickGoLibrespotPath([binPath, "go-librespot"], () => false)).toBe(
+      "go-librespot",
+    );
+  });
+
+  it("returns bare command names without touching the filesystem", () => {
+    const exists = vi.fn(() => false);
+    expect(pickGoLibrespotPath(["go-librespot"], exists)).toBe("go-librespot");
+    expect(exists).not.toHaveBeenCalled();
+  });
+});
+
+describe("findGoLibrespot", () => {
+  it("returns the bare command name when bin/go-librespot is absent", () => {
+    // No go-librespot binary is committed under bin/, so resolution must
+    // fall back to the bare PATH name (execFile resolves it at run time).
+    expect(findGoLibrespot()).toBe("go-librespot");
+  });
+});
+
+describe("checkGoLibrespotAvailable", () => {
+  it("returns false immediately on unsupported platforms without probing", async () => {
+    setPlatform("darwin");
+    const probe = vi.fn(async () => {});
+    __setGoLibrespotVersionProbe(probe);
+    expect(await checkGoLibrespotAvailable()).toBe(false);
+    expect(probe).not.toHaveBeenCalled();
+  });
+
+  it("returns true when the binary responds to --version on linux", async () => {
+    setPlatform("linux");
+    __setGoLibrespotVersionProbe(async () => {});
+    expect(await checkGoLibrespotAvailable()).toBe(true);
+  });
+
+  it("caches only positive results and probes once", async () => {
+    setPlatform("linux");
+    const probe = vi.fn(async () => {});
+    __setGoLibrespotVersionProbe(probe);
+    expect(await checkGoLibrespotAvailable()).toBe(true);
+    expect(await checkGoLibrespotAvailable()).toBe(true);
+    expect(probe).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not cache a failed probe (retries on the next call)", async () => {
+    setPlatform("linux");
+    __setGoLibrespotVersionProbe(async () => {
+      throw new Error("ENOENT");
+    });
+    expect(await checkGoLibrespotAvailable()).toBe(false);
+    // A later successful probe must now succeed — negatives are not cached.
+    __setGoLibrespotVersionProbe(async () => {});
+    expect(await checkGoLibrespotAvailable()).toBe(true);
+  });
+
+  it("resetGoLibrespotBinaryCache clears a cached positive", async () => {
+    setPlatform("linux");
+    __setGoLibrespotVersionProbe(async () => {});
+    expect(await checkGoLibrespotAvailable()).toBe(true);
+    resetGoLibrespotBinaryCache();
+    __setGoLibrespotVersionProbe(async () => {
+      throw new Error("gone");
+    });
+    expect(await checkGoLibrespotAvailable()).toBe(false);
+  });
+});
+
+describe("isRustLibrespotSupported", () => {
+  it("is true on every platform (pipe->stdout works everywhere)", () => {
+    setPlatform("linux");
+    expect(isRustLibrespotSupported()).toBe(true);
+    setPlatform("win32");
+    expect(isRustLibrespotSupported()).toBe(true);
+    setPlatform("darwin");
+    expect(isRustLibrespotSupported()).toBe(true);
+  });
+});
+
+describe("pickLibrespotPath (bin/ then PATH ordering, win32 exe)", () => {
+  it("prefers the bin/ path when the file exists", () => {
+    const binPath = join("some", "root", "bin", "librespot");
+    expect(
+      pickLibrespotPath([binPath, "librespot"], (p) => p === binPath),
+    ).toBe(binPath);
+  });
+
+  it("prefers the bin/librespot.exe path on win32 when it exists", () => {
+    setPlatform("win32");
+    const binExe = join("some", "root", "bin", "librespot.exe");
+    expect(
+      pickLibrespotPath([binExe, "librespot.exe"], (p) => p === binExe),
+    ).toBe(binExe);
+  });
+
+  it("falls through to the bare PATH name (librespot) on posix when bin/ is missing", () => {
+    setPlatform("linux");
+    const binPath = join("some", "root", "bin", "librespot");
+    expect(pickLibrespotPath([binPath, "librespot"], () => false)).toBe(
+      "librespot",
+    );
+  });
+
+  it("falls through to librespot.exe on win32 when bin/ is missing", () => {
+    setPlatform("win32");
+    const binExe = join("some", "root", "bin", "librespot.exe");
+    expect(pickLibrespotPath([binExe], () => false)).toBe("librespot.exe");
+  });
+
+  it("returns bare command names without touching the filesystem", () => {
+    const exists = vi.fn(() => false);
+    expect(pickLibrespotPath(["librespot"], exists)).toBe("librespot");
+    expect(exists).not.toHaveBeenCalled();
+  });
+});
+
+describe("findLibrespot", () => {
+  it("returns the bare command name when bin/librespot is absent", () => {
+    // No librespot binary is committed under bin/, so resolution must fall
+    // back to the bare PATH name (execFile resolves it at run time).
+    setPlatform("linux");
+    expect(findLibrespot()).toBe("librespot");
+  });
+
+  it("returns librespot.exe on win32 when bin/librespot.exe is absent", () => {
+    setPlatform("win32");
+    expect(findLibrespot()).toBe("librespot.exe");
+  });
+});
+
+describe("checkLibrespotAvailable", () => {
+  it("returns true when the binary responds to --version (any platform)", async () => {
+    setPlatform("win32");
+    __setLibrespotVersionProbe(async () => {});
+    expect(await checkLibrespotAvailable()).toBe(true);
+  });
+
+  it("returns true on darwin too (no platform gate)", async () => {
+    setPlatform("darwin");
+    __setLibrespotVersionProbe(async () => {});
+    expect(await checkLibrespotAvailable()).toBe(true);
+  });
+
+  it("caches only positive results and probes once", async () => {
+    setPlatform("linux");
+    const probe = vi.fn(async () => {});
+    __setLibrespotVersionProbe(probe);
+    expect(await checkLibrespotAvailable()).toBe(true);
+    expect(await checkLibrespotAvailable()).toBe(true);
+    expect(probe).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not cache a failed probe (retries on the next call)", async () => {
+    setPlatform("win32");
+    __setLibrespotVersionProbe(async () => {
+      throw new Error("ENOENT");
+    });
+    expect(await checkLibrespotAvailable()).toBe(false);
+    __setLibrespotVersionProbe(async () => {});
+    expect(await checkLibrespotAvailable()).toBe(true);
+  });
+
+  it("resetLibrespotBinaryCache clears a cached positive", async () => {
+    setPlatform("linux");
+    __setLibrespotVersionProbe(async () => {});
+    expect(await checkLibrespotAvailable()).toBe(true);
+    resetLibrespotBinaryCache();
+    __setLibrespotVersionProbe(async () => {
+      throw new Error("gone");
+    });
+    expect(await checkLibrespotAvailable()).toBe(false);
+  });
+
+  it("de-dupes concurrent in-flight probes", async () => {
+    setPlatform("linux");
+    const probe = vi.fn(async () => {});
+    __setLibrespotVersionProbe(probe);
+    const [a, b] = await Promise.all([
+      checkLibrespotAvailable(),
+      checkLibrespotAvailable(),
+    ]);
+    expect(a).toBe(true);
+    expect(b).toBe(true);
+    expect(probe).toHaveBeenCalledTimes(1);
+  });
+});
+
+describe("resolveExecutable", () => {
+  it("returns a bin/-style path (contains a separator) that exists, unchanged", () => {
+    const dir = mkTmpDir("tsmb-resolve-");
+    const full = join(dir, "some-binary");
+    writeFileSync(full, "#!/bin/sh\n");
+    // The path contains a separator so it is checked directly, not via PATH.
+    expect(resolveExecutable(full)).toBe(full);
+  });
+
+  it("returns null for a bin/-style path that does not exist", () => {
+    const full = join(tmpdir(), "tsmb-resolve-missing", "nope-binary");
+    expect(resolveExecutable(full)).toBeNull();
+  });
+
+  it("resolves a BARE command name found in a PATH directory (the I3 fix)", () => {
+    // This is the regression the fix targets: a PATH-installed binary (bare
+    // name, no separator) must resolve against $PATH, not process.cwd().
+    const dir = mkTmpDir("tsmb-resolve-path-");
+    const exeName = process.platform === "win32" ? "faketool.exe" : "faketool";
+    const full = join(dir, exeName);
+    writeFileSync(full, "#!/bin/sh\n");
+    process.env.PATH = dir;
+    expect(resolveExecutable(exeName)).toBe(full);
+  });
+
+  it("returns null for a bare name not present in any PATH directory", () => {
+    process.env.PATH = mkTmpDir("tsmb-resolve-empty-");
+    expect(resolveExecutable("definitely-not-a-real-binary-xyz")).toBeNull();
+  });
+
+  it("appends PATHEXT extensions on win32 to resolve a bare (extensionless) name", () => {
+    setPlatform("win32");
+    const dir = mkTmpDir("tsmb-resolve-pathext-");
+    const full = join(dir, "mytool.CMD");
+    writeFileSync(full, "@echo hi\n");
+    process.env.PATH = dir;
+    process.env.PATHEXT = ".EXE;.CMD;.BAT;.COM";
+    // Bare "mytool" has no extension; win32 resolution must try PATHEXT and
+    // find mytool.CMD.
+    expect(resolveExecutable("mytool")).toBe(full);
+  });
+
+  it("does not touch PATH for a bin/-style relative path with a separator", () => {
+    // A candidate with a separator is checked directly even if PATH is empty.
+    process.env.PATH = "";
+    const dir = mkTmpDir("tsmb-resolve-direct-");
+    const full = join(dir, "direct-binary");
+    writeFileSync(full, "#!/bin/sh\n");
+    expect(resolveExecutable(full)).toBe(full);
+  });
+});
+
+describe("isLibrespotPresent (PATH-aware, sync)", () => {
+  it("true when the bare librespot name resolves on PATH (posix)", () => {
+    setPlatform("linux");
+    const dir = mkTmpDir("tsmb-librespot-path-");
+    writeFileSync(join(dir, "librespot"), "#!/bin/sh\n");
+    process.env.PATH = dir;
+    expect(isLibrespotPresent()).toBe(true);
+  });
+
+  it("true when librespot.exe resolves on PATH (win32)", () => {
+    setPlatform("win32");
+    const dir = mkTmpDir("tsmb-librespot-win-");
+    writeFileSync(join(dir, "librespot.exe"), "MZ\n");
+    process.env.PATH = dir;
+    process.env.PATHEXT = ".EXE;.CMD;.BAT;.COM";
+    expect(isLibrespotPresent()).toBe(true);
+  });
+
+  it("false when librespot is not on PATH", () => {
+    setPlatform("linux");
+    process.env.PATH = mkTmpDir("tsmb-librespot-empty-");
+    expect(isLibrespotPresent()).toBe(false);
+  });
+});
+
+describe("isGoLibrespotPresent (PATH-aware, sync)", () => {
+  it("true when go-librespot resolves on PATH (linux)", () => {
+    setPlatform("linux");
+    const dir = mkTmpDir("tsmb-go-path-");
+    writeFileSync(join(dir, "go-librespot"), "#!/bin/sh\n");
+    process.env.PATH = dir;
+    expect(isGoLibrespotPresent()).toBe(true);
+  });
+
+  it("false on non-linux platforms regardless of PATH (unsupported gate)", () => {
+    setPlatform("win32");
+    const dir = mkTmpDir("tsmb-go-win-");
+    writeFileSync(join(dir, "go-librespot"), "#!/bin/sh\n");
+    process.env.PATH = dir;
+    expect(isGoLibrespotPresent()).toBe(false);
+  });
+
+  it("false on linux when go-librespot is not on PATH", () => {
+    setPlatform("linux");
+    process.env.PATH = mkTmpDir("tsmb-go-empty-");
+    expect(isGoLibrespotPresent()).toBe(false);
+  });
+});

src/music/spotify/binary.ts

@@ -0,0 +1,244 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import { existsSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, join, delimiter } from "node:path";
+
+const execFileAsync = promisify(execFile);
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
+/**
+ * Resolve a command to an existing absolute path, SYNCHRONOUSLY. A candidate
+ * that already contains a path separator (a project bin/ path) is checked
+ * directly; a BARE command name (e.g. "librespot" / "go-librespot") is searched
+ * across the $PATH directories — with PATHEXT extensions appended on win32 — so
+ * a scoop/choco/cargo/apt PATH install resolves. Returns the resolved path, or
+ * null when nothing exists.
+ *
+ * This is the sync counterpart to checkLibrespotAvailable()/…GoLibrespot… and
+ * is what the hot-path presence gates (chooseBackend/isAvailable) rely on: a
+ * bare name must NOT be handed to existsSync() directly, since existsSync
+ * resolves it against process.cwd() rather than PATH (Bug I3/m1).
+ */
+export function resolveExecutable(cmd: string): string | null {
+  if (cmd.includes("/") || cmd.includes("\\")) {
+    return existsSync(cmd) ? cmd : null;
+  }
+  const dirs = (process.env.PATH || "").split(delimiter).filter(Boolean);
+  const exts =
+    process.platform === "win32"
+      ? (process.env.PATHEXT || ".EXE;.CMD;.BAT;.COM").split(";").filter(Boolean)
+      : [""];
+  for (const dir of dirs) {
+    for (const ext of exts) {
+      const hasExt = ext && cmd.toLowerCase().endsWith(ext.toLowerCase());
+      const full = join(dir, hasExt ? cmd : cmd + ext);
+      if (existsSync(full)) return full;
+    }
+  }
+  return null;
+}
+
+/**
+ * True only on Linux. go-librespot ships Linux-only release binaries and the
+ * sidecar relies on a POSIX FIFO (mkfifo), so the Spotify audio backend is
+ * gated to Linux/Docker. Everywhere else the caller falls back to the Stage-1
+ * sentinel-skip message.
+ */
+export function isGoLibrespotSupported(): boolean {
+  return process.platform === "linux";
+}
+
+/**
+ * Pure resolver core behind findGoLibrespot(). Returns the first candidate
+ * that is either a bare command name (left for execFile to resolve via PATH)
+ * or an existing bin/ file. Exported so tests can inject candidates + a fake
+ * existence predicate and need no real binary on disk.
+ */
+export function pickGoLibrespotPath(
+  candidates: string[],
+  exists: (p: string) => boolean,
+): string {
+  for (const c of candidates) {
+    // bin/ paths only count when the file is actually present; bare names are
+    // returned unconditionally and resolved later via PATH.
+    const isBinPath = c.includes(join("bin", "go-librespot"));
+    if (!isBinPath || exists(c)) return c;
+  }
+  return "go-librespot";
+}
+
+/** Resolve the go-librespot binary path: project bin/ dir first, then PATH. */
+export function findGoLibrespot(): string {
+  // src/music/spotify -> ../../../bin (one level deeper than youtube.ts).
+  const binPath = join(__dirname, "..", "..", "..", "bin", "go-librespot");
+  return pickGoLibrespotPath([binPath, "go-librespot"], existsSync);
+}
+
+/**
+ * SYNCHRONOUS presence gate for go-librespot: supported platform (linux) AND
+ * the resolved binary (project bin/ OR a PATH install) actually exists. This is
+ * what chooseBackend()/isAvailable() must use — NOT existsSync(findGoLibrespot())
+ * which cannot see a bare PATH name (Bug I3/m1).
+ */
+export function isGoLibrespotPresent(): boolean {
+  return isGoLibrespotSupported() && resolveExecutable(findGoLibrespot()) !== null;
+}
+
+// Injectable `--version` probe. Defaults to the real execFile call; tests
+// override it so checkGoLibrespotAvailable() needs no real binary. Keeps the
+// public checkGoLibrespotAvailable() signature param-free per the contract.
+type VersionProbe = (bin: string) => Promise<void>;
+const realProbe: VersionProbe = async (bin) => {
+  await execFileAsync(bin, ["--version"], { timeout: 5_000, maxBuffer: 1024 });
+};
+let versionProbe: VersionProbe = realProbe;
+
+/** Test hook: override the `--version` probe, or restore the default with null. */
+export function __setGoLibrespotVersionProbe(
+  probe: VersionProbe | null,
+): void {
+  versionProbe = probe ?? realProbe;
+}
+
+/**
+ * Availability check for go-librespot. Returns false immediately on non-Linux
+ * platforms (unsupported). Otherwise runs `go-librespot --version` (5s timeout)
+ * and caches ONLY the positive result — a missing binary is retried on the
+ * next call so the operator can install it without restarting the server.
+ */
+let cachedAvailable = false;
+let pendingCheck: Promise<boolean> | null = null;
+export async function checkGoLibrespotAvailable(): Promise<boolean> {
+  if (!isGoLibrespotSupported()) return false;
+  if (cachedAvailable) return true;
+  if (pendingCheck) return pendingCheck;
+  pendingCheck = (async () => {
+    try {
+      await versionProbe(findGoLibrespot());
+      cachedAvailable = true;
+      return true;
+    } catch {
+      return false;
+    } finally {
+      pendingCheck = null;
+    }
+  })();
+  return pendingCheck;
+}
+
+/** Force re-detection on the next call (for tests). */
+export function resetGoLibrespotBinaryCache(): void {
+  cachedAvailable = false;
+  pendingCheck = null;
+}
+
+// ---------------------------------------------------------------------------
+// Rust librespot (librespot-org) resolver — mirrors the go-librespot fns
+// above. Unlike go-librespot, Rust librespot's `--backend pipe` writes PCM to
+// *stdout* on every platform (no FIFO, no audio device), so it is supported
+// on Windows/macOS/Linux alike and the binary is named librespot.exe on win32.
+// ---------------------------------------------------------------------------
+
+/**
+ * True on ALL platforms. The Rust librespot pipe backend writes raw bytes to
+ * process stdout, which Node's spawned child.stdout receives unmodified on
+ * Windows too — so there is no platform gate here (contrast
+ * isGoLibrespotSupported, which is Linux-only).
+ */
+export function isRustLibrespotSupported(): boolean {
+  return true;
+}
+
+/**
+ * Pure resolver core behind findLibrespot(). Returns the first candidate that
+ * is either a bare command name (left for execFile to resolve via PATH) or an
+ * existing bin/ file. Exported so tests can inject candidates + a fake
+ * existence predicate and need no real binary on disk. Mirrors
+ * pickGoLibrespotPath but keys off the win32 exe name.
+ */
+export function pickLibrespotPath(
+  candidates: string[],
+  exists: (p: string) => boolean,
+): string {
+  const exe = process.platform === "win32" ? "librespot.exe" : "librespot";
+  for (const c of candidates) {
+    // bin/ paths only count when the file is actually present; bare names are
+    // returned unconditionally and resolved later via PATH.
+    const isBinPath = c.includes(join("bin", "librespot"));
+    if (!isBinPath || exists(c)) return c;
+  }
+  return exe;
+}
+
+/**
+ * Resolve the Rust librespot binary path: project bin/ dir first, then PATH.
+ * On win32 both the bin/librespot.exe candidate and the bare "librespot.exe"
+ * fallback are used so a PATH-installed librespot.exe (scoop/choco) resolves.
+ */
+export function findLibrespot(): string {
+  const exe = process.platform === "win32" ? "librespot.exe" : "librespot";
+  // src/music/spotify -> ../../../bin (same depth as findGoLibrespot).
+  const binExe = join(__dirname, "..", "..", "..", "bin", exe);
+  const binBare = join(__dirname, "..", "..", "..", "bin", "librespot");
+  return pickLibrespotPath([binExe, binBare, exe], existsSync);
+}
+
+/**
+ * SYNCHRONOUS presence gate for Rust librespot: supported everywhere AND the
+ * resolved binary (project bin/ OR a scoop/choco/cargo PATH install) actually
+ * exists. This is what chooseBackend()/isAvailable() must use — NOT
+ * existsSync(findLibrespot()) which cannot see a bare PATH name (Bug I3/m1).
+ */
+export function isLibrespotPresent(): boolean {
+  return isRustLibrespotSupported() && resolveExecutable(findLibrespot()) !== null;
+}
+
+// Injectable `--version` probe. Defaults to the real execFile call; tests
+// override it so checkLibrespotAvailable() needs no real binary. Keeps the
+// public checkLibrespotAvailable() signature param-free per the contract.
+type LibrespotVersionProbe = (bin: string) => Promise<void>;
+const realLibrespotProbe: LibrespotVersionProbe = async (bin) => {
+  await execFileAsync(bin, ["--version"], { timeout: 5_000, maxBuffer: 1024 });
+};
+let librespotVersionProbe: LibrespotVersionProbe = realLibrespotProbe;
+
+/** Test hook: override the `--version` probe, or restore the default with null. */
+export function __setLibrespotVersionProbe(
+  probe: LibrespotVersionProbe | null,
+): void {
+  librespotVersionProbe = probe ?? realLibrespotProbe;
+}
+
+/**
+ * Availability check for Rust librespot. No platform gate (supported
+ * everywhere). Runs `librespot --version` (5s timeout) and caches ONLY the
+ * positive result — a missing binary is retried on the next call so the
+ * operator can install it (cargo/scoop/choco) without restarting the server.
+ */
+let rustCachedAvailable = false;
+let rustPendingCheck: Promise<boolean> | null = null;
+export async function checkLibrespotAvailable(): Promise<boolean> {
+  if (!isRustLibrespotSupported()) return false;
+  if (rustCachedAvailable) return true;
+  if (rustPendingCheck) return rustPendingCheck;
+  rustPendingCheck = (async () => {
+    try {
+      await librespotVersionProbe(findLibrespot());
+      rustCachedAvailable = true;
+      return true;
+    } catch {
+      return false;
+    } finally {
+      rustPendingCheck = null;
+    }
+  })();
+  return rustPendingCheck;
+}
+
+/** Force re-detection on the next call (for tests). */
+export function resetLibrespotBinaryCache(): void {
+  rustCachedAvailable = false;
+  rustPendingCheck = null;
+}

src/music/spotify/connect-api.test.ts

@@ -0,0 +1,396 @@
+import { describe, it, expect, vi } from "vitest";
+import type { AxiosInstance } from "axios";
+import { SpotifyConnectApi } from "./connect-api.js";
+
+/** Minimal axios stub: only get/put are exercised by the Connect client. */
+function makeHttp(overrides?: Partial<Record<"get" | "put", any>>) {
+  return {
+    get: vi.fn().mockResolvedValue({ status: 200, data: {} }),
+    put: vi.fn().mockResolvedValue({ status: 200, data: {} }),
+    ...overrides,
+  } as unknown as AxiosInstance;
+}
+
+const AUTH = { headers: { Authorization: "Bearer tok123" } };
+const token = () =>
+  vi.fn<() => Promise<string | null>>().mockResolvedValue("tok123");
+
+describe("SpotifyConnectApi.getDevices", () => {
+  it("GETs /v1/me/player/devices with the bearer header and maps the list", async () => {
+    const http = makeHttp({
+      get: vi.fn().mockResolvedValue({
+        status: 200,
+        data: {
+          devices: [
+            { id: "dev-1", name: "TS Bot", is_active: true, type: "Speaker" },
+            { id: "dev-2", name: "Phone", is_active: false },
+          ],
+        },
+      }),
+    });
+    const api = new SpotifyConnectApi(token(), { http });
+    const devices = await api.getDevices();
+    expect(http.get).toHaveBeenCalledWith("/v1/me/player/devices", AUTH);
+    expect(devices).toEqual([
+      { id: "dev-1", name: "TS Bot", is_active: true },
+      { id: "dev-2", name: "Phone", is_active: false },
+    ]);
+  });
+
+  it("returns [] when getToken() is null (unauthorized) without calling http", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(vi.fn().mockResolvedValue(null), { http });
+    await expect(api.getDevices()).resolves.toEqual([]);
+    expect(http.get).not.toHaveBeenCalled();
+  });
+
+  it("returns [] on a 401/network rejection (graceful)", async () => {
+    const err: any = new Error("unauthorized");
+    err.response = { status: 401 };
+    const http = makeHttp({ get: vi.fn().mockRejectedValue(err) });
+    const api = new SpotifyConnectApi(token(), { http });
+    await expect(api.getDevices()).resolves.toEqual([]);
+  });
+});
+
+describe("SpotifyConnectApi.findDeviceByName", () => {
+  it("returns the matching device id", async () => {
+    const http = makeHttp({
+      get: vi.fn().mockResolvedValue({
+        status: 200,
+        data: { devices: [{ id: "dev-1", name: "TS Bot", is_active: false }] },
+      }),
+    });
+    const api = new SpotifyConnectApi(token(), { http });
+    await expect(api.findDeviceByName("TS Bot")).resolves.toBe("dev-1");
+  });
+
+  it("returns null when no device name matches", async () => {
+    const http = makeHttp({
+      get: vi.fn().mockResolvedValue({
+        status: 200,
+        data: { devices: [{ id: "dev-1", name: "Other", is_active: false }] },
+      }),
+    });
+    const api = new SpotifyConnectApi(token(), { http });
+    await expect(api.findDeviceByName("TS Bot")).resolves.toBeNull();
+  });
+});
+
+describe("SpotifyConnectApi mutating calls", () => {
+  it("transfer() PUTs /v1/me/player with device_ids + play=false default", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.transfer("dev-1");
+    expect(http.put).toHaveBeenCalledWith(
+      "/v1/me/player",
+      { device_ids: ["dev-1"], play: false },
+      AUTH,
+    );
+  });
+
+  it("transfer(id, true) forwards play=true", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.transfer("dev-1", true);
+    expect(http.put).toHaveBeenCalledWith(
+      "/v1/me/player",
+      { device_ids: ["dev-1"], play: true },
+      AUTH,
+    );
+  });
+
+  it("play() PUTs /v1/me/player/play?device_id= with the uris body", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.play("dev-1", "spotify:track:abc");
+    expect(http.put).toHaveBeenCalledWith(
+      "/v1/me/player/play",
+      { uris: ["spotify:track:abc"] },
+      { headers: { Authorization: "Bearer tok123" }, params: { device_id: "dev-1" } },
+    );
+  });
+
+  it("pause() PUTs /v1/me/player/pause (no params) with no body", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.pause();
+    expect(http.put).toHaveBeenCalledWith("/v1/me/player/pause", undefined, {
+      headers: { Authorization: "Bearer tok123" },
+      params: undefined,
+    });
+  });
+
+  it("pause(id) forwards device_id param", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.pause("dev-1");
+    expect(http.put).toHaveBeenCalledWith("/v1/me/player/pause", undefined, {
+      headers: { Authorization: "Bearer tok123" },
+      params: { device_id: "dev-1" },
+    });
+  });
+
+  it("resume() PUTs /v1/me/player/play with no uris body (resume)", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.resume();
+    expect(http.put).toHaveBeenCalledWith("/v1/me/player/play", undefined, {
+      headers: { Authorization: "Bearer tok123" },
+      params: undefined,
+    });
+  });
+
+  it("resume(id) forwards device_id param", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.resume("dev-1");
+    expect(http.put).toHaveBeenCalledWith("/v1/me/player/play", undefined, {
+      headers: { Authorization: "Bearer tok123" },
+      params: { device_id: "dev-1" },
+    });
+  });
+
+  it("seek() PUTs /v1/me/player/seek?position_ms=", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.seek(42000);
+    expect(http.put).toHaveBeenCalledWith("/v1/me/player/seek", undefined, {
+      headers: { Authorization: "Bearer tok123" },
+      params: { position_ms: 42000 },
+    });
+  });
+
+  it("seek(ms, id) adds device_id param", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(token(), { http });
+    await api.seek(1000, "dev-1");
+    expect(http.put).toHaveBeenCalledWith("/v1/me/player/seek", undefined, {
+      headers: { Authorization: "Bearer tok123" },
+      params: { position_ms: 1000, device_id: "dev-1" },
+    });
+  });
+
+  it("mutating calls no-op (no http.put) when unauthorized", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(vi.fn().mockResolvedValue(null), { http });
+    await api.transfer("dev-1");
+    await api.play("dev-1", "spotify:track:x");
+    await api.pause();
+    expect(http.put).not.toHaveBeenCalled();
+  });
+});
+
+/**
+ * REQUIRED CORRECTION C3.6: mutating calls must NOT reject up the queue-advance
+ * path. A transient 403 (non-Premium) / 404 (no active device) / 429
+ * (rate-limited) from Spotify must be swallowed (resolve to void), never thrown,
+ * so a failed play() degrades to "couldn't play" instead of an unhandled
+ * rejection that crashes the backend.
+ */
+describe("SpotifyConnectApi C3.6 — mutating calls are resilient (no throw)", () => {
+  // S4.6: transient statuses (404/429) now retry with backoff; inject a no-op
+  // sleep so these swallow-guarantee tests stay instant (no real timers). The
+  // no-throw/swallow contract asserted here is unchanged.
+  const noSleep = async () => {};
+
+  function rejectingHttp(status: number) {
+    const err: any = new Error(`http ${status}`);
+    err.response = { status };
+    return makeHttp({ put: vi.fn().mockRejectedValue(err) });
+  }
+
+  it("play() does NOT throw on a 404 (no active device)", async () => {
+    const api = new SpotifyConnectApi(token(), {
+      http: rejectingHttp(404),
+      sleep: noSleep,
+    });
+    await expect(api.play("dev-1", "spotify:track:abc")).resolves.toBeUndefined();
+  });
+
+  it("play() does NOT throw on a 429 (rate-limited)", async () => {
+    const api = new SpotifyConnectApi(token(), {
+      http: rejectingHttp(429),
+      sleep: noSleep,
+    });
+    await expect(api.play("dev-1", "spotify:track:abc")).resolves.toBeUndefined();
+  });
+
+  it("transfer() does NOT throw on a 403 (non-Premium)", async () => {
+    const api = new SpotifyConnectApi(token(), { http: rejectingHttp(403) });
+    await expect(api.transfer("dev-1", true)).resolves.toBeUndefined();
+  });
+
+  it("pause/resume/seek do NOT throw on a rejection", async () => {
+    const api = new SpotifyConnectApi(token(), {
+      http: rejectingHttp(404),
+      sleep: noSleep,
+    });
+    await expect(api.pause("dev-1")).resolves.toBeUndefined();
+    await expect(api.resume("dev-1")).resolves.toBeUndefined();
+    await expect(api.seek(1000, "dev-1")).resolves.toBeUndefined();
+  });
+
+  it("play() does NOT throw on a raw network error (no response)", async () => {
+    const http = makeHttp({ put: vi.fn().mockRejectedValue(new Error("ECONNRESET")) });
+    const api = new SpotifyConnectApi(token(), { http });
+    await expect(api.play("dev-1", "spotify:track:abc")).resolves.toBeUndefined();
+  });
+});
+
+/**
+ * Task S4.6: bounded retry/backoff on the mutating Connect commands
+ * (spec §4.3/§13 recovery/watchdog). Transient statuses {404,429,500,502,503}
+ * retry up to MAX_ATTEMPTS (3) with exponential backoff; non-transient statuses
+ * are NOT retried. Every path still preserves C3.6 (swallow, never throw). A
+ * no-op injected `sleep` keeps the tests instant (no real timers).
+ */
+describe("SpotifyConnectApi S4.6 — retry/backoff on mutating commands", () => {
+  const MAX_ATTEMPTS = 3;
+  const noSleep = async () => {};
+
+  function rejectStatus(status: number, headers?: Record<string, string>) {
+    const err: any = new Error(`http ${status}`);
+    err.response = { status, headers };
+    return err;
+  }
+
+  it("play() retries a transient 404 then succeeds (2 calls, no throw)", async () => {
+    const put = vi
+      .fn()
+      .mockRejectedValueOnce(rejectStatus(404))
+      .mockResolvedValueOnce({ status: 200, data: {} });
+    const http = makeHttp({ put });
+    const api = new SpotifyConnectApi(token(), { http, sleep: noSleep });
+    await expect(api.play("dev-1", "spotify:track:abc")).resolves.toBeUndefined();
+    expect(put).toHaveBeenCalledTimes(2);
+  });
+
+  it("play() exhausts on a persistent 500 (MAX_ATTEMPTS calls, swallowed, warns once)", async () => {
+    const put = vi.fn().mockRejectedValue(rejectStatus(500));
+    const http = makeHttp({ put });
+    const warn = vi.fn();
+    const logger = { warn } as any;
+    const api = new SpotifyConnectApi(token(), { http, sleep: noSleep, logger });
+    await expect(api.play("dev-1", "spotify:track:abc")).resolves.toBeUndefined();
+    expect(put).toHaveBeenCalledTimes(MAX_ATTEMPTS);
+    expect(warn).toHaveBeenCalledTimes(1);
+  });
+
+  it("does NOT retry a non-transient 403 (exactly ONE call, no throw)", async () => {
+    const put = vi.fn().mockRejectedValue(rejectStatus(403));
+    const http = makeHttp({ put });
+    const api = new SpotifyConnectApi(token(), { http, sleep: noSleep });
+    await expect(api.play("dev-1", "spotify:track:abc")).resolves.toBeUndefined();
+    expect(put).toHaveBeenCalledTimes(1);
+  });
+
+  it("429 honors a CAPPED Retry-After then succeeds (2 calls, bounded sleep)", async () => {
+    const put = vi
+      .fn()
+      .mockRejectedValueOnce(rejectStatus(429, { "retry-after": "1" }))
+      .mockResolvedValueOnce({ status: 200, data: {} });
+    const http = makeHttp({ put });
+    const sleep = vi.fn<(ms: number) => Promise<void>>(async () => {});
+    const api = new SpotifyConnectApi(token(), { http, sleep });
+    await expect(api.play("dev-1", "spotify:track:abc")).resolves.toBeUndefined();
+    expect(put).toHaveBeenCalledTimes(2);
+    expect(sleep).toHaveBeenCalledTimes(1);
+    const delay = sleep.mock.calls[0][0];
+    expect(delay).toBe(1000);
+    expect(delay).toBeLessThanOrEqual(2000);
+  });
+
+  it("transfer() shares the retry path — 404 then success (2 calls)", async () => {
+    const put = vi
+      .fn()
+      .mockRejectedValueOnce(rejectStatus(404))
+      .mockResolvedValueOnce({ status: 200, data: {} });
+    const http = makeHttp({ put });
+    const api = new SpotifyConnectApi(token(), { http, sleep: noSleep });
+    await expect(api.transfer("dev-1", true)).resolves.toBeUndefined();
+    expect(put).toHaveBeenCalledTimes(2);
+  });
+});
+
+describe("SpotifyConnectApi.getPlaybackState", () => {
+  it("GETs /v1/me/player and maps is_playing/progress/item", async () => {
+    const http = makeHttp({
+      get: vi.fn().mockResolvedValue({
+        status: 200,
+        data: {
+          is_playing: true,
+          progress_ms: 12345,
+          item: { uri: "spotify:track:abc", duration_ms: 200000 },
+        },
+      }),
+    });
+    const api = new SpotifyConnectApi(token(), { http });
+    const state = await api.getPlaybackState();
+    expect(http.get).toHaveBeenCalledWith("/v1/me/player", AUTH);
+    expect(state).toEqual({
+      isPlaying: true,
+      progressMs: 12345,
+      trackUri: "spotify:track:abc",
+      durationMs: 200000,
+    });
+  });
+
+  // R4-4 (multi-bot): the account-wide /v1/me/player response names the single
+  // ACTIVE Connect device. Expose it as activeDeviceId so the Rust backend can
+  // tell "our device" from a foreign device another bot stole the session with.
+  it("maps device.id -> activeDeviceId", async () => {
+    const http = makeHttp({
+      get: vi.fn().mockResolvedValue({
+        status: 200,
+        data: {
+          is_playing: true,
+          progress_ms: 1000,
+          item: { uri: "spotify:track:abc", duration_ms: 200000 },
+          device: { id: "dev-1", name: "TS Bot", is_active: true },
+        },
+      }),
+    });
+    const api = new SpotifyConnectApi(token(), { http });
+    const state = await api.getPlaybackState();
+    expect(state).toEqual({
+      isPlaying: true,
+      progressMs: 1000,
+      trackUri: "spotify:track:abc",
+      durationMs: 200000,
+      activeDeviceId: "dev-1",
+    });
+  });
+
+  it("returns null on 204 (no active device)", async () => {
+    const http = makeHttp({ get: vi.fn().mockResolvedValue({ status: 204, data: "" }) });
+    const api = new SpotifyConnectApi(token(), { http });
+    await expect(api.getPlaybackState()).resolves.toBeNull();
+  });
+
+  it("returns null when item is missing / trackUri null", async () => {
+    const http = makeHttp({
+      get: vi.fn().mockResolvedValue({ status: 200, data: { is_playing: false, progress_ms: 0, item: null } }),
+    });
+    const api = new SpotifyConnectApi(token(), { http });
+    await expect(api.getPlaybackState()).resolves.toEqual({
+      isPlaying: false,
+      progressMs: 0,
+      trackUri: null,
+      durationMs: 0,
+    });
+  });
+
+  it("returns null on rejection (e.g. 401) instead of throwing", async () => {
+    const http = makeHttp({ get: vi.fn().mockRejectedValue(new Error("boom")) });
+    const api = new SpotifyConnectApi(token(), { http });
+    await expect(api.getPlaybackState()).resolves.toBeNull();
+  });
+
+  it("returns null when getToken() is null (unauthorized) without calling http", async () => {
+    const http = makeHttp();
+    const api = new SpotifyConnectApi(vi.fn().mockResolvedValue(null), { http });
+    await expect(api.getPlaybackState()).resolves.toBeNull();
+    expect(http.get).not.toHaveBeenCalled();
+  });
+});

src/music/spotify/connect-api.ts

@@ -0,0 +1,205 @@
+import axios, { type AxiosInstance } from "axios";
+
+const API_BASE = "https://api.spotify.com";
+
+/**
+ * Task S4.6 (spec §4.3/§13 recovery/watchdog): transient Connect-command
+ * failures that warrant a bounded retry with exponential backoff. 404 is the
+ * device-visibility latency case (device not yet enumerated), 429 is rate-limit,
+ * 5xx are Spotify-side flakiness. Everything else (401/403/network) is NOT
+ * retried — it is swallowed immediately (C3.6).
+ */
+const TRANSIENT = new Set([404, 429, 500, 502, 503]);
+const MAX_ATTEMPTS = 3;
+const BASE_DELAY_MS = 150;
+const MAX_DELAY_MS = 2_000;
+
+export interface SpotifyDevice {
+  id: string;
+  name: string;
+  is_active: boolean;
+}
+
+export interface PlaybackState {
+  isPlaying: boolean;
+  progressMs: number;
+  trackUri: string | null;
+  durationMs: number;
+  /**
+   * R4-4 (multi-bot): the id of the single ACTIVE Connect device the
+   * account-wide GET /v1/me/player is reporting (from `device.id`). Absent when
+   * Spotify omits the device block. The Rust backend uses it to tell "our
+   * device" from a foreign device another bot stole the shared session with, so
+   * it never misattributes that bot's track/stop as ours.
+   */
+  activeDeviceId?: string;
+}
+
+/**
+ * Spotify Web API "Connect" remote-control client. Wraps an axios instance and
+ * attaches a live user Bearer token from getToken() to every request.
+ *
+ * Error policy:
+ * - Read-only calls (getDevices/getPlaybackState) degrade to []/null on error.
+ * - Mutating calls (transfer/play/pause/resume/seek) no-op when unauthorized.
+ * - REQUIRED CORRECTION C3.6: mutating calls ALSO swallow transport errors
+ *   (403 non-Premium / 404 no active device / 429 rate-limited / network) and
+ *   resolve to void instead of rejecting. The contract keeps the Promise<void>
+ *   signatures, so the backend treats a failed play as "couldn't play" and
+ *   falls back — a transient Spotify error can never surface as an unhandled
+ *   rejection that crashes the queue-advance path.
+ */
+export class SpotifyConnectApi {
+  private getToken: () => Promise<string | null>;
+  private http: AxiosInstance;
+  private sleep: (ms: number) => Promise<void>;
+  private logger?: import("pino").Logger;
+
+  constructor(
+    getToken: () => Promise<string | null>,
+    deps?: {
+      http?: AxiosInstance;
+      sleep?: (ms: number) => Promise<void>;
+      logger?: import("pino").Logger;
+    },
+  ) {
+    this.getToken = getToken;
+    this.http = deps?.http ?? axios.create({ baseURL: API_BASE, timeout: 15_000 });
+    this.sleep = deps?.sleep ?? ((ms) => new Promise((r) => setTimeout(r, ms)));
+    this.logger = deps?.logger;
+  }
+
+  /** Bearer auth headers, or null when no valid user token is available. */
+  private async authHeaders(): Promise<{ Authorization: string } | null> {
+    const token = await this.getToken();
+    if (!token) return null;
+    return { Authorization: `Bearer ${token}` };
+  }
+
+  /**
+   * S4.6 recovery/watchdog: run a mutating PUT with bounded retry + exponential
+   * backoff on TRANSIENT statuses only. On exhaustion OR a non-transient error
+   * it SWALLOWS and warns once — preserving C3.6 (a mutating command NEVER
+   * rejects up the queue-advance path). Tokens are never logged.
+   */
+  private async mutateWithRetry(put: () => Promise<unknown>): Promise<void> {
+    for (let attempt = 1; attempt <= MAX_ATTEMPTS; attempt++) {
+      try {
+        await put();
+        return;
+      } catch (err: any) {
+        const status = err?.response?.status;
+        if (!TRANSIENT.has(status) || attempt === MAX_ATTEMPTS) {
+          // C3.6: never reject up the queue path — swallow, but surface once.
+          this.logger?.warn(
+            { status },
+            "Spotify Connect command failed (exhausted/non-retryable)",
+          );
+          return;
+        }
+        let delay = Math.min(BASE_DELAY_MS * 2 ** (attempt - 1), MAX_DELAY_MS);
+        if (status === 429) {
+          const ra = Number(err?.response?.headers?.["retry-after"]);
+          if (Number.isFinite(ra) && ra > 0) delay = Math.min(ra * 1000, MAX_DELAY_MS);
+        }
+        await this.sleep(delay);
+      }
+    }
+  }
+
+  async getDevices(): Promise<SpotifyDevice[]> {
+    const headers = await this.authHeaders();
+    if (!headers) return [];
+    try {
+      const { data } = await this.http.get("/v1/me/player/devices", { headers });
+      const list = Array.isArray(data?.devices) ? data.devices : [];
+      return list.map((d: any) => ({
+        id: d?.id ?? "",
+        name: d?.name ?? "",
+        is_active: Boolean(d?.is_active),
+      }));
+    } catch {
+      return [];
+    }
+  }
+
+  async findDeviceByName(name: string): Promise<string | null> {
+    const devices = await this.getDevices();
+    const match = devices.find((d) => d.name === name);
+    return match ? match.id : null;
+  }
+
+  async transfer(deviceId: string, play = false): Promise<void> {
+    const headers = await this.authHeaders();
+    if (!headers) return;
+    await this.mutateWithRetry(() =>
+      this.http.put("/v1/me/player", { device_ids: [deviceId], play }, { headers }),
+    );
+  }
+
+  async play(deviceId: string, trackUri: string): Promise<void> {
+    const headers = await this.authHeaders();
+    if (!headers) return;
+    await this.mutateWithRetry(() =>
+      this.http.put(
+        "/v1/me/player/play",
+        { uris: [trackUri] },
+        { headers, params: { device_id: deviceId } },
+      ),
+    );
+  }
+
+  async pause(deviceId?: string): Promise<void> {
+    const headers = await this.authHeaders();
+    if (!headers) return;
+    await this.mutateWithRetry(() =>
+      this.http.put("/v1/me/player/pause", undefined, {
+        headers,
+        params: deviceId ? { device_id: deviceId } : undefined,
+      }),
+    );
+  }
+
+  async resume(deviceId?: string): Promise<void> {
+    const headers = await this.authHeaders();
+    if (!headers) return;
+    await this.mutateWithRetry(() =>
+      this.http.put("/v1/me/player/play", undefined, {
+        headers,
+        params: deviceId ? { device_id: deviceId } : undefined,
+      }),
+    );
+  }
+
+  async seek(ms: number, deviceId?: string): Promise<void> {
+    const headers = await this.authHeaders();
+    if (!headers) return;
+    const params: Record<string, unknown> = { position_ms: ms };
+    if (deviceId) params.device_id = deviceId;
+    await this.mutateWithRetry(() =>
+      this.http.put("/v1/me/player/seek", undefined, { headers, params }),
+    );
+  }
+
+  async getPlaybackState(): Promise<PlaybackState | null> {
+    const headers = await this.authHeaders();
+    if (!headers) return null;
+    try {
+      const res = await this.http.get("/v1/me/player", { headers });
+      // 204 = no active device / playback; body is empty.
+      if (res.status === 204 || !res.data) return null;
+      const d = res.data;
+      return {
+        isPlaying: Boolean(d.is_playing),
+        progressMs: Number(d.progress_ms ?? 0),
+        trackUri: d.item?.uri ?? null,
+        durationMs: Number(d.item?.duration_ms ?? 0),
+        // R4-4: expose the account's single active device so a foreign steal is
+        // distinguishable from our own device. Left undefined when absent.
+        activeDeviceId: d.device?.id ?? undefined,
+      };
+    } catch {
+      return null;
+    }
+  }
+}

src/music/spotify/controller.test.ts

@@ -0,0 +1,709 @@
+import { describe, it, expect, beforeAll, afterAll, beforeEach, vi } from "vitest";
+import { EventEmitter } from "node:events";
+import { Readable } from "node:stream";
+import { writeFileSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import type { Logger } from "pino";
+import type { SpotifyConfig } from "../../data/config.js";
+import type {
+  SpotifyAudioBackend,
+  SpotifyTrackEndedEvent,
+  SpotifyNowPlaying,
+} from "./backend.js";
+import type { SpotifyOAuth } from "./spotify-oauth.js";
+
+// Controllable, hoisted so the vi.mock factory can close over it.
+// go-* keys keep their Stage-2 names (`supported`/`path`) so existing tests are
+// untouched; rust* keys drive the new librespot selection paths.
+const bin = vi.hoisted(() => ({
+  supported: true,
+  path: "",
+  rustSupported: true,
+  rustPath: "",
+}));
+vi.mock("./binary.js", async () => {
+  // existsSync mirrors the real resolveExecutable(find*()) result for the
+  // bin/-style temp paths this suite uses (existingBin exists, missingBin does
+  // not), keeping the chooseBackend/isAvailable matrix behavior-identical.
+  const { existsSync } = await import("node:fs");
+  return {
+    isGoLibrespotSupported: () => bin.supported,
+    findGoLibrespot: () => bin.path,
+    resetGoLibrespotBinaryCache: () => {},
+    checkGoLibrespotAvailable: async () => bin.supported && !!bin.path,
+    isRustLibrespotSupported: () => bin.rustSupported,
+    findLibrespot: () => bin.rustPath,
+    resetLibrespotBinaryCache: () => {},
+    checkLibrespotAvailable: async () => bin.rustSupported && !!bin.rustPath,
+    // PATH-aware presence gates (Bug I3): supported gate AND the resolved
+    // binary actually exists on disk.
+    isGoLibrespotPresent: () => bin.supported && existsSync(bin.path),
+    isLibrespotPresent: () => bin.rustSupported && existsSync(bin.rustPath),
+  };
+});
+
+// Capture options the DEFAULT factory hands to the real GoLibrespotBackend so
+// we can assert per-bot ports (Fix 3) are threaded through. Every other test
+// injects its own backendFactory, so this mock is inert for them.
+const goLibrespotCtor = vi.hoisted(() => vi.fn());
+vi.mock("./go-librespot.js", async () => {
+  const { EventEmitter } = await import("node:events");
+  return {
+    GoLibrespotBackend: class extends EventEmitter {
+      constructor(opts: any) {
+        super();
+        goLibrespotCtor(opts);
+      }
+      async start(): Promise<void> {}
+      isReady(): boolean {
+        return true;
+      }
+      stop(): void {}
+      async playTrack(): Promise<void> {}
+      async pause(): Promise<void> {}
+      async resume(): Promise<void> {}
+      async seek(): Promise<void> {}
+      getPcmStream(): any {
+        return null;
+      }
+      getPositionMs(): number {
+        return 0;
+      }
+    },
+  };
+});
+
+// Import AFTER vi.mock so the mocked binary module is used.
+const { SpotifyController, perBotDeviceName } = await import("./controller.js");
+
+const existingBin = join(tmpdir(), `tsmb-golibrespot-${process.pid}`);
+const missingBin = join(tmpdir(), `tsmb-golibrespot-missing-${process.pid}`);
+
+beforeAll(() => {
+  writeFileSync(existingBin, "#!/bin/sh\n");
+});
+afterAll(() => {
+  try {
+    rmSync(existingBin);
+  } catch {
+    /* ignore */
+  }
+});
+beforeEach(() => {
+  bin.supported = true;
+  bin.path = existingBin;
+  bin.rustSupported = true;
+  bin.rustPath = missingBin;
+});
+
+class FakeBackend extends EventEmitter implements SpotifyAudioBackend {
+  startCalls = 0;
+  stopCalls = 0;
+  playCalls: string[] = [];
+  pauseCalls = 0;
+  resumeCalls = 0;
+  seekCalls: number[] = [];
+  ready = false;
+  startShouldReject = false;
+  playShouldReject = false;
+  /** When set, start() awaits this before resolving — lets a test interleave
+   *  stop()/error DURING a mid-flight start (Bug I1). */
+  startGate?: Promise<void>;
+  readonly pcm = Readable.from([Buffer.alloc(0)]);
+
+  async start(): Promise<void> {
+    this.startCalls++;
+    if (this.startGate) await this.startGate;
+    if (this.startShouldReject) throw new Error("start boom");
+    this.ready = true;
+  }
+  stop(): void {
+    this.stopCalls++;
+    this.ready = false;
+  }
+  isReady(): boolean {
+    return this.ready;
+  }
+  async playTrack(uri: string): Promise<void> {
+    this.playCalls.push(uri);
+    if (this.playShouldReject) throw new Error("play boom");
+  }
+  async pause(): Promise<void> {
+    this.pauseCalls++;
+  }
+  async resume(): Promise<void> {
+    this.resumeCalls++;
+  }
+  async seek(ms: number): Promise<void> {
+    this.seekCalls.push(ms);
+  }
+  getPcmStream(): Readable {
+    return this.pcm;
+  }
+  getPositionMs(): number {
+    return 0;
+  }
+}
+
+const silentLogger = {
+  info() {},
+  error() {},
+  warn() {},
+  debug() {},
+  trace() {},
+  fatal() {},
+  child() {
+    return silentLogger;
+  },
+} as unknown as Logger;
+
+function cfg(over: Partial<SpotifyConfig> = {}): SpotifyConfig {
+  return {
+    enabled: true,
+    backend: "auto",
+    clientId: "",
+    clientSecret: "",
+    deviceName: "TSMusicBot",
+    bitrate: 320,
+    ...over,
+  };
+}
+
+function makeCtrl(over: {
+  config?: Partial<SpotifyConfig>;
+  backendFactory?: () => SpotifyAudioBackend;
+  oauth?: import("./spotify-oauth.js").SpotifyOAuth;
+} = {}) {
+  const be = new FakeBackend();
+  const ctrl = new SpotifyController({
+    config: cfg(over.config),
+    workDir: "/tmp/work",
+    configDir: "/tmp/cfg",
+    logger: silentLogger,
+    backendFactory: over.backendFactory ?? (() => be),
+    oauth: over.oauth,
+  });
+  return { ctrl, be };
+}
+
+function fakeOAuth(
+  authorized: boolean,
+  hooks: { onIsAuthorized?: () => void } = {},
+): SpotifyOAuth {
+  return {
+    isAuthorized: () => {
+      hooks.onIsAuthorized?.();
+      return authorized;
+    },
+    getAccessToken: async () => (authorized ? "tok" : null),
+    getClientId: () => "cid",
+    getRedirectUri: () => "http://127.0.0.1:5588/login",
+    buildAuthorizeUrl: () => ({ url: "https://accounts.spotify.com/authorize", state: "s" }),
+    handleCallback: async () => true,
+  } as unknown as SpotifyOAuth;
+}
+
+describe("SpotifyController.isAvailable", () => {
+  it("true when enabled + supported + binary present", () => {
+    const { ctrl } = makeCtrl();
+    expect(ctrl.isAvailable()).toBe(true);
+  });
+  it("false when config disabled", () => {
+    const { ctrl } = makeCtrl({ config: { enabled: false } });
+    expect(ctrl.isAvailable()).toBe(false);
+  });
+  it("false when platform unsupported", () => {
+    bin.supported = false;
+    const { ctrl } = makeCtrl();
+    expect(ctrl.isAvailable()).toBe(false);
+  });
+  it("false when binary file is absent", () => {
+    bin.path = missingBin;
+    const { ctrl } = makeCtrl();
+    expect(ctrl.isAvailable()).toBe(false);
+  });
+});
+
+describe("SpotifyController.ensureStarted", () => {
+  it("starts the backend exactly once across repeated calls", async () => {
+    let built = 0;
+    const be = new FakeBackend();
+    const { ctrl } = makeCtrl({
+      backendFactory: () => {
+        built++;
+        return be;
+      },
+    });
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(built).toBe(1);
+    expect(be.startCalls).toBe(1);
+  });
+
+  it("is idempotent under concurrent calls (single start)", async () => {
+    let built = 0;
+    const be = new FakeBackend();
+    const { ctrl } = makeCtrl({
+      backendFactory: () => {
+        built++;
+        return be;
+      },
+    });
+    const [a, b] = await Promise.all([ctrl.ensureStarted(), ctrl.ensureStarted()]);
+    expect(a).toBe(true);
+    expect(b).toBe(true);
+    expect(built).toBe(1);
+    expect(be.startCalls).toBe(1);
+  });
+
+  it("returns false and does not build a backend when unavailable", async () => {
+    let built = 0;
+    const { ctrl } = makeCtrl({
+      config: { enabled: false },
+      backendFactory: () => {
+        built++;
+        return new FakeBackend();
+      },
+    });
+    expect(await ctrl.ensureStarted()).toBe(false);
+    expect(built).toBe(0);
+  });
+
+  it("returns false when backend.start() throws, and allows a later retry", async () => {
+    const be = new FakeBackend();
+    be.startShouldReject = true;
+    let built = 0;
+    const { ctrl } = makeCtrl({
+      backendFactory: () => {
+        built++;
+        return be;
+      },
+    });
+    expect(await ctrl.ensureStarted()).toBe(false);
+    // start failure clears the cached promise so a subsequent call retries.
+    be.startShouldReject = false;
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(built).toBe(2);
+    expect(be.startCalls).toBe(2);
+  });
+});
+
+describe("SpotifyController.playTrack", () => {
+  it("ensures started then delegates the uri, returning true", async () => {
+    const { ctrl, be } = makeCtrl();
+    expect(await ctrl.playTrack("spotify:track:abc")).toBe(true);
+    expect(be.startCalls).toBe(1);
+    expect(be.playCalls).toEqual(["spotify:track:abc"]);
+  });
+  it("returns false when the controller is unavailable", async () => {
+    const { ctrl, be } = makeCtrl({ config: { enabled: false } });
+    expect(await ctrl.playTrack("spotify:track:abc")).toBe(false);
+    expect(be.playCalls).toEqual([]);
+  });
+  it("returns false when backend.playTrack rejects", async () => {
+    const be = new FakeBackend();
+    be.playShouldReject = true;
+    const { ctrl } = makeCtrl({ backendFactory: () => be });
+    expect(await ctrl.playTrack("spotify:track:abc")).toBe(false);
+  });
+});
+
+describe("SpotifyController transport delegation", () => {
+  it("pause/resume/seek forward to the backend after start", async () => {
+    const { ctrl, be } = makeCtrl();
+    await ctrl.ensureStarted();
+    await ctrl.pause();
+    await ctrl.resume();
+    await ctrl.seek(4200);
+    expect(be.pauseCalls).toBe(1);
+    expect(be.resumeCalls).toBe(1);
+    expect(be.seekCalls).toEqual([4200]);
+  });
+  it("pause/resume/seek are safe no-ops before start", async () => {
+    const { ctrl, be } = makeCtrl();
+    await expect(ctrl.pause()).resolves.toBeUndefined();
+    await expect(ctrl.resume()).resolves.toBeUndefined();
+    await expect(ctrl.seek(10)).resolves.toBeUndefined();
+    expect(be.pauseCalls).toBe(0);
+  });
+  it("getPcmStream returns the backend stream", async () => {
+    const { ctrl, be } = makeCtrl();
+    await ctrl.ensureStarted();
+    expect(ctrl.getPcmStream()).toBe(be.pcm);
+  });
+  it("getPcmStream throws before the backend is started", () => {
+    const { ctrl } = makeCtrl();
+    expect(() => ctrl.getPcmStream()).toThrow();
+  });
+});
+
+// R4-1: the web progress bar computes seekTime = ratio * duration (fractional
+// seconds); BotInstance routes Spotify seeks through seek(seconds * 1000),
+// yielding a NON-integer ms (e.g. 71610.00000000001). go-librespot decodes
+// `position` into an int64 and Go's encoding/json REJECTS a JSON number with a
+// decimal point -> HTTP 400 -> the error is swallowed -> the track never seeks.
+// The controller must round ONCE so BOTH backends get a valid integer ms.
+describe("SpotifyController.seek integer rounding (R4-1)", () => {
+  it("rounds a fractional seek to an integer ms before forwarding to the backend", async () => {
+    const { ctrl, be } = makeCtrl();
+    await ctrl.ensureStarted();
+    await ctrl.seek(71610.00000000001);
+    expect(be.seekCalls).toHaveLength(1);
+    expect(Number.isInteger(be.seekCalls[0])).toBe(true);
+    expect(be.seekCalls[0]).toBe(71610);
+  });
+
+  it("clamps a negative seek to 0", async () => {
+    const { ctrl, be } = makeCtrl();
+    await ctrl.ensureStarted();
+    await ctrl.seek(-5);
+    expect(be.seekCalls).toEqual([0]);
+  });
+
+  it("passes an already-integer seek through unchanged", async () => {
+    const { ctrl, be } = makeCtrl();
+    await ctrl.ensureStarted();
+    await ctrl.seek(4200);
+    expect(be.seekCalls).toEqual([4200]);
+  });
+});
+
+describe("SpotifyController event re-emission", () => {
+  it("re-emits backend trackEnded with the same payload", async () => {
+    const { ctrl, be } = makeCtrl();
+    await ctrl.ensureStarted();
+    const got: SpotifyTrackEndedEvent[] = [];
+    ctrl.on("trackEnded", (e) => got.push(e));
+    const evt: SpotifyTrackEndedEvent = { uri: "spotify:track:x", reason: "ended" };
+    be.emit("trackEnded", evt);
+    expect(got).toEqual([evt]);
+  });
+  it("re-emits backend metadata with the same payload", async () => {
+    const { ctrl, be } = makeCtrl();
+    await ctrl.ensureStarted();
+    const got: SpotifyNowPlaying[] = [];
+    ctrl.on("metadata", (m) => got.push(m));
+    const meta: SpotifyNowPlaying = {
+      uri: "spotify:track:x",
+      name: "Song",
+      artist: "Artist",
+      album: "Album",
+      coverUrl: "http://img",
+      durationMs: 1000,
+    };
+    be.emit("metadata", meta);
+    expect(got).toEqual([meta]);
+  });
+});
+
+describe("SpotifyController backend error handling (C3)", () => {
+  it("does NOT throw on a backend 'error' with no controller listener, and marks not-ready", async () => {
+    const be1 = new FakeBackend();
+    const be2 = new FakeBackend();
+    const backends = [be1, be2];
+    let built = 0;
+    const { ctrl } = makeCtrl({
+      backendFactory: () => {
+        built++;
+        return backends.shift()!;
+      },
+    });
+    await ctrl.ensureStarted();
+    expect(built).toBe(1);
+    // The controller itself has NO "error" listener. A raw re-emit would make
+    // Node throw here; the controller must swallow+log instead.
+    expect(() => be1.emit("error", new Error("sidecar boom"))).not.toThrow();
+    // Marked not-ready: a fresh ensureStarted relaunches a new backend.
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(built).toBe(2);
+    expect(be2.startCalls).toBe(1);
+  });
+
+  it("tears down the errored backend (stop + detach + null) so it is not orphaned", async () => {
+    const be1 = new FakeBackend();
+    const be2 = new FakeBackend();
+    const backends = [be1, be2];
+    let built = 0;
+    const { ctrl } = makeCtrl({
+      backendFactory: () => {
+        built++;
+        return backends.shift()!;
+      },
+    });
+    await ctrl.ensureStarted();
+    expect(built).toBe(1);
+
+    // On "error" the controller must stop() the errored backend (cleans its
+    // ffmpeg/go-librespot children + FIFO) and detach ALL its listeners.
+    expect(() => be1.emit("error", new Error("sidecar boom"))).not.toThrow();
+    expect(be1.stopCalls).toBe(1);
+    expect(be1.listenerCount("error")).toBe(0);
+    expect(be1.listenerCount("trackEnded")).toBe(0);
+    expect(be1.listenerCount("metadata")).toBe(0);
+
+    // Internal backend is null: a transport call no-ops (delegates to nothing)
+    // and getPcmStream throws until a rebuild.
+    await ctrl.pause();
+    expect(be1.pauseCalls).toBe(0);
+    expect(() => ctrl.getPcmStream()).toThrow();
+
+    // A fresh ensureStarted builds a NEW backend instance.
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(built).toBe(2);
+    expect(be2.startCalls).toBe(1);
+    expect(ctrl.getPcmStream()).toBe(be2.pcm);
+  });
+
+  it("does not cross-talk: a later error from the ORPHANED backend leaves the healthy rebuilt controller ready", async () => {
+    const be1 = new FakeBackend();
+    const be2 = new FakeBackend();
+    const backends = [be1, be2];
+    let built = 0;
+    const { ctrl } = makeCtrl({
+      backendFactory: () => {
+        built++;
+        return backends.shift()!;
+      },
+    });
+    await ctrl.ensureStarted();
+
+    // First error tears down be1 and the controller rebuilds onto healthy be2.
+    be1.emit("error", new Error("sidecar boom"));
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(built).toBe(2);
+    await ctrl.pause();
+    expect(be2.pauseCalls).toBe(1);
+
+    // A SECOND error emitted by the OLD (orphaned) backend must NOT reach the
+    // controller: the healthy be2 stays owned, ready, and untouched.
+    be1.on("error", () => {}); // controller detached; re-arm to avoid unhandled throw
+    expect(() => be1.emit("error", new Error("orphan boom"))).not.toThrow();
+    expect(be2.stopCalls).toBe(0); // healthy backend not torn down
+    // Still ready: ensureStarted returns true without rebuilding a third time.
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(built).toBe(2);
+    await ctrl.pause();
+    expect(be2.pauseCalls).toBe(2);
+    expect(ctrl.getPcmStream()).toBe(be2.pcm);
+  });
+});
+
+describe("SpotifyController.stop", () => {
+  it("stops the backend and tears down state so a later start rebuilds", async () => {
+    const be1 = new FakeBackend();
+    const be2 = new FakeBackend();
+    const backends = [be1, be2];
+    const { ctrl } = makeCtrl({ backendFactory: () => backends.shift()! });
+    await ctrl.ensureStarted();
+    ctrl.stop();
+    expect(be1.stopCalls).toBe(1);
+    // After teardown getPcmStream is invalid again until re-started.
+    expect(() => ctrl.getPcmStream()).toThrow();
+    // A fresh ensureStarted builds a new backend.
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(be2.startCalls).toBe(1);
+  });
+  it("stop before start is a safe no-op", () => {
+    const { ctrl, be } = makeCtrl();
+    expect(() => ctrl.stop()).not.toThrow();
+    expect(be.stopCalls).toBe(0);
+  });
+
+  it("Bug I1: stop() DURING a mid-flight start tears the sidecar down instead of resurrecting it", async () => {
+    // A Deferred the test resolves to complete backend.start() on demand.
+    let resolveStart!: () => void;
+    const startGate = new Promise<void>((res) => {
+      resolveStart = res;
+    });
+    const be = new FakeBackend();
+    be.startGate = startGate;
+    const { ctrl } = makeCtrl({ backendFactory: () => be });
+
+    // Kick off ensureStarted but DO NOT await — start() is parked on the gate.
+    const startedP = ctrl.ensureStarted();
+    // Let the ensureStarted IIFE run up to `await backend.start()`.
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(be.startCalls).toBe(1); // start() was entered and is now pending
+
+    // Caller tears the controller down while start() is still in flight
+    // (a user `!stop`/disconnect). Pre-fix this.backend is still null so this
+    // is a no-op and the spawned sidecar is orphaned.
+    ctrl.stop();
+
+    // Now let start() finally resolve. Pre-fix the IIFE would set this.backend
+    // and started=true, RESURRECTING the sidecar the caller already stopped.
+    resolveStart();
+    const result = await startedP;
+
+    // Post-fix: the mid-flight backend is torn down, not promoted.
+    expect(result).toBe(false);
+    expect(be.stopCalls).toBeGreaterThanOrEqual(1); // the fake WAS stopped
+    expect(be.listenerCount("trackEnded")).toBe(0); // listeners detached
+    expect(() => ctrl.getPcmStream()).toThrow(); // not resurrected
+  });
+});
+
+describe("SpotifyController per-bot ports (Fix 3)", () => {
+  it("threads apiPort + callbackPort into the default GoLibrespotBackend", async () => {
+    goLibrespotCtor.mockClear();
+    // No injected backendFactory → the controller builds the (mocked) real backend.
+    const ctrl = new SpotifyController({
+      config: cfg(),
+      workDir: "/tmp/work",
+      configDir: "/tmp/cfg",
+      logger: silentLogger,
+      apiPort: 3712,
+      callbackPort: 8712,
+    });
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(goLibrespotCtor).toHaveBeenCalledWith(
+      expect.objectContaining({ apiPort: 3712, callbackPort: 8712 }),
+    );
+  });
+});
+
+describe("perBotDeviceName (corner-case R2-5)", () => {
+  it("suffixes the base name with the instanceId", () => {
+    expect(perBotDeviceName("TSMusicBot", "bot1")).toBe("TSMusicBot-bot1");
+  });
+  it("returns the base name unchanged when no instanceId is given", () => {
+    expect(perBotDeviceName("TSMusicBot", undefined)).toBe("TSMusicBot");
+  });
+});
+
+describe("SpotifyController per-bot device name (corner-case R2-5)", () => {
+  it("applies the per-bot suffix to the backend deviceName when instanceId is set", async () => {
+    goLibrespotCtor.mockClear();
+    // No injected backendFactory → the controller builds the (mocked) real backend.
+    const ctrl = new SpotifyController({
+      config: cfg(),
+      workDir: "/tmp/work",
+      configDir: "/tmp/cfg",
+      logger: silentLogger,
+      instanceId: "bot1",
+    });
+    expect(await ctrl.ensureStarted()).toBe(true);
+    // The Connect identity is UNIQUE per bot ("<base>-<id>") so two bots under
+    // one account never register the same name (misroute / false-readiness).
+    expect(goLibrespotCtor).toHaveBeenCalledWith(
+      expect.objectContaining({ deviceName: "TSMusicBot-bot1" }),
+    );
+  });
+
+  it("leaves the base deviceName unchanged when no instanceId (behavior-preserving)", async () => {
+    goLibrespotCtor.mockClear();
+    const ctrl = new SpotifyController({
+      config: cfg(),
+      workDir: "/tmp/work",
+      configDir: "/tmp/cfg",
+      logger: silentLogger,
+    });
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(goLibrespotCtor).toHaveBeenCalledWith(
+      expect.objectContaining({ deviceName: "TSMusicBot" }),
+    );
+  });
+});
+
+describe("SpotifyController.chooseBackend (platform x config matrix)", () => {
+  function pick(
+    backend: SpotifyConfig["backend"],
+    opts: { go: boolean; goSupported?: boolean; rust: boolean; rustSupported?: boolean },
+  ) {
+    bin.supported = opts.goSupported ?? true;
+    bin.path = opts.go ? existingBin : missingBin;
+    bin.rustSupported = opts.rustSupported ?? true;
+    bin.rustPath = opts.rust ? existingBin : missingBin;
+    const { ctrl } = makeCtrl({ config: { backend } });
+    return ctrl.chooseBackend();
+  }
+
+  it("auto: prefers go-librespot when linux + go binary present", () => {
+    expect(pick("auto", { go: true, rust: true })).toBe("go-librespot");
+  });
+  it("auto: falls back to librespot when go unsupported (e.g. Windows) but librespot present", () => {
+    expect(pick("auto", { go: true, goSupported: false, rust: true })).toBe("librespot");
+  });
+  it("auto: falls back to librespot when go binary is absent", () => {
+    expect(pick("auto", { go: false, rust: true })).toBe("librespot");
+  });
+  it("auto: null when neither backend is usable", () => {
+    expect(pick("auto", { go: false, goSupported: false, rust: false })).toBeNull();
+  });
+  it("go-librespot: selected when supported + present", () => {
+    expect(pick("go-librespot", { go: true, rust: true })).toBe("go-librespot");
+  });
+  it("go-librespot: null when unsupported, even if librespot is present", () => {
+    expect(pick("go-librespot", { go: true, goSupported: false, rust: true })).toBeNull();
+  });
+  it("librespot: selected when the librespot binary is present", () => {
+    expect(pick("librespot", { go: true, rust: true })).toBe("librespot");
+  });
+  it("librespot: null when the librespot binary is absent, even if go is present", () => {
+    expect(pick("librespot", { go: true, rust: false })).toBeNull();
+  });
+});
+
+describe("SpotifyController Rust-backend auth gate", () => {
+  it("isAvailable is true for a present librespot binary regardless of auth", () => {
+    bin.rustPath = existingBin;
+    const { ctrl } = makeCtrl({
+      config: { backend: "librespot" },
+      oauth: fakeOAuth(false),
+    });
+    expect(ctrl.isAvailable()).toBe(true);
+  });
+
+  it("ensureStarted returns false (no backend built) when Rust chosen but unauthorized", async () => {
+    bin.rustPath = existingBin;
+    let built = 0;
+    const { ctrl } = makeCtrl({
+      config: { backend: "librespot" },
+      oauth: fakeOAuth(false),
+      backendFactory: () => {
+        built++;
+        return new FakeBackend();
+      },
+    });
+    expect(await ctrl.ensureStarted()).toBe(false);
+    expect(built).toBe(0);
+  });
+
+  it("ensureStarted starts the Rust backend once authorized", async () => {
+    bin.rustPath = existingBin;
+    const be = new FakeBackend();
+    let built = 0;
+    const { ctrl } = makeCtrl({
+      config: { backend: "librespot" },
+      oauth: fakeOAuth(true),
+      backendFactory: () => {
+        built++;
+        return be;
+      },
+    });
+    expect(await ctrl.ensureStarted()).toBe(true);
+    expect(built).toBe(1);
+    expect(be.startCalls).toBe(1);
+  });
+
+  it("go-librespot path never consults oauth.isAuthorized()", async () => {
+    // auto + go present -> go-librespot; the auth gate must be skipped so a
+    // throwing isAuthorized() is never reached.
+    const oauth = fakeOAuth(false, {
+      onIsAuthorized: () => {
+        throw new Error("isAuthorized must not be called on the go path");
+      },
+    });
+    const { ctrl } = makeCtrl({ config: { backend: "auto" }, oauth });
+    expect(await ctrl.ensureStarted()).toBe(true);
+  });
+
+  it("exposes the shared oauth + connect instances", () => {
+    const oauth = fakeOAuth(true);
+    const { ctrl } = makeCtrl({ config: { backend: "auto" }, oauth });
+    expect(ctrl.getOAuth()).toBe(oauth);
+    expect(ctrl.getConnect()).toBeDefined();
+  });
+});

src/music/spotify/controller.ts

@@ -0,0 +1,405 @@
+import { EventEmitter } from "node:events";
+import {
+  existsSync,
+  readFileSync,
+  writeFileSync,
+  mkdirSync,
+  rmSync,
+} from "node:fs";
+import { dirname, join } from "node:path";
+import type { Readable } from "node:stream";
+import type { Logger } from "pino";
+import type { SpotifyConfig } from "../../data/config.js";
+import type {
+  SpotifyAudioBackend,
+  SpotifyTrackEndedEvent,
+  SpotifyNowPlaying,
+} from "./backend.js";
+import { isGoLibrespotPresent, isLibrespotPresent } from "./binary.js";
+import { GoLibrespotBackend } from "./go-librespot.js";
+import { RustLibrespotBackend } from "./rust-librespot.js";
+import {
+  SpotifyOAuth,
+  type OAuthTokens,
+  type OAuthTokenStore,
+} from "./spotify-oauth.js";
+import { SpotifyConnectApi } from "./connect-api.js";
+import {
+  resolveSpotifyBackendKind,
+  type SpotifyBackendKind,
+} from "./backend-select.js";
+export type { SpotifyBackendKind }; // keep the name exported for existing importers
+
+/**
+ * Derive the per-bot Spotify Connect device name from the user-configured base.
+ *
+ * `config.spotify` is a single process-wide object shared by every BotInstance,
+ * so `config.spotify.deviceName` is identical for all bots. On the Rust
+ * (librespot) backend each bot would otherwise spawn `librespot --name <base>`
+ * with NO uniqueness, registering TWO Connect devices with the SAME name under
+ * the one shared Premium account — so `findDeviceByName` / `waitForDevice`
+ * (which match by name) could target the OTHER bot's device, misrouting
+ * transfer()+play() and reporting false readiness (corner-case R2-5).
+ *
+ * Suffixing the base with the bot's instanceId makes the Connect identity
+ * unique per bot. Pure + deterministic: same inputs → same name across
+ * restarts. Returns the base unchanged when no instanceId is supplied (keeps
+ * existing single-bot / non-injected behavior byte-for-byte).
+ */
+export function perBotDeviceName(base: string, instanceId?: string): string {
+  return instanceId ? `${base}-${instanceId}` : base;
+}
+
+/**
+ * Minimal file-backed OAuth token store used when the caller does not inject a
+ * SpotifyOAuth. Persists the rotating refresh-token JSON next to the bot config
+ * (0600). All IO is lazy + guarded so construction never throws and a
+ * missing/corrupt file simply reads as "unauthorized".
+ */
+class FileOAuthTokenStore implements OAuthTokenStore {
+  constructor(private readonly file: string) {}
+  load(): OAuthTokens | null {
+    try {
+      if (!existsSync(this.file)) return null;
+      return JSON.parse(readFileSync(this.file, "utf8")) as OAuthTokens;
+    } catch {
+      return null;
+    }
+  }
+  save(t: OAuthTokens): void {
+    mkdirSync(dirname(this.file), { recursive: true });
+    writeFileSync(this.file, JSON.stringify(t), { mode: 0o600 });
+  }
+  clear(): void {
+    try {
+      rmSync(this.file, { force: true });
+    } catch {
+      /* ignore */
+    }
+  }
+}
+
+export interface SpotifyControllerOptions {
+  config: SpotifyConfig;
+  workDir: string;
+  configDir: string;
+  logger: Logger;
+  /** Owning bot's id; suffixed onto the shared base deviceName so each bot's
+   *  Spotify Connect identity is unique (avoids multi-bot device collision /
+   *  misroute — corner-case R2-5). Omitted → the base name is used unchanged. */
+  instanceId?: string;
+  /** Per-bot go-librespot control-API port (distinct per bot to avoid binds). */
+  apiPort?: number;
+  /** Per-bot go-librespot OAuth callback port (distinct per bot). */
+  callbackPort?: number;
+  /** Injected for tests; when set it overrides the per-kind default builders. */
+  backendFactory?: () => SpotifyAudioBackend;
+  /** Injected for tests; defaults to a file-backed SpotifyOAuth in configDir. */
+  oauth?: SpotifyOAuth;
+  /** Injected for tests; defaults to a SpotifyConnectApi wired to oauth. */
+  connect?: SpotifyConnectApi;
+}
+
+/**
+ * Per-bot orchestrator for the Spotify sidecar. Selects a backend for this
+ * host+config (chooseBackend), owns backend lifecycle, gates on availability
+ * (config + platform + binary) plus — for the Rust librespot backend — OAuth
+ * authorization, delegates transport, and re-emits "trackEnded"/"metadata" so
+ * BotInstance can advance the queue exactly as for the ffmpeg path.
+ *
+ * Correction C3 (unchanged): this controller does NOT re-emit a raw "error"
+ * event. It subscribes to the backend's "error", logs it, tears the backend
+ * down, and marks itself not-ready so the next ensureStarted() relaunches a
+ * fresh backend. getPcmStream() proxies the backend's SINGLE persistent stream.
+ */
+export class SpotifyController extends EventEmitter {
+  private readonly config: SpotifyConfig;
+  private readonly workDir: string;
+  private readonly configDir: string;
+  private readonly logger: Logger;
+  private readonly instanceId?: string;
+  private readonly apiPort?: number;
+  private readonly callbackPort?: number;
+  private readonly injectedFactory?: () => SpotifyAudioBackend;
+  private readonly oauth: SpotifyOAuth;
+  private readonly connect: SpotifyConnectApi;
+
+  private backend: SpotifyAudioBackend | null = null;
+  // The in-flight backend during a start() that has not yet completed. A
+  // stop()/handleBackendError() DURING start() clears (or replaces) this so the
+  // mid-start sidecar is torn down and a completing start is discarded by the
+  // ensureStarted post-await guard instead of resurrecting a backend the caller
+  // already tore down (Bug I1).
+  private pendingBackend: SpotifyAudioBackend | null = null;
+  private started = false;
+  private startPromise: Promise<boolean> | null = null;
+
+  constructor(o: SpotifyControllerOptions) {
+    super();
+    this.config = o.config;
+    this.workDir = o.workDir;
+    this.configDir = o.configDir;
+    this.logger = o.logger;
+    this.instanceId = o.instanceId;
+    this.apiPort = o.apiPort;
+    this.callbackPort = o.callbackPort;
+    this.injectedFactory = o.backendFactory;
+    // The controller OWNS a shared OAuth + Connect pair (Task 6 web router and
+    // the Rust backend reuse these exact instances). Constructing the defaults
+    // performs no IO/network — the file store loads lazily on first use.
+    this.oauth =
+      o.oauth ??
+      new SpotifyOAuth({
+        store: new FileOAuthTokenStore(
+          join(this.configDir, "spotify-oauth.json"),
+        ),
+      });
+    this.connect =
+      o.connect ??
+      new SpotifyConnectApi(() => this.oauth.getAccessToken(), {
+        logger: this.logger,
+      });
+  }
+
+  /** Shared OAuth client (web router + Rust backend reuse this instance). */
+  getOAuth(): SpotifyOAuth {
+    return this.oauth;
+  }
+
+  /** Shared Connect API client (Rust backend reuses this instance). */
+  getConnect(): SpotifyConnectApi {
+    return this.connect;
+  }
+
+  private goPresent(): boolean {
+    // PATH-aware, sync presence gate (Bug I3): a bare PATH-installed binary is
+    // resolved against $PATH, not existsSync()'d against process.cwd().
+    return isGoLibrespotPresent();
+  }
+
+  private rustPresent(): boolean {
+    return isLibrespotPresent();
+  }
+
+  /**
+   * Resolve which backend to run for this platform + config, or null when none
+   * is usable (caller falls back to the Stage-1 sentinel message):
+   *   "go-librespot" -> GoLibrespot iff supported (linux) + binary present
+   *   "librespot"    -> Rust iff librespot(.exe) present (all platforms)
+   *   "auto"         -> GoLibrespot when (linux + go binary), else Rust when
+   *                     librespot present, else null.
+   */
+  chooseBackend(): SpotifyBackendKind | null {
+    return resolveSpotifyBackendKind(
+      this.config.backend,
+      this.goPresent(),
+      this.rustPresent(),
+    );
+  }
+
+  /** enabled in config AND a backend is selectable (platform + binary present). */
+  isAvailable(): boolean {
+    return this.config.enabled && this.chooseBackend() !== null;
+  }
+
+  /** Build the concrete backend for the chosen kind (or the injected fake). */
+  private buildBackend(kind: SpotifyBackendKind): SpotifyAudioBackend {
+    if (this.injectedFactory) return this.injectedFactory();
+    // Compute the effective (per-bot-unique) Connect device name ONCE and pass
+    // the SAME value to whichever backend we build, so `librespot --name`,
+    // findDeviceByName(), and waitForDevice() all key on one identity — that
+    // consistency is what prevents the multi-bot misroute / false-readiness
+    // (corner-case R2-5). The user-configured base (config.deviceName) is left
+    // untouched; the suffix is applied only to the backend/Connect identity.
+    const deviceName = perBotDeviceName(this.config.deviceName, this.instanceId);
+    if (kind === "librespot") {
+      return new RustLibrespotBackend({
+        deviceName,
+        bitrate: this.config.bitrate,
+        cacheDir: join(this.workDir, "librespot-cache"),
+        oauth: this.oauth,
+        connect: this.connect,
+        logger: this.logger,
+      });
+    }
+    return new GoLibrespotBackend({
+      deviceName,
+      bitrate: this.config.bitrate,
+      workDir: this.workDir,
+      configDir: this.configDir,
+      apiPort: this.apiPort,
+      callbackPort: this.callbackPort,
+      logger: this.logger,
+    });
+  }
+
+  /**
+   * Idempotently start the selected backend. Returns false (without building a
+   * backend) when unavailable, or — for the Rust backend — when OAuth is not
+   * yet authorized, so callers show the login-needed / fallback message. A
+   * failed start clears the cached promise so a later call can retry.
+   */
+  async ensureStarted(): Promise<boolean> {
+    if (!this.isAvailable()) return false;
+    const kind = this.chooseBackend();
+    if (!kind) return false;
+    // The Rust librespot device only appears in Spotify Connect once the user
+    // has authorized OAuth; without it, do not spawn a dead sidecar.
+    if (kind === "librespot" && !this.oauth.isAuthorized()) return false;
+
+    if (this.started) {
+      if (this.backend?.isReady()) return true;
+      this.stop();
+    }
+    if (this.startPromise) return this.startPromise;
+
+    this.startPromise = (async () => {
+      const backend = this.buildBackend(kind);
+      // Publish the in-flight backend BEFORE the (potentially ~20s) start()
+      // await so a concurrent stop()/handleBackendError() can reach and tear
+      // down this mid-start sidecar (Bug I1).
+      this.pendingBackend = backend;
+      backend.on("trackEnded", (e: SpotifyTrackEndedEvent) =>
+        this.emit("trackEnded", e),
+      );
+      backend.on("metadata", (m: SpotifyNowPlaying) =>
+        this.emit("metadata", m),
+      );
+      // C3: do NOT re-emit "error". Log and mark not-ready so the next
+      // ensureStarted() relaunches a fresh backend.
+      backend.on("error", (err?: unknown) => this.handleBackendError(err));
+      try {
+        await backend.start();
+      } catch (err) {
+        this.logger.error({ err }, "Spotify backend failed to start");
+        if (this.pendingBackend === backend) this.pendingBackend = null;
+        this.startPromise = null;
+        return false;
+      }
+      // Post-await guard (Bug I1): if teardown ran DURING start() — stop() or
+      // handleBackendError() cleared/replaced pendingBackend — do NOT promote
+      // this backend. Tear the just-started sidecar down so it is neither
+      // leaked nor resurrected, and report failure to the caller.
+      if (this.pendingBackend !== backend) {
+        this.teardownBackend(
+          backend,
+          "Spotify backend stop() threw tearing down a superseded start",
+        );
+        return false;
+      }
+      this.pendingBackend = null;
+      this.backend = backend;
+      this.started = true;
+      return true;
+    })();
+    return this.startPromise;
+  }
+
+  /**
+   * Stop a backend and detach ALL its listeners, swallowing+logging any throw
+   * from stop() so teardown never propagates. Shared by the error/stop paths
+   * and the ensureStarted post-await guard.
+   */
+  private teardownBackend(be: SpotifyAudioBackend, stopMsg: string): void {
+    try {
+      be.stop();
+    } catch (stopErr) {
+      this.logger.error({ err: stopErr }, stopMsg);
+    }
+    (be as unknown as EventEmitter).removeAllListeners();
+  }
+
+  /**
+   * Tear down an in-flight (mid-start) backend so a start() still awaiting is
+   * discarded by ensureStarted's post-await guard rather than promoted, and its
+   * spawned sidecar is killed rather than orphaned (Bug I1).
+   */
+  private teardownPendingBackend(): void {
+    if (!this.pendingBackend) return;
+    this.teardownBackend(
+      this.pendingBackend,
+      "Spotify backend stop() threw tearing down in-flight start",
+    );
+    this.pendingBackend = null;
+  }
+
+  /**
+   * C3 backend-error handler. Never re-emits "error" (an unhandled "error" on
+   * an EventEmitter throws). Logs, tears the errored backend down, and marks
+   * the controller not-ready so the next ensureStarted() relaunches it.
+   */
+  private handleBackendError(err: unknown): void {
+    this.logger.error({ err }, "Spotify backend error; marking not-ready");
+    if (this.backend) {
+      this.teardownBackend(
+        this.backend,
+        "Spotify backend stop() threw during error teardown",
+      );
+    }
+    this.backend = null;
+    // Also kill an in-flight start so its post-await guard discards it.
+    this.teardownPendingBackend();
+    this.started = false;
+    this.startPromise = null;
+  }
+
+  /** Ensure started, then play the spotify: URI. False on any failure. */
+  async playTrack(uri: string): Promise<boolean> {
+    const ok = await this.ensureStarted();
+    if (!ok || !this.backend) return false;
+    try {
+      await this.backend.playTrack(uri);
+      return true;
+    } catch (err) {
+      this.logger.error({ err, uri }, "Spotify playTrack failed");
+      return false;
+    }
+  }
+
+  async pause(): Promise<void> {
+    if (this.backend) await this.backend.pause();
+  }
+
+  async resume(): Promise<void> {
+    if (this.backend) await this.backend.resume();
+  }
+
+  async seek(ms: number): Promise<void> {
+    // R4-1: round to an integer ms ONCE here so BOTH backends receive a valid
+    // integer position. The web progress bar computes seekTime = ratio *
+    // duration (fractional seconds), so seek(seconds * 1000) is a NON-integer ms
+    // (e.g. 71610.00000000001). go-librespot decodes `position` into an int64
+    // and Go's encoding/json rejects a JSON number with a decimal point → HTTP
+    // 400 → the error is swallowed → the track never seeks. The Spotify Web API
+    // (Rust path) likewise expects an integer position_ms. Clamp negatives to 0.
+    const position = Math.max(0, Math.round(ms));
+    if (this.backend) await this.backend.seek(position);
+  }
+
+  getPcmStream(): Readable {
+    if (!this.backend) {
+      throw new Error("Spotify backend not started");
+    }
+    return this.backend.getPcmStream();
+  }
+
+  /**
+   * Tear down the backend and reset lifecycle state (safe before start).
+   * Mirrors handleBackendError's teardown so the NEXT ensureStarted() rebuilds
+   * a fresh backend.
+   */
+  stop(): void {
+    if (this.backend) {
+      this.teardownBackend(
+        this.backend,
+        "Spotify backend stop() threw during teardown",
+      );
+    }
+    this.backend = null;
+    // Also kill an in-flight start so its post-await guard discards it rather
+    // than resurrecting the sidecar this stop() just tore down (Bug I1).
+    this.teardownPendingBackend();
+    this.started = false;
+    this.startPromise = null;
+  }
+}

src/music/spotify/go-librespot-api.test.ts

@@ -0,0 +1,299 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { EventEmitter } from "node:events";
+import type { AxiosInstance } from "axios";
+import { GoLibrespotRestClient, GoLibrespotEventClient } from "./go-librespot-api.js";
+
+/** Minimal axios stub: only get/post are exercised by the client. */
+function makeHttp(overrides?: Partial<Record<"get" | "post", any>>) {
+  return {
+    get: vi.fn().mockResolvedValue({ status: 200, data: {} }),
+    post: vi.fn().mockResolvedValue({ status: 200, data: {} }),
+    ...overrides,
+  } as unknown as AxiosInstance;
+}
+
+describe("GoLibrespotRestClient", () => {
+  it("ping() returns true on GET / -> 200", async () => {
+    const http = makeHttp();
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    await expect(client.ping()).resolves.toBe(true);
+    expect(http.get).toHaveBeenCalledWith("/");
+  });
+
+  it("ping() returns false when GET / rejects (daemon not up)", async () => {
+    const http = makeHttp({ get: vi.fn().mockRejectedValue(new Error("ECONNREFUSED")) });
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    await expect(client.ping()).resolves.toBe(false);
+  });
+
+  it("playTrack() POSTs /player/play with the uri body", async () => {
+    const http = makeHttp();
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    await client.playTrack("spotify:track:abc123");
+    expect(http.post).toHaveBeenCalledWith("/player/play", { uri: "spotify:track:abc123" });
+  });
+
+  it("pause/resume/stop POST their bodyless endpoints", async () => {
+    const http = makeHttp();
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    await client.pause();
+    await client.resume();
+    await client.stop();
+    expect(http.post).toHaveBeenNthCalledWith(1, "/player/pause");
+    expect(http.post).toHaveBeenNthCalledWith(2, "/player/resume");
+    expect(http.post).toHaveBeenNthCalledWith(3, "/player/stop");
+  });
+
+  it("seek() POSTs /player/seek with position(ms) and relative:false", async () => {
+    const http = makeHttp();
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    await client.seek(42000);
+    expect(http.post).toHaveBeenCalledWith("/player/seek", { position: 42000, relative: false });
+  });
+
+  it("playTrack() rejects when the POST fails (surfaced to caller)", async () => {
+    const http = makeHttp({ post: vi.fn().mockRejectedValue(new Error("boom")) });
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    await expect(client.playTrack("spotify:track:x")).rejects.toThrow("boom");
+  });
+
+  it("getStatus() normalizes the /status shape (ms position/duration)", async () => {
+    const http = makeHttp({
+      get: vi.fn().mockResolvedValue({
+        status: 200,
+        data: {
+          stopped: false,
+          paused: false,
+          buffering: false,
+          track: {
+            uri: "spotify:track:abc",
+            name: "Song",
+            artist_names: ["A", "B"],
+            album_name: "Alb",
+            album_cover_url: "https://i.scdn.co/c.jpg",
+            position: 12345,
+            duration: 200000,
+          },
+        },
+      }),
+    });
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    const status = await client.getStatus();
+    expect(http.get).toHaveBeenCalledWith("/status");
+    expect(status).toEqual({
+      stopped: false,
+      paused: false,
+      buffering: false,
+      track: {
+        uri: "spotify:track:abc",
+        name: "Song",
+        artist_names: ["A", "B"],
+        album_name: "Alb",
+        album_cover_url: "https://i.scdn.co/c.jpg",
+        position: 12345,
+        duration: 200000,
+      },
+    });
+  });
+
+  it("getStatus() returns null with a null track when nothing is loaded", async () => {
+    const http = makeHttp({ get: vi.fn().mockResolvedValue({ status: 200, data: { stopped: true, paused: false, buffering: false, track: null } }) });
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    const status = await client.getStatus();
+    expect(status).toEqual({ stopped: true, paused: false, buffering: false, track: null });
+  });
+
+  it("getStatus() returns null when GET /status rejects", async () => {
+    const http = makeHttp({ get: vi.fn().mockRejectedValue(new Error("down")) });
+    const client = new GoLibrespotRestClient("http://127.0.0.1:3678", { http });
+    await expect(client.getStatus()).resolves.toBeNull();
+  });
+});
+
+/** Fake ws: records instances, lets tests drive open/message/close/error. */
+class FakeWebSocket extends EventEmitter {
+  static instances: FakeWebSocket[] = [];
+  closed = false;
+  constructor(public url: string) {
+    super();
+    FakeWebSocket.instances.push(this);
+  }
+  close() {
+    this.closed = true;
+    this.emit("close");
+  }
+}
+
+function frame(type: string, data: unknown): Buffer {
+  return Buffer.from(JSON.stringify({ type, data }));
+}
+
+describe("GoLibrespotEventClient", () => {
+  beforeEach(() => {
+    FakeWebSocket.instances = [];
+  });
+
+  it("emits 'not_playing' (track-end) with its data payload", () => {
+    const client = new GoLibrespotEventClient("ws://127.0.0.1:3678/events", {
+      WebSocketCtor: FakeWebSocket as any,
+    });
+    const onEnded = vi.fn();
+    client.on("not_playing", onEnded);
+    client.start();
+
+    const ws = FakeWebSocket.instances[0];
+    expect(ws.url).toBe("ws://127.0.0.1:3678/events");
+    ws.emit("message", frame("not_playing", { uri: "spotify:track:abc", play_origin: "go-librespot" }));
+
+    expect(onEnded).toHaveBeenCalledTimes(1);
+    expect(onEnded).toHaveBeenCalledWith({ uri: "spotify:track:abc", play_origin: "go-librespot" });
+    client.stop();
+  });
+
+  it("emits 'metadata' with the now-playing object", () => {
+    const client = new GoLibrespotEventClient("ws://127.0.0.1:3678/events", {
+      WebSocketCtor: FakeWebSocket as any,
+    });
+    const onMeta = vi.fn();
+    client.on("metadata", onMeta);
+    client.start();
+
+    FakeWebSocket.instances[0].emit(
+      "message",
+      frame("metadata", { uri: "spotify:track:xyz", name: "Song", artist_names: ["Q"], duration: 200000 }),
+    );
+
+    expect(onMeta).toHaveBeenCalledWith({ uri: "spotify:track:xyz", name: "Song", artist_names: ["Q"], duration: 200000 });
+    client.stop();
+  });
+
+  it("ignores non-JSON frames without throwing", () => {
+    const client = new GoLibrespotEventClient("ws://x/events", { WebSocketCtor: FakeWebSocket as any });
+    const onAny = vi.fn();
+    client.on("metadata", onAny);
+    client.start();
+    expect(() => FakeWebSocket.instances[0].emit("message", Buffer.from("not json"))).not.toThrow();
+    expect(onAny).not.toHaveBeenCalled();
+    client.stop();
+  });
+
+  it("reconnects with backoff after the socket closes", () => {
+    vi.useFakeTimers();
+    try {
+      const client = new GoLibrespotEventClient("ws://x/events", { WebSocketCtor: FakeWebSocket as any });
+      client.start();
+      expect(FakeWebSocket.instances).toHaveLength(1);
+
+      FakeWebSocket.instances[0].emit("close");
+      expect(FakeWebSocket.instances).toHaveLength(1); // not immediate
+      vi.advanceTimersByTime(500);
+      expect(FakeWebSocket.instances).toHaveLength(2); // reconnected
+      client.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("stop() closes the socket and prevents reconnect", () => {
+    vi.useFakeTimers();
+    try {
+      const client = new GoLibrespotEventClient("ws://x/events", { WebSocketCtor: FakeWebSocket as any });
+      client.start();
+      const ws = FakeWebSocket.instances[0];
+      client.stop();
+      expect(ws.closed).toBe(true);
+      vi.advanceTimersByTime(60000);
+      expect(FakeWebSocket.instances).toHaveLength(1); // no new socket
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("does not throw on socket 'error' when no error listener is attached", () => {
+    const client = new GoLibrespotEventClient("ws://x/events", { WebSocketCtor: FakeWebSocket as any });
+    client.start();
+    expect(() => FakeWebSocket.instances[0].emit("error", new Error("net"))).not.toThrow();
+    client.stop();
+  });
+
+  // R4-3: a WS drop at a track boundary can lose the not_playing/stopped event.
+  // After a successful RE-open the client emits "reconnected" so the backend can
+  // re-query GET /status and reconcile. The FIRST connect must NOT emit it (there
+  // is nothing to reconcile yet).
+  it("emits 'reconnected' after a reconnect open, but NOT on the initial connect", () => {
+    vi.useFakeTimers();
+    try {
+      const client = new GoLibrespotEventClient("ws://x/events", { WebSocketCtor: FakeWebSocket as any });
+      const onReconnected = vi.fn();
+      client.on("reconnected", onReconnected);
+      client.start();
+
+      FakeWebSocket.instances[0].emit("open"); // initial connect
+      expect(onReconnected).not.toHaveBeenCalled(); // no re-sync on first connect
+
+      FakeWebSocket.instances[0].emit("close");
+      vi.advanceTimersByTime(500);
+      expect(FakeWebSocket.instances).toHaveLength(2); // reconnected socket
+      FakeWebSocket.instances[1].emit("open"); // reconnect open
+      expect(onReconnected).toHaveBeenCalledTimes(1);
+      client.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  // R4-5: a socket that is accepted then immediately closed (a flap) must let the
+  // exponential backoff GROW — the old code reset it to 500ms on every 'open',
+  // pinning reconnects at ~2 Hz.
+  it("grows the reconnect backoff across open→immediate-close flaps (no 500ms pin)", () => {
+    vi.useFakeTimers();
+    try {
+      const client = new GoLibrespotEventClient("ws://x/events", { WebSocketCtor: FakeWebSocket as any });
+      client.start();
+      // Flap #1: accept then immediately drop -> next reconnect at 500ms.
+      FakeWebSocket.instances[0].emit("open");
+      FakeWebSocket.instances[0].emit("close");
+      vi.advanceTimersByTime(500);
+      expect(FakeWebSocket.instances).toHaveLength(2);
+
+      // Flap #2: accept then immediately drop -> backoff has doubled to 1000ms.
+      FakeWebSocket.instances[1].emit("open");
+      FakeWebSocket.instances[1].emit("close");
+      vi.advanceTimersByTime(500);
+      expect(FakeWebSocket.instances).toHaveLength(2); // still 2: 500ms is NOT enough now
+      vi.advanceTimersByTime(500);
+      expect(FakeWebSocket.instances).toHaveLength(3); // reconnects only after 1000ms
+      client.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  // R4-5: once a connection has been STABLE (up for >= STABLE_CONNECTION_MS) the
+  // backoff resets, so a later drop reconnects promptly again.
+  it("resets the reconnect backoff after a stable connection", () => {
+    vi.useFakeTimers();
+    try {
+      const client = new GoLibrespotEventClient("ws://x/events", { WebSocketCtor: FakeWebSocket as any });
+      client.start();
+      // Flap once to grow the backoff to 1000ms.
+      FakeWebSocket.instances[0].emit("open");
+      FakeWebSocket.instances[0].emit("close");
+      vi.advanceTimersByTime(500);
+      expect(FakeWebSocket.instances).toHaveLength(2);
+
+      // Now a STABLE connection: open and stay up past the stability threshold.
+      FakeWebSocket.instances[1].emit("open");
+      vi.advanceTimersByTime(5000); // >= STABLE_CONNECTION_MS -> backoff reset to 500
+
+      FakeWebSocket.instances[1].emit("close");
+      vi.advanceTimersByTime(499);
+      expect(FakeWebSocket.instances).toHaveLength(2); // not yet
+      vi.advanceTimersByTime(1);
+      expect(FakeWebSocket.instances).toHaveLength(3); // reconnected at 500ms -> backoff was reset
+      client.stop();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+});

src/music/spotify/go-librespot-api.ts

@@ -0,0 +1,235 @@
+import { EventEmitter } from "node:events";
+import axios, { type AxiosInstance } from "axios";
+import WebSocket from "ws";
+
+export interface GoLibrespotStatusTrack {
+  uri: string;
+  name: string;
+  artist_names: string[];
+  album_name: string;
+  album_cover_url: string | null;
+  position: number;
+  duration: number;
+}
+
+export interface GoLibrespotStatus {
+  stopped: boolean;
+  paused: boolean;
+  buffering: boolean;
+  track: GoLibrespotStatusTrack | null;
+}
+
+export class GoLibrespotRestClient {
+  private http: AxiosInstance;
+
+  constructor(baseUrl: string, deps?: { http?: AxiosInstance }) {
+    this.http =
+      deps?.http ??
+      axios.create({
+        baseURL: baseUrl,
+        timeout: 10000,
+        headers: { "Content-Type": "application/json" },
+      });
+  }
+
+  async ping(): Promise<boolean> {
+    try {
+      const res = await this.http.get("/");
+      return res.status === 200;
+    } catch {
+      return false;
+    }
+  }
+
+  async playTrack(uri: string): Promise<void> {
+    await this.http.post("/player/play", { uri });
+  }
+
+  async pause(): Promise<void> {
+    await this.http.post("/player/pause");
+  }
+
+  async resume(): Promise<void> {
+    await this.http.post("/player/resume");
+  }
+
+  async stop(): Promise<void> {
+    await this.http.post("/player/stop");
+  }
+
+  async seek(ms: number): Promise<void> {
+    await this.http.post("/player/seek", { position: ms, relative: false });
+  }
+
+  async getStatus(): Promise<GoLibrespotStatus | null> {
+    try {
+      const res = await this.http.get("/status");
+      const d = res.data ?? {};
+      const t = d.track;
+      return {
+        stopped: Boolean(d.stopped),
+        paused: Boolean(d.paused),
+        buffering: Boolean(d.buffering),
+        track: t
+          ? {
+              uri: t.uri ?? "",
+              name: t.name ?? "",
+              artist_names: Array.isArray(t.artist_names) ? t.artist_names : [],
+              album_name: t.album_name ?? "",
+              album_cover_url: t.album_cover_url ?? null,
+              position: t.position ?? 0,
+              duration: t.duration ?? 0,
+            }
+          : null,
+      };
+    } catch {
+      return null;
+    }
+  }
+}
+
+export type GoLibrespotEventType =
+  | "metadata"
+  | "playing"
+  | "paused"
+  | "not_playing"
+  | "stopped"
+  | "will_play"
+  | "seek"
+  | "active"
+  | "inactive"
+  | "volume"
+  | "playback_ready";
+
+interface WsLike {
+  on(event: string, cb: (...args: any[]) => void): void;
+  close(): void;
+}
+type WebSocketCtor = new (url: string) => WsLike;
+
+const INITIAL_RECONNECT_MS = 500;
+const MAX_RECONNECT_MS = 10000;
+// R4-5: a connection must stay up at least this long before we treat it as
+// "stable" and reset the reconnect backoff. A socket that is accepted and then
+// immediately closed (a flap) never reaches this, so the exponential backoff
+// keeps growing instead of pinning the reconnect interval at INITIAL_RECONNECT_MS.
+const STABLE_CONNECTION_MS = 5000;
+
+/**
+ * Emitted (in addition to the go-librespot event types) after the socket has
+ * SUCCESSFULLY re-opened following a drop — never on the very first connect.
+ * Consumers use it to re-query GET /status and reconcile any track-end that was
+ * emitted by go-librespot during the WS-down window (R4-3).
+ */
+export type GoLibrespotSyntheticEvent = "reconnected";
+
+export class GoLibrespotEventClient extends EventEmitter {
+  private wsUrl: string;
+  private WebSocketCtor: WebSocketCtor;
+  private ws: WsLike | null = null;
+  private stopped = false;
+  private reconnectDelay = INITIAL_RECONNECT_MS;
+  private reconnectTimer: ReturnType<typeof setTimeout> | null = null;
+  // R4-3: false until the FIRST successful open. A later open is therefore a
+  // reconnect and warrants a "reconnected" re-sync signal.
+  private hasConnected = false;
+  // R4-5: fires STABLE_CONNECTION_MS after an open; only then is the backoff reset.
+  private stableTimer: ReturnType<typeof setTimeout> | null = null;
+
+  constructor(wsUrl: string, deps?: { WebSocketCtor?: WebSocketCtor }) {
+    super();
+    this.wsUrl = wsUrl;
+    this.WebSocketCtor = deps?.WebSocketCtor ?? (WebSocket as unknown as WebSocketCtor);
+  }
+
+  start(): void {
+    this.stopped = false;
+    this.connect();
+  }
+
+  stop(): void {
+    this.stopped = true;
+    this.clearStableTimer();
+    if (this.reconnectTimer) {
+      clearTimeout(this.reconnectTimer);
+      this.reconnectTimer = null;
+    }
+    if (this.ws) {
+      this.ws.close();
+      this.ws = null;
+    }
+  }
+
+  private connect(): void {
+    if (this.stopped) return;
+    const ws = new this.WebSocketCtor(this.wsUrl);
+    this.ws = ws;
+    ws.on("open", () => this.onOpen());
+    ws.on("message", (buf: unknown) => this.handleMessage(buf));
+    ws.on("close", () => {
+      this.ws = null;
+      // R4-5: the connection is gone — cancel the pending stability reset so a
+      // short-lived (flapping) socket never resets the backoff.
+      this.clearStableTimer();
+      this.scheduleReconnect();
+    });
+    ws.on("error", (err: unknown) => {
+      if (this.listenerCount("error") > 0) this.emit("error", err);
+    });
+  }
+
+  private onOpen(): void {
+    // R4-3: only a RE-open (a socket that had connected before, then dropped)
+    // needs reconciliation; the initial connect has nothing to catch up on.
+    const isReconnect = this.hasConnected;
+    this.hasConnected = true;
+    // R4-5: do NOT reset the backoff here. Arm a timer that resets it only once
+    // the connection has stayed up for STABLE_CONNECTION_MS; a flap that closes
+    // before then leaves the exponential backoff to keep growing.
+    this.armStableTimer();
+    if (isReconnect) this.emit("reconnected");
+  }
+
+  private armStableTimer(): void {
+    this.clearStableTimer();
+    this.stableTimer = setTimeout(() => {
+      this.stableTimer = null;
+      this.reconnectDelay = INITIAL_RECONNECT_MS;
+    }, STABLE_CONNECTION_MS);
+    (this.stableTimer as { unref?: () => void }).unref?.();
+  }
+
+  private clearStableTimer(): void {
+    if (this.stableTimer) {
+      clearTimeout(this.stableTimer);
+      this.stableTimer = null;
+    }
+  }
+
+  private handleMessage(buf: unknown): void {
+    let parsed: unknown;
+    try {
+      const text = Buffer.isBuffer(buf)
+        ? buf.toString("utf8")
+        : typeof buf === "string"
+          ? buf
+          : String(buf);
+      parsed = JSON.parse(text);
+    } catch {
+      return;
+    }
+    if (parsed && typeof (parsed as any).type === "string") {
+      this.emit((parsed as any).type, (parsed as any).data ?? {});
+    }
+  }
+
+  private scheduleReconnect(): void {
+    if (this.stopped || this.reconnectTimer) return;
+    const delay = this.reconnectDelay;
+    this.reconnectDelay = Math.min(delay * 2, MAX_RECONNECT_MS);
+    this.reconnectTimer = setTimeout(() => {
+      this.reconnectTimer = null;
+      this.connect();
+    }, delay);
+  }
+}

src/music/spotify/go-librespot-config.test.ts

@@ -0,0 +1,119 @@
+// src/music/spotify/go-librespot-config.test.ts
+import { describe, it, expect } from "vitest";
+import { renderConfigYml, type GoLibrespotConfigOptions } from "./go-librespot-config.js";
+
+const OPTS: GoLibrespotConfigOptions = {
+  deviceName: "TeamSpeak Music Bot",
+  bitrate: 320,
+  fifoPath: "/tmp/go-librespot.fifo",
+  apiAddress: "0.0.0.0",
+  apiPort: 3678,
+  callbackPort: 8080,
+};
+
+/**
+ * Minimal 2-level YAML reader for the exact shape renderConfigYml emits
+ * (flat scalars + one level of nesting under `server:` / `credentials:`).
+ * Avoids adding a yaml dependency while still proving the output round-trips.
+ */
+function parseTinyYaml(src: string): Record<string, unknown> {
+  const root: Record<string, unknown> = {};
+  const stack: Array<{ indent: number; obj: Record<string, unknown> }> = [
+    { indent: -1, obj: root },
+  ];
+  for (const rawLine of src.split("\n")) {
+    if (rawLine.trim() === "") continue;
+    const indent = rawLine.length - rawLine.trimStart().length;
+    const line = rawLine.trim();
+    const idx = line.indexOf(":");
+    const key = line.slice(0, idx).trim();
+    let valRaw = line.slice(idx + 1).trim();
+    while (stack.length > 1 && indent <= stack[stack.length - 1].indent) {
+      stack.pop();
+    }
+    const parent = stack[stack.length - 1].obj;
+    if (valRaw === "") {
+      const child: Record<string, unknown> = {};
+      parent[key] = child;
+      stack.push({ indent, obj: child });
+      continue;
+    }
+    let val: unknown = valRaw;
+    if (valRaw.startsWith('"') && valRaw.endsWith('"')) val = JSON.parse(valRaw);
+    else if (valRaw === "true") val = true;
+    else if (valRaw === "false") val = false;
+    else if (/^-?\d+$/.test(valRaw)) val = Number(valRaw);
+    parent[key] = val;
+  }
+  return root;
+}
+
+describe("renderConfigYml", () => {
+  it("emits the exact top-level go-librespot keys/values", () => {
+    const lines = renderConfigYml(OPTS).split("\n");
+    expect(lines).toContain('device_name: "TeamSpeak Music Bot"');
+    expect(lines).toContain("device_type: computer");
+    expect(lines).toContain("bitrate: 320");
+    expect(lines).toContain("audio_backend: pipe");
+    expect(lines).toContain("audio_output_pipe: /tmp/go-librespot.fifo");
+    expect(lines).toContain("audio_output_pipe_format: s16le");
+  });
+
+  it("emits a server block with enabled/address/port set explicitly", () => {
+    const parsed = parseTinyYaml(renderConfigYml(OPTS));
+    expect(parsed.server).toEqual({
+      enabled: true,
+      address: "0.0.0.0",
+      port: 3678,
+    });
+  });
+
+  it("emits interactive OAuth credentials with the callback port", () => {
+    const parsed = parseTinyYaml(renderConfigYml(OPTS));
+    expect(parsed.credentials).toEqual({
+      type: "interactive",
+      interactive: { callback_port: 8080 },
+    });
+  });
+
+  it("full round-trip reflects every provided option", () => {
+    const parsed = parseTinyYaml(renderConfigYml(OPTS));
+    expect(parsed).toEqual({
+      device_name: "TeamSpeak Music Bot",
+      device_type: "computer",
+      bitrate: 320,
+      audio_backend: "pipe",
+      audio_output_pipe: "/tmp/go-librespot.fifo",
+      audio_output_pipe_format: "s16le",
+      server: { enabled: true, address: "0.0.0.0", port: 3678 },
+      credentials: { type: "interactive", interactive: { callback_port: 8080 } },
+    });
+  });
+
+  it("threads distinct option values through unchanged (no hard-coded ports)", () => {
+    const parsed = parseTinyYaml(
+      renderConfigYml({
+        deviceName: "Other Bot",
+        bitrate: 160,
+        fifoPath: "/run/librespot/pipe",
+        apiAddress: "127.0.0.1",
+        apiPort: 4000,
+        callbackPort: 9099,
+      }),
+    );
+    expect(parsed).toMatchObject({
+      device_name: "Other Bot",
+      bitrate: 160,
+      audio_output_pipe: "/run/librespot/pipe",
+      server: { address: "127.0.0.1", port: 4000 },
+      credentials: { interactive: { callback_port: 9099 } },
+    });
+  });
+
+  it("safely quotes device names containing special characters", () => {
+    const yml = renderConfigYml({ ...OPTS, deviceName: 'My "Cool" Bot' });
+    expect(yml.split("\n")).toContain('device_name: "My \\"Cool\\" Bot"');
+    // and still round-trips back to the original string
+    expect(parseTinyYaml(yml).device_name).toBe('My "Cool" Bot');
+  });
+});

src/music/spotify/go-librespot-config.ts

@@ -0,0 +1,42 @@
+// src/music/spotify/go-librespot-config.ts
+// Hand-built go-librespot config.yml. Keys/values verified against
+// devgianlu/go-librespot cmd/daemon/cli_config.go koanf tags. No yaml
+// dependency is used; the file is a small, fixed-shape document.
+
+export interface GoLibrespotConfigOptions {
+  deviceName: string;
+  bitrate: number;
+  fifoPath: string;
+  apiAddress: string;
+  apiPort: number;
+  callbackPort: number;
+}
+
+/**
+ * Render a headless go-librespot config.yml:
+ *  - pipe audio backend writing raw 44.1kHz/s16le stereo PCM to a FIFO,
+ *  - HTTP+WebSocket control server enabled (port has NO built-in default,
+ *    so it is always written explicitly),
+ *  - interactive OAuth credentials (persisted automatically to
+ *    <config_dir>/credentials.json after first login).
+ */
+export function renderConfigYml(o: GoLibrespotConfigOptions): string {
+  return (
+    [
+      `device_name: ${JSON.stringify(o.deviceName)}`,
+      `device_type: computer`,
+      `bitrate: ${o.bitrate}`,
+      `audio_backend: pipe`,
+      `audio_output_pipe: ${o.fifoPath}`,
+      `audio_output_pipe_format: s16le`,
+      `server:`,
+      `  enabled: true`,
+      `  address: ${o.apiAddress}`,
+      `  port: ${o.apiPort}`,
+      `credentials:`,
+      `  type: interactive`,
+      `  interactive:`,
+      `    callback_port: ${o.callbackPort}`,
+    ].join("\n") + "\n"
+  );
+}

src/music/spotify/go-librespot.test.ts

@@ -0,0 +1,411 @@
+import { describe, it, expect, vi } from "vitest";
+import { EventEmitter } from "node:events";
+import { PassThrough } from "node:stream";
+import pino from "pino";
+import { GoLibrespotBackend } from "./go-librespot.js";
+
+const log = pino({ level: "silent" });
+
+/** A minimal stand-in for a spawned ChildProcess with real Readable stdout/stderr. */
+function makeFakeChild() {
+  const child: any = new EventEmitter();
+  child.stdout = new PassThrough();
+  child.stderr = new PassThrough();
+  child.kill = vi.fn();
+  return child;
+}
+
+function makeHarness(portOpts: { apiPort?: number; callbackPort?: number } = {}) {
+  const calls: string[] = [];
+  const ffmpegChild = makeFakeChild();
+  const gliChild = makeFakeChild();
+
+  const spawn = vi.fn((cmd: string, ..._rest: any[]) => {
+    const isGli = cmd.includes("go-librespot");
+    calls.push(`spawn:${isGli ? "go-librespot" : cmd}`);
+    return isGli ? gliChild : ffmpegChild;
+  });
+  const execFileSync = vi.fn((cmd: string) => {
+    calls.push(`exec:${cmd}`);
+    return Buffer.from("");
+  });
+  const writeFileSync = vi.fn(() => calls.push("write:config"));
+  const mkdirSync = vi.fn();
+  const unlinkSync = vi.fn(() => calls.push("unlink:fifo"));
+  const existsSync = vi.fn(() => false);
+
+  const rest = {
+    ping: vi.fn(async () => true),
+    playTrack: vi.fn(async () => {}),
+    pause: vi.fn(async () => {}),
+    resume: vi.fn(async () => {}),
+    stop: vi.fn(async () => {}),
+    seek: vi.fn(async () => {}),
+    getStatus: vi.fn(async () => null),
+  };
+  const events: any = new EventEmitter();
+  events.start = vi.fn(() => calls.push("ws:start"));
+  events.stop = vi.fn();
+
+  const backend = new GoLibrespotBackend({
+    deviceName: "Test Bot",
+    bitrate: 320,
+    workDir: "/tmp/work",
+    configDir: "/tmp/cfg",
+    apiPort: portOpts.apiPort ?? 3678,
+    callbackPort: portOpts.callbackPort,
+    logger: log,
+    deps: {
+      spawn,
+      execFileSync,
+      writeFileSync,
+      mkdirSync,
+      unlinkSync,
+      existsSync,
+      // C1: pin the ffmpeg command so the arg-array/order assertions below stay
+      // stable while production resolves ffmpeg via getFfmpegCommand() (which
+      // falls back to bundled ffmpeg-static when `ffmpeg` isn't on PATH).
+      ffmpegCommand: "ffmpeg",
+      findBinary: () => "/bin/go-librespot",
+      makeRest: () => rest,
+      makeEvents: () => events,
+      sleep: async () => {},
+      pollIntervalMs: 1,
+      pollTimeoutMs: 100,
+    } as any,
+  });
+
+  return { backend, calls, spawn, execFileSync, writeFileSync, existsSync, unlinkSync, rest, events, ffmpegChild, gliChild };
+}
+
+describe("GoLibrespotBackend.start", () => {
+  it("creates the FIFO with mkfifo before spawning ffmpeg, and spawns ffmpeg BEFORE go-librespot", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+
+    expect(h.execFileSync).toHaveBeenCalledWith("mkfifo", ["/tmp/work/go-librespot.fifo"]);
+    expect(h.writeFileSync).toHaveBeenCalled();
+
+    const mkfifoIdx = h.calls.indexOf("exec:mkfifo");
+    const ffmpegIdx = h.calls.indexOf("spawn:ffmpeg");
+    const gliIdx = h.calls.indexOf("spawn:go-librespot");
+    expect(mkfifoIdx).toBeGreaterThanOrEqual(0);
+    expect(ffmpegIdx).toBeGreaterThan(mkfifoIdx); // ffmpeg attaches to the FIFO first
+    expect(gliIdx).toBeGreaterThan(ffmpegIdx);     // then the writer (go-librespot)
+    expect(h.calls.indexOf("ws:start")).toBeGreaterThan(gliIdx); // WS connects last
+  });
+
+  it("passes --config_dir to go-librespot using the resolved binary path", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.spawn).toHaveBeenCalledWith(
+      "/bin/go-librespot",
+      ["--config_dir", "/tmp/cfg"],
+      expect.anything(),
+    );
+  });
+
+  it("uses the 44100->48000 s16le ffmpeg command reading the FIFO", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const ffmpegArgs = h.spawn.mock.calls.find((c) => c[0] === "ffmpeg")![1] as string[];
+    expect(ffmpegArgs).toEqual([
+      "-hide_banner", "-loglevel", "error",
+      "-f", "s16le", "-ar", "44100", "-ac", "2", "-i", "/tmp/work/go-librespot.fifo",
+      "-f", "s16le", "-ar", "48000", "-ac", "2", "-acodec", "pcm_s16le", "pipe:1",
+    ]);
+  });
+
+  it("emits 'ready' and reports isReady() true once the REST ping succeeds", async () => {
+    const h = makeHarness();
+    const ready = vi.fn();
+    h.backend.on("ready", ready);
+    await h.backend.start();
+    expect(h.rest.ping).toHaveBeenCalled();
+    expect(ready).toHaveBeenCalledTimes(1);
+    expect(h.backend.isReady()).toBe(true);
+  });
+
+  it("keeps polling ping() until it returns true", async () => {
+    const h = makeHarness();
+    h.rest.ping.mockResolvedValueOnce(false).mockResolvedValueOnce(false).mockResolvedValue(true);
+    await h.backend.start();
+    expect(h.rest.ping).toHaveBeenCalledTimes(3);
+    expect(h.backend.isReady()).toBe(true);
+  });
+});
+
+describe("GoLibrespotBackend config binding (Fix 1 loopback + Fix 3 ports)", () => {
+  function writtenYml(h: ReturnType<typeof makeHarness>): string {
+    const calls = h.writeFileSync.mock.calls as unknown as any[][];
+    const call = calls.find((c) => String(c[0]).endsWith("config.yml"));
+    expect(call).toBeDefined();
+    return call![1] as string;
+  }
+
+  it("binds the control API to loopback (127.0.0.1), never 0.0.0.0", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const yml = writtenYml(h);
+    expect(yml).toContain("address: 127.0.0.1");
+    expect(yml).not.toContain("0.0.0.0");
+  });
+
+  it("threads apiPort + callbackPort into the rendered config", async () => {
+    const h = makeHarness({ apiPort: 3712, callbackPort: 8712 });
+    await h.backend.start();
+    const yml = writtenYml(h);
+    expect(yml).toContain("port: 3712");
+    expect(yml).toContain("callback_port: 8712");
+  });
+
+  it("defaults callbackPort to 8080 when unset", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(writtenYml(h)).toContain("callback_port: 8080");
+  });
+});
+
+describe("GoLibrespotBackend WebSocket event mapping", () => {
+  it("maps a not_playing event to trackEnded{reason:'ended'}", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.events.emit("not_playing", { uri: "spotify:track:abc" });
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:abc", reason: "ended" });
+  });
+
+  it("maps a stopped event to trackEnded{reason:'stopped'}", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.events.emit("stopped", { uri: "spotify:track:xyz" });
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:xyz", reason: "stopped" });
+  });
+
+  it("maps a metadata event to a SpotifyNowPlaying and updates getPositionMs()", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const meta = vi.fn();
+    h.backend.on("metadata", meta);
+    h.events.emit("metadata", {
+      uri: "spotify:track:abc",
+      name: "Song",
+      artist_names: ["A", "B"],
+      album_name: "Alb",
+      album_cover_url: "http://x/y.jpg",
+      position: 1234,
+      duration: 200000,
+    });
+    expect(meta).toHaveBeenCalledWith({
+      uri: "spotify:track:abc",
+      name: "Song",
+      artist: "A, B",
+      album: "Alb",
+      coverUrl: "http://x/y.jpg",
+      durationMs: 200000,
+    });
+    expect(h.backend.getPositionMs()).toBe(1234);
+  });
+});
+
+describe("GoLibrespotBackend transport delegation + PCM", () => {
+  it("playTrack delegates to the REST client", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:go");
+    expect(h.rest.playTrack).toHaveBeenCalledWith("spotify:track:go");
+  });
+
+  it("pause/resume/seek delegate to the REST client and seek updates position", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.pause();
+    await h.backend.resume();
+    await h.backend.seek(5000);
+    expect(h.rest.pause).toHaveBeenCalled();
+    expect(h.rest.resume).toHaveBeenCalled();
+    expect(h.rest.seek).toHaveBeenCalledWith(5000);
+    expect(h.backend.getPositionMs()).toBe(5000);
+  });
+
+  it("getPcmStream() returns the ffmpeg stdout Readable", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.backend.getPcmStream()).toBe(h.ffmpegChild.stdout);
+  });
+});
+
+describe("GoLibrespotBackend.stop", () => {
+  it("kills ffmpeg + go-librespot, stops the WS, removes the FIFO, and clears ready", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    h.existsSync.mockReturnValue(true); // FIFO now present, so stop() unlinks it
+    h.backend.stop();
+    expect(h.ffmpegChild.kill).toHaveBeenCalled();
+    expect(h.gliChild.kill).toHaveBeenCalled();
+    expect(h.events.stop).toHaveBeenCalled();
+    expect(h.unlinkSync).toHaveBeenCalledWith("/tmp/work/go-librespot.fifo");
+    expect(h.backend.isReady()).toBe(false);
+  });
+});
+
+describe("GoLibrespotBackend.start failure cleanup", () => {
+  it("tears down ffmpeg, go-librespot, and the FIFO when readiness polling never succeeds", async () => {
+    const h = makeHarness();
+    // ping() never returns true → waitUntilReady() times out → start() rejects
+    // AFTER both processes were spawned and the FIFO was created.
+    h.rest.ping.mockResolvedValue(false);
+    // FIFO present so the cleanup path unlinks it.
+    h.existsSync.mockReturnValue(true);
+
+    await expect(h.backend.start()).rejects.toThrow(/did not become ready/);
+
+    expect(h.ffmpegChild.kill).toHaveBeenCalled(); // ffmpeg killed on failed startup
+    expect(h.gliChild.kill).toHaveBeenCalled(); // go-librespot killed on failed startup
+    expect(h.unlinkSync).toHaveBeenCalledWith("/tmp/work/go-librespot.fifo"); // FIFO removed
+    expect(h.backend.isReady()).toBe(false);
+  });
+});
+
+describe("GoLibrespotBackend child-process error handling", () => {
+  it("does not throw when a child 'error' is emitted with no backend 'error' listener attached", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    // No "error" listener on the backend: an unhandled 'error' event would crash
+    // Node, so the backend must swallow+log it instead of re-emitting.
+    expect(h.backend.listenerCount("error")).toBe(0);
+    expect(() => h.ffmpegChild.emit("error", new Error("ffmpeg boom"))).not.toThrow();
+    expect(() => h.gliChild.emit("error", new Error("gli boom"))).not.toThrow();
+  });
+
+  it("re-emits a child 'error' to an attached backend 'error' listener", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const onErr = vi.fn();
+    h.backend.on("error", onErr);
+    const err = new Error("ffmpeg boom");
+    h.ffmpegChild.emit("error", err);
+    expect(onErr).toHaveBeenCalledWith(err);
+  });
+});
+
+// Let queued microtasks (the async reconnect re-sync) settle.
+const flush = () => new Promise<void>((r) => setTimeout(r, 0));
+
+describe("GoLibrespotBackend R4-2: unexpected sidecar exit recovery", () => {
+  it("degrades to trackEnded{reason:'error'}, surfaces 'error', and stops the WS on an UNEXPECTED exit", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:cur"); // sets the current uri
+
+    const ended = vi.fn();
+    const onErr = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.backend.on("error", onErr);
+
+    // Sidecar dies under us (nonzero exit, no stop() from us).
+    h.gliChild.emit("exit", 1, null);
+
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:cur", reason: "error" });
+    expect(onErr).toHaveBeenCalledTimes(1); // controller told -> rebuilds on next ensureStarted
+    expect(h.events.stop).toHaveBeenCalled(); // WS reconnect loop stopped (no dead-port hammer)
+    expect(h.backend.isReady()).toBe(false);
+  });
+
+  it("emits trackEnded BEFORE error so a listener that tears down still receives the skip", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:cur");
+
+    const order: string[] = [];
+    h.backend.on("trackEnded", () => order.push("trackEnded"));
+    h.backend.on("error", () => order.push("error"));
+    h.gliChild.emit("exit", null, "SIGKILL");
+
+    expect(order).toEqual(["trackEnded", "error"]);
+  });
+
+  it("a stop()-initiated exit emits NEITHER trackEnded NOR error", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:cur");
+
+    const ended = vi.fn();
+    const onErr = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.backend.on("error", onErr);
+
+    h.backend.stop(); // intentional teardown -> the ensuing 'exit' must be silent
+    h.gliChild.emit("exit", 0, "SIGTERM");
+
+    expect(ended).not.toHaveBeenCalled();
+    expect(onErr).not.toHaveBeenCalled();
+  });
+});
+
+describe("GoLibrespotBackend R4-3: WS reconnect re-sync", () => {
+  const notPlaying = { stopped: true, paused: false, buffering: false, track: null };
+  const stillPlaying = {
+    stopped: false,
+    paused: false,
+    buffering: false,
+    track: {
+      uri: "spotify:track:cur",
+      name: "Song",
+      artist_names: ["A"],
+      album_name: "Alb",
+      album_cover_url: null,
+      position: 1000,
+      duration: 200000,
+    },
+  };
+
+  it("re-queries GET /status on reconnect and emits trackEnded when playback is no longer active", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:cur");
+    h.rest.getStatus.mockResolvedValue(notPlaying as any);
+
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.events.emit("reconnected");
+    await flush();
+
+    expect(h.rest.getStatus).toHaveBeenCalled();
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:cur", reason: "ended" });
+  });
+
+  it("does NOT emit a spurious trackEnded on reconnect when status shows still-playing", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:cur");
+    h.rest.getStatus.mockResolvedValue(stillPlaying as any);
+
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.events.emit("reconnected");
+    await flush();
+
+    expect(h.rest.getStatus).toHaveBeenCalled();
+    expect(ended).not.toHaveBeenCalled();
+  });
+
+  it("re-sync is idempotent: a reconnect then a live not_playing emits trackEnded only once", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    await h.backend.playTrack("spotify:track:cur");
+    h.rest.getStatus.mockResolvedValue(notPlaying as any);
+
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.events.emit("reconnected");
+    await flush();
+    // A duplicate live event for the same track must not double-advance the queue.
+    h.events.emit("not_playing", { uri: "spotify:track:cur" });
+
+    expect(ended).toHaveBeenCalledTimes(1);
+  });
+});

src/music/spotify/go-librespot.ts

@@ -0,0 +1,402 @@
+import { EventEmitter } from "node:events";
+// The go-librespot sidecar is Linux/Docker-gated (mkfifo + Linux-only binary),
+// so the FIFO and config-dir paths are ALWAYS POSIX. Use posix.join so the
+// separators are correct on the Linux target regardless of the host OS.
+import { posix as posixPath } from "node:path";
+import type { Readable } from "node:stream";
+import type { ChildProcess } from "node:child_process";
+import {
+  execFileSync as realExecFileSync,
+  spawn as realSpawn,
+} from "node:child_process";
+import {
+  existsSync as realExistsSync,
+  mkdirSync as realMkdirSync,
+  unlinkSync as realUnlinkSync,
+  writeFileSync as realWriteFileSync,
+} from "node:fs";
+import type { Logger } from "pino";
+import type {
+  SpotifyAudioBackend,
+  SpotifyTrackEndedEvent,
+  SpotifyNowPlaying,
+} from "./backend.js";
+import { findGoLibrespot } from "./binary.js";
+import { renderConfigYml } from "./go-librespot-config.js";
+import { GoLibrespotRestClient, GoLibrespotEventClient } from "./go-librespot-api.js";
+import { getFfmpegCommand } from "../../audio/player.js";
+
+export interface GoLibrespotBackendOptions {
+  deviceName: string;
+  bitrate: number;
+  workDir: string;
+  configDir: string;
+  apiPort?: number;
+  callbackPort?: number;
+  logger: Logger;
+  deps?: GoLibrespotBackendDeps;
+}
+
+/** Injectable seams so the whole lifecycle is testable without a real binary/FIFO/network. */
+export interface GoLibrespotBackendDeps {
+  spawn?: typeof realSpawn;
+  execFileSync?: typeof realExecFileSync;
+  existsSync?: typeof realExistsSync;
+  mkdirSync?: typeof realMkdirSync;
+  unlinkSync?: typeof realUnlinkSync;
+  writeFileSync?: typeof realWriteFileSync;
+  findBinary?: () => string;
+  /**
+   * C1: override the ffmpeg command. Production resolves it via
+   * getFfmpegCommand() (bundled ffmpeg-static fallback when `ffmpeg` isn't on
+   * PATH, the Docker case); tests pin it to "ffmpeg" for stable arg assertions.
+   */
+  ffmpegCommand?: string;
+  makeRest?: (baseUrl: string) => GoLibrespotRestClient;
+  makeEvents?: (wsUrl: string) => GoLibrespotEventClient;
+  sleep?: (ms: number) => Promise<void>;
+  pollIntervalMs?: number;
+  pollTimeoutMs?: number;
+}
+
+const DEFAULT_API_PORT = 3678;
+const DEFAULT_CALLBACK_PORT = 8080;
+const FIFO_NAME = "go-librespot.fifo";
+
+export class GoLibrespotBackend extends EventEmitter implements SpotifyAudioBackend {
+  private readonly opts: GoLibrespotBackendOptions;
+  private readonly log: Logger;
+  private readonly deps: GoLibrespotBackendDeps;
+  private readonly apiPort: number;
+  private readonly callbackPort: number;
+  private readonly fifoPath: string;
+
+  private ffmpeg: ChildProcess | null = null;
+  private proc: ChildProcess | null = null;
+  private rest: GoLibrespotRestClient | null = null;
+  private events: GoLibrespotEventClient | null = null;
+  private ready = false;
+  private positionMs = 0;
+  // R4-2: distinguishes an INTENTIONAL teardown (stop()/failed start()) — where a
+  // sidecar exit is expected and must be silent — from an UNEXPECTED death that
+  // must degrade-to-skip + surface an error so the controller relaunches.
+  private stopping = false;
+  // The uri of the track currently loaded in the sidecar (set by playTrack and
+  // refreshed from metadata). Used as the trackEnded uri on an unexpected death
+  // (R4-2) and on reconnect reconciliation (R4-3).
+  private currentUri = "";
+  // At-most-one trackEnded per track. Set when we emit a track-end, cleared when
+  // a new track begins. Keeps the reconnect re-sync (R4-3) from double-emitting
+  // with a live not_playing/stopped event.
+  private endLatched = false;
+
+  constructor(o: GoLibrespotBackendOptions) {
+    super();
+    this.opts = o;
+    this.log = o.logger;
+    this.deps = o.deps ?? {};
+    this.apiPort = o.apiPort ?? DEFAULT_API_PORT;
+    this.callbackPort = o.callbackPort ?? DEFAULT_CALLBACK_PORT;
+    this.fifoPath = posixPath.join(o.workDir, FIFO_NAME);
+  }
+
+  async start(): Promise<void> {
+    // Fresh lifecycle (also covers a start() after a previous stop() on the same
+    // instance): clear the teardown flag and per-track end state.
+    this.stopping = false;
+    this.currentUri = "";
+    this.endLatched = false;
+    const spawn = this.deps.spawn ?? realSpawn;
+    const execFileSync = this.deps.execFileSync ?? realExecFileSync;
+    const existsSync = this.deps.existsSync ?? realExistsSync;
+    const mkdirSync = this.deps.mkdirSync ?? realMkdirSync;
+    const unlinkSync = this.deps.unlinkSync ?? realUnlinkSync;
+    const writeFileSync = this.deps.writeFileSync ?? realWriteFileSync;
+    const findBinary = this.deps.findBinary ?? findGoLibrespot;
+    // C1: resolve ffmpeg via the repo's getFfmpegCommand() (ffmpeg-static
+    // fallback) unless a command is injected for tests.
+    const ffmpegCommand = this.deps.ffmpegCommand ?? getFfmpegCommand();
+
+    // 1. Ensure work + config directories exist.
+    mkdirSync(this.opts.workDir, { recursive: true });
+    mkdirSync(this.opts.configDir, { recursive: true });
+
+    // 2. (Re)create the FIFO — mkfifo fails if the path already exists.
+    if (existsSync(this.fifoPath)) unlinkSync(this.fifoPath);
+    execFileSync("mkfifo", [this.fifoPath]);
+
+    // Everything past this point spawns processes / opens sockets. If any step
+    // throws (e.g. the readiness poll times out), tear down whatever was already
+    // created via stop() (kill ffmpeg + go-librespot, close WS, remove FIFO) so
+    // we don't leak child processes or leave the FIFO on disk, then rethrow.
+    try {
+      // 3. Spawn ffmpeg FIRST so the PCM reader is attached to the FIFO before
+      //    go-librespot (the writer) starts pushing raw 44.1k s16le into it.
+      //    Opening the FIFO for writing before a reader exists errors with ENXIO.
+      this.ffmpeg = spawn(
+        ffmpegCommand,
+        [
+          "-hide_banner", "-loglevel", "error",
+          "-f", "s16le", "-ar", "44100", "-ac", "2", "-i", this.fifoPath,
+          "-f", "s16le", "-ar", "48000", "-ac", "2", "-acodec", "pcm_s16le", "pipe:1",
+        ],
+        { stdio: ["ignore", "pipe", "pipe"] },
+      );
+      this.ffmpeg.stderr?.on("data", (b: Buffer) =>
+        this.log.debug({ ffmpeg: b.toString().trim() }, "ffmpeg"),
+      );
+      this.ffmpeg.on("error", (err) => this.emitError(err));
+
+      // 4. Render + write config.yml into the config dir.
+      const yml = renderConfigYml({
+        deviceName: this.opts.deviceName,
+        bitrate: this.opts.bitrate,
+        fifoPath: this.fifoPath,
+        // The go-librespot control API is UNAUTHENTICATED and the client only
+        // ever connects via 127.0.0.1; the sidecar runs in the SAME container
+        // as the bot, so bind to loopback rather than exposing it on 0.0.0.0.
+        apiAddress: "127.0.0.1",
+        apiPort: this.apiPort,
+        callbackPort: this.callbackPort,
+      });
+      writeFileSync(posixPath.join(this.opts.configDir, "config.yml"), yml, "utf8");
+
+      // 5. Spawn go-librespot AFTER ffmpeg is listening on the FIFO. Its stdout/
+      //    stderr carry the interactive OAuth URL on first run — surface via logger.
+      const bin = findBinary();
+      this.proc = spawn(bin, ["--config_dir", this.opts.configDir], {
+        stdio: ["ignore", "pipe", "pipe"],
+      });
+      const onLog = (b: Buffer) => this.log.info({ golibrespot: b.toString().trim() }, "go-librespot");
+      this.proc.stdout?.on("data", onLog);
+      this.proc.stderr?.on("data", onLog);
+      this.proc.on("error", (err) => this.emitError(err));
+      this.proc.on("exit", (code, signal) => {
+        this.ready = false;
+        this.log.warn({ code, signal }, "go-librespot exited");
+        // R4-2: a stop()-initiated exit is expected — stay silent. Any other exit
+        // is the sidecar dying under us and must be recovered.
+        if (this.stopping) return;
+        this.onUnexpectedExit(code, signal);
+      });
+
+      // 6. REST client, then poll GET / until the HTTP server answers.
+      const baseUrl = `http://127.0.0.1:${this.apiPort}`;
+      this.rest = this.deps.makeRest
+        ? this.deps.makeRest(baseUrl)
+        : new GoLibrespotRestClient(baseUrl);
+      await this.waitUntilReady();
+
+      // 7. Connect the WebSocket event stream and wire event mapping.
+      const wsUrl = `ws://127.0.0.1:${this.apiPort}/events`;
+      this.events = this.deps.makeEvents
+        ? this.deps.makeEvents(wsUrl)
+        : new GoLibrespotEventClient(wsUrl);
+      this.wireEvents(this.events);
+      this.events.start();
+
+      this.ready = true;
+      this.emit("ready");
+    } catch (e) {
+      this.stop();
+      throw e;
+    }
+  }
+
+  /**
+   * Re-emit a child-process "error" only when a consumer is listening; Node
+   * throws on an unhandled "error" event (can crash the process), so with no
+   * listener we log via the injected logger instead. Mirrors the WS client's
+   * listenerCount("error") gate in go-librespot-api.ts.
+   */
+  private emitError(err: unknown): void {
+    if (this.listenerCount("error") > 0) {
+      this.emit("error", err);
+    } else {
+      this.log.error({ err }, "go-librespot backend error (no listener)");
+    }
+  }
+
+  /**
+   * At-most-one track-end per track. The WS not_playing/stopped events, the
+   * unexpected-death degrade (R4-2) and the reconnect re-sync (R4-3) all funnel
+   * through here so a track can only advance the queue once.
+   */
+  private emitTrackEnded(e: SpotifyTrackEndedEvent): void {
+    if (this.endLatched) return;
+    this.endLatched = true;
+    this.emit("trackEnded", e);
+  }
+
+  /**
+   * R4-2: the go-librespot sidecar died without a stop() from us. Spotify
+   * auto-advance is driven ONLY by the WS trackEnded event (the player-side stall
+   * watchdog is disabled in external mode), so a silent death would stall the
+   * queue forever. Mirror the Rust I4 degrade-to-skip:
+   *  (a) emit trackEnded{reason:"error"} so BotInstance advances the queue;
+   *  (b) stop the WS reconnect loop so it stops hammering the now-dead API port;
+   *  (c) surface via emitError so the controller tears down + relaunches a fresh
+   *      backend on the next ensureStarted().
+   * trackEnded MUST be emitted BEFORE emitError: the controller's error handler
+   * removeAllListeners() on teardown, so a trackEnded emitted after would be lost.
+   */
+  private onUnexpectedExit(code: number | null, signal: NodeJS.Signals | null): void {
+    // (b) Kill the reconnect loop first — the API port is dead.
+    try {
+      this.events?.stop();
+    } catch {
+      /* ignore */
+    }
+    this.events = null;
+    // (a) Degrade-to-skip only if a track was actually loaded; a death during
+    // startup with nothing playing has no queue item to advance.
+    if (this.currentUri) {
+      this.emitTrackEnded({ uri: this.currentUri, reason: "error" });
+    }
+    // (c) Surface so the controller rebuilds.
+    this.emitError(
+      new Error(
+        `go-librespot exited unexpectedly (code=${code ?? "null"}, signal=${signal ?? "null"})`,
+      ),
+    );
+  }
+
+  /**
+   * R4-3: the WS reconnected after a drop. go-librespot only pushes events (it is
+   * never polled after startup), so a not_playing/stopped emitted during the
+   * down window is lost and the queue would stall. Re-query GET /status and, if
+   * playback is no longer active (the track ended in the gap), emit trackEnded so
+   * the queue advances. Idempotent via the end-latch (no double-emit with a live
+   * event). Only fired on a real reconnect — never the initial connect.
+   */
+  private async reconcileAfterReconnect(): Promise<void> {
+    const rest = this.rest;
+    if (!rest) return;
+    const status = await rest.getStatus();
+    // Couldn't read status — don't guess a track-end.
+    if (!status) return;
+    const active = status.track != null && !status.stopped;
+    if (!active) {
+      this.emitTrackEnded({ uri: this.currentUri, reason: "ended" });
+    }
+  }
+
+  private async waitUntilReady(): Promise<void> {
+    const sleep = this.deps.sleep ?? ((ms: number) => new Promise<void>((r) => setTimeout(r, ms)));
+    const interval = this.deps.pollIntervalMs ?? 200;
+    const timeout = this.deps.pollTimeoutMs ?? 15_000;
+    const deadline = Date.now() + timeout;
+    while (Date.now() < deadline) {
+      if (this.rest && (await this.rest.ping())) return;
+      await sleep(interval);
+    }
+    throw new Error("go-librespot API did not become ready within timeout");
+  }
+
+  private wireEvents(ev: GoLibrespotEventClient): void {
+    ev.on("metadata", (d: any) => {
+      const np: SpotifyNowPlaying = {
+        uri: typeof d?.uri === "string" ? d.uri : "",
+        name: typeof d?.name === "string" ? d.name : "",
+        artist: Array.isArray(d?.artist_names) ? d.artist_names.join(", ") : "",
+        album: typeof d?.album_name === "string" ? d.album_name : "",
+        coverUrl: typeof d?.album_cover_url === "string" ? d.album_cover_url : "",
+        durationMs: typeof d?.duration === "number" ? d.duration : 0,
+      };
+      if (typeof d?.position === "number") this.positionMs = d.position;
+      // A new track is now playing: remember it (for R4-2/R4-3) and clear the
+      // per-track end-latch so its eventual end can advance the queue.
+      if (np.uri && np.uri !== this.currentUri) {
+        this.currentUri = np.uri;
+        this.endLatched = false;
+      }
+      this.emit("metadata", np);
+    });
+    ev.on("seek", (d: any) => {
+      if (typeof d?.position === "number") this.positionMs = d.position;
+    });
+    ev.on("not_playing", (d: any) => {
+      this.emitTrackEnded({ uri: typeof d?.uri === "string" ? d.uri : "", reason: "ended" });
+    });
+    ev.on("stopped", (d: any) => {
+      this.emitTrackEnded({ uri: typeof d?.uri === "string" ? d.uri : "", reason: "stopped" });
+    });
+    // R4-3: reconcile any track-end missed while the WS was down.
+    ev.on("reconnected", () => {
+      void this.reconcileAfterReconnect();
+    });
+  }
+
+  isReady(): boolean {
+    return this.ready;
+  }
+
+  async playTrack(uri: string): Promise<void> {
+    if (!this.rest) throw new Error("go-librespot backend not started");
+    // Track what is loaded so an unexpected death (R4-2) / reconnect re-sync
+    // (R4-3) can advance the queue for the right uri, and reset the end-latch.
+    this.currentUri = uri;
+    this.endLatched = false;
+    await this.rest.playTrack(uri);
+  }
+
+  async pause(): Promise<void> {
+    if (this.rest) await this.rest.pause();
+  }
+
+  async resume(): Promise<void> {
+    if (this.rest) await this.rest.resume();
+  }
+
+  async seek(ms: number): Promise<void> {
+    if (this.rest) await this.rest.seek(ms);
+    this.positionMs = ms;
+  }
+
+  getPcmStream(): Readable {
+    const out = this.ffmpeg?.stdout;
+    if (!out) throw new Error("PCM stream unavailable (go-librespot backend not started)");
+    return out;
+  }
+
+  getPositionMs(): number {
+    return this.positionMs;
+  }
+
+  stop(): void {
+    // R4-2: mark this an INTENTIONAL teardown so the go-librespot 'exit' handler
+    // (fired by the SIGTERM below) stays silent instead of degrading-to-skip.
+    this.stopping = true;
+    this.ready = false;
+    try {
+      this.events?.stop();
+    } catch {
+      /* ignore */
+    }
+    this.events = null;
+    this.rest = null;
+    if (this.proc) {
+      try {
+        this.proc.kill("SIGTERM");
+      } catch {
+        /* ignore */
+      }
+      this.proc = null;
+    }
+    if (this.ffmpeg) {
+      try {
+        this.ffmpeg.kill("SIGTERM");
+      } catch {
+        /* ignore */
+      }
+      this.ffmpeg = null;
+    }
+    const existsSync = this.deps.existsSync ?? realExistsSync;
+    const unlinkSync = this.deps.unlinkSync ?? realUnlinkSync;
+    try {
+      if (existsSync(this.fifoPath)) unlinkSync(this.fifoPath);
+    } catch {
+      /* ignore */
+    }
+  }
+}

src/music/spotify/rust-librespot.test.ts

@@ -0,0 +1,814 @@
+import { describe, it, expect, vi } from "vitest";
+import { EventEmitter } from "node:events";
+import { PassThrough } from "node:stream";
+import pino from "pino";
+import { RustLibrespotBackend, type RustLibrespotBackendDeps } from "./rust-librespot.js";
+
+const log = pino({ level: "silent" });
+
+/** ChildProcess stand-in with real Readable/Writable pipes so stdout->stdin piping works. */
+function makeFakeChild() {
+  const child: any = new EventEmitter();
+  child.stdout = new PassThrough();
+  child.stderr = new PassThrough();
+  child.stdin = new PassThrough();
+  child.kill = vi.fn();
+  return child;
+}
+
+function makeConnect() {
+  return {
+    getDevices: vi.fn(async () => [{ id: "dev1", name: "Test Bot", is_active: false }]),
+    findDeviceByName: vi.fn(async () => "dev1"),
+    transfer: vi.fn(async () => {}),
+    play: vi.fn(async () => {}),
+    pause: vi.fn(async () => {}),
+    resume: vi.fn(async () => {}),
+    seek: vi.fn(async () => {}),
+    getPlaybackState: vi.fn(async () => null as any),
+  };
+}
+
+function makeOAuth() {
+  return {
+    getAccessToken: vi.fn(async () => "tok-123" as string | null),
+    isAuthorized: () => true,
+  };
+}
+
+function makeHarness(
+  over: { connect?: any; oauth?: any; deps?: Partial<RustLibrespotBackendDeps> } = {},
+) {
+  const calls: string[] = [];
+  const librespotChild = makeFakeChild();
+  const ffmpegChild = makeFakeChild();
+
+  const spawn = vi.fn((cmd: string, ..._rest: any[]) => {
+    const isLibrespot = cmd.includes("librespot");
+    calls.push(`spawn:${isLibrespot ? "librespot" : cmd}`);
+    return isLibrespot ? librespotChild : ffmpegChild;
+  });
+  const mkdirSync = vi.fn();
+  const connect = over.connect ?? makeConnect();
+  const oauth = over.oauth ?? makeOAuth();
+
+  const backend = new RustLibrespotBackend({
+    deviceName: "Test Bot",
+    bitrate: 320,
+    cacheDir: "/tmp/cache",
+    oauth: oauth as any,
+    connect: connect as any,
+    logger: log,
+    deps: {
+      spawn: spawn as any,
+      mkdirSync: mkdirSync as any,
+      findBinary: () => "/bin/librespot",
+      // C1: pin ffmpeg so arg-array assertions stay stable while prod uses getFfmpegCommand().
+      ffmpegCommand: "ffmpeg",
+      sleep: async () => {},
+      readyPollIntervalMs: 1,
+      readyTimeoutMs: 100,
+      // huge so the background setInterval never fires; tests drive pollState() directly.
+      statePollIntervalMs: 10_000_000,
+      ...over.deps,
+    },
+  });
+
+  return { backend, calls, spawn, mkdirSync, connect, oauth, librespotChild, ffmpegChild };
+}
+
+describe("RustLibrespotBackend.start", () => {
+  it("spawns librespot with the pipe/stdout arg set and the OAuth access token", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.spawn).toHaveBeenCalledWith(
+      "/bin/librespot",
+      [
+        "--name", "Test Bot",
+        "--backend", "pipe",
+        "--bitrate", "320",
+        "--format", "S16",
+        "--cache", "/tmp/cache",
+        "--device-type", "speaker",
+        "--access-token", "tok-123",
+      ],
+      expect.anything(),
+    );
+    // NO --device (=> stdout) and NO --passthrough (=> decoded PCM, not Ogg).
+    const args = h.spawn.mock.calls.find((c) => String(c[0]).includes("librespot"))![1] as string[];
+    expect(args).not.toContain("--device");
+    expect(args).not.toContain("--passthrough");
+    h.backend.stop();
+  });
+
+  it("spawns ffmpeg (reader) before librespot (writer) with the exact 44100->48000 s16le args", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const ffmpegArgs = h.spawn.mock.calls.find((c) => c[0] === "ffmpeg")![1] as string[];
+    expect(ffmpegArgs).toEqual([
+      "-f", "s16le", "-ar", "44100", "-ac", "2", "-i", "pipe:0",
+      "-f", "s16le", "-ar", "48000", "-ac", "2", "-acodec", "pcm_s16le", "pipe:1",
+    ]);
+    const ffmpegIdx = h.calls.indexOf("spawn:ffmpeg");
+    const librespotIdx = h.calls.indexOf("spawn:librespot");
+    expect(ffmpegIdx).toBeGreaterThanOrEqual(0);
+    expect(librespotIdx).toBeGreaterThan(ffmpegIdx);
+    h.backend.stop();
+  });
+
+  it("getPcmStream() returns the ffmpeg stdout Readable", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.backend.getPcmStream()).toBe(h.ffmpegChild.stdout);
+    h.backend.stop();
+  });
+
+  it("emits 'ready' and reports isReady() true once our device appears in getDevices()", async () => {
+    const h = makeHarness();
+    const ready = vi.fn();
+    h.backend.on("ready", ready);
+    await h.backend.start();
+    expect(h.connect.getDevices).toHaveBeenCalled();
+    expect(ready).toHaveBeenCalledTimes(1);
+    expect(h.backend.isReady()).toBe(true);
+    h.backend.stop();
+  });
+
+  it("keeps polling getDevices() until the device name appears", async () => {
+    const h = makeHarness();
+    h.connect.getDevices
+      .mockResolvedValueOnce([])
+      .mockResolvedValueOnce([{ id: "other", name: "Someone else", is_active: true }])
+      .mockResolvedValue([{ id: "dev1", name: "Test Bot", is_active: false }]);
+    await h.backend.start();
+    expect(h.connect.getDevices).toHaveBeenCalledTimes(3);
+    expect(h.backend.isReady()).toBe(true);
+    h.backend.stop();
+  });
+
+  it("throws (and does not spawn) when the OAuth token is null", async () => {
+    const oauth = makeOAuth();
+    oauth.getAccessToken.mockResolvedValue(null);
+    const h = makeHarness({ oauth });
+    await expect(h.backend.start()).rejects.toThrow(/authorized|token/i);
+    expect(h.spawn).not.toHaveBeenCalled();
+  });
+});
+
+describe("RustLibrespotBackend transport delegation (Connect API)", () => {
+  it("playTrack resolves the device then transfer(false) then play(uri)", async () => {
+    const h = makeHarness();
+    await h.backend.playTrack("spotify:track:go");
+    expect(h.connect.findDeviceByName).toHaveBeenCalledWith("Test Bot");
+    expect(h.connect.transfer).toHaveBeenCalledWith("dev1", false);
+    expect(h.connect.play).toHaveBeenCalledWith("dev1", "spotify:track:go");
+    // ordering: transfer before play
+    expect(h.connect.transfer.mock.invocationCallOrder[0])
+      .toBeLessThan(h.connect.play.mock.invocationCallOrder[0]);
+  });
+
+  it("playTrack throws when the device cannot be found", async () => {
+    const h = makeHarness();
+    h.connect.findDeviceByName.mockResolvedValue(null);
+    await expect(h.backend.playTrack("spotify:track:x")).rejects.toThrow(/device/i);
+  });
+
+  // R4-4 (multi-bot): control must be scoped to OUR device. playTrack resolves
+  // and stores our device id (dev1); pause/resume/seek then pass it to the
+  // Connect API so bot A's pause/resume/seek can't act on bot B's playback (the
+  // account-wide default would pause whatever device is currently active).
+  it("pause/resume/seek delegate to the Connect API scoped to OUR device, and seek updates position", async () => {
+    const h = makeHarness();
+    await h.backend.playTrack("spotify:track:go"); // stores our device id (dev1)
+    await h.backend.pause();
+    await h.backend.resume();
+    await h.backend.seek(5000);
+    expect(h.connect.pause).toHaveBeenCalledWith("dev1");
+    expect(h.connect.resume).toHaveBeenCalledWith("dev1");
+    expect(h.connect.seek).toHaveBeenCalledWith(5000, "dev1");
+    expect(h.backend.getPositionMs()).toBe(5000);
+  });
+});
+
+describe("RustLibrespotBackend track-end poll loop", () => {
+  it("emits trackEnded when progress reaches the end-of-track window", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    const meta = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.backend.on("metadata", meta);
+    // Arm detection the way production does — via our own playTrack().
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 1000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 199000, trackUri: "spotify:track:A", durationMs: 200000 });
+    await (h.backend as any).pollState();
+    expect(meta).toHaveBeenCalledWith(expect.objectContaining({ uri: "spotify:track:A", durationMs: 200000 }));
+    expect(h.backend.getPositionMs()).toBe(1000);
+    expect(ended).not.toHaveBeenCalled();
+    await (h.backend as any).pollState();
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  // C1(pause-skip): a USER pause reports is_playing:false with the SAME uri on
+  // the Rust backend. That MUST NOT be read as a track end (it would skip the
+  // paused track and break pause + occupancy auto-pause). Formerly the
+  // "!isPlaying after having played" test asserted the opposite — that encoded
+  // the bug; it is now split into this pause-no-skip test plus the two-poll
+  // external-stop test below.
+  it("does NOT emit trackEnded when the user PAUSES (self-initiated pause is not a track end)", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    // Observe our track actually playing first.
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    // User pauses: the Connect device stays loaded but reports is_playing:false
+    // with the SAME uri across every subsequent poll while paused.
+    await h.backend.pause();
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: false, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    await (h.backend as any).pollState(); // stays paused across multiple polls
+    expect(ended).not.toHaveBeenCalled();
+    // Resuming keeps the same track playing — still no spurious end.
+    await h.backend.resume();
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: true, progressMs: 6000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    expect(ended).not.toHaveBeenCalled();
+  });
+
+  it("emits trackEnded once on an EXTERNAL stop only after TWO consecutive !isPlaying polls (a transient mid-track !isPlaying is not a skip)", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValueOnce({ isPlaying: false, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValue({ isPlaying: false, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000 });
+    await (h.backend as any).pollState(); // observed playing
+    await (h.backend as any).pollState(); // FIRST !isPlaying -> unconfirmed (could be transient buffering)
+    expect(ended).not.toHaveBeenCalled();
+    await (h.backend as any).pollState(); // SECOND consecutive !isPlaying -> confirmed external stop
+    await (h.backend as any).pollState(); // idempotent: no second emit for same track
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  it("a transient single !isPlaying poll followed by playing again does NOT emit trackEnded (buffering hiccup)", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValueOnce({ isPlaying: false, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValue({ isPlaying: true, progressMs: 6000, trackUri: "spotify:track:A", durationMs: 200000 });
+    await (h.backend as any).pollState(); // playing
+    await (h.backend as any).pollState(); // momentary !isPlaying (buffering)
+    await (h.backend as any).pollState(); // playing again -> stop confirmation reset
+    expect(ended).not.toHaveBeenCalled();
+  });
+
+  // m(sub-window): a track SHORTER than the end-of-track window must not be
+  // declared finished on its first observed-playing poll (durationMs - window
+  // is negative, so the old near-end check fired unconditionally).
+  it("does NOT false-finish a sub-window (< END_OF_TRACK_WINDOW_MS) duration on the first playing poll", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:short");
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: true, progressMs: 100, trackUri: "spotify:track:short", durationMs: 1200,
+    });
+    await (h.backend as any).pollState();
+    expect(ended).not.toHaveBeenCalled();
+  });
+
+  // C1(pause-skip) residual: a self-initiated pause within the FINAL
+  // END_OF_TRACK_WINDOW_MS freezes progress at >= dur-window with is_playing:false
+  // and the SAME uri. The finishedByProgress near-end heuristic must NOT fire
+  // while paused (it would skip the paused track). After resume(), the track
+  // plays on and finishes exactly once.
+  it("does NOT skip when the user PAUSES within the final end-of-track window, but ends once after resume", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    // Observe our track actually playing first (mid-track).
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    // User pauses within the final ~1.5s: frozen progress >= dur-window,
+    // is_playing:false, SAME uri, across every subsequent paused poll.
+    await h.backend.pause();
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: false, progressMs: 199000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    await (h.backend as any).pollState(); // stays paused across multiple polls
+    expect(ended).not.toHaveBeenCalled();
+    // Resume -> the track plays on to its natural end and finishes exactly once.
+    await h.backend.resume();
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: true, progressMs: 199000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  // C1(pause-skip) residual: during a LONG self-initiated pause the Connect
+  // device can idle out to a 204 / null playback state. That null state while
+  // paused must NOT be read as a track end (it would skip the paused track).
+  // After resume() a genuinely dead device (persistent null) IS detected.
+  it("does NOT skip a PAUSED track when the device idles out to null/204, but DOES after resume", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    // Observe our track actually playing first.
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    // Pause, then the Connect device idles out to a 204 / null state.
+    await h.backend.pause();
+    h.connect.getPlaybackState.mockResolvedValue(null as any);
+    await (h.backend as any).pollState();
+    await (h.backend as any).pollState(); // stays paused across multiple null polls
+    expect(ended).not.toHaveBeenCalled();
+    // Resume -> a genuinely dead device (persistent null) is now detected once.
+    await h.backend.resume();
+    await (h.backend as any).pollState();
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  // Regression: a NON-paused track that idles out to a null/204 state after
+  // having played must STILL emit exactly one trackEnded (the pause gate must
+  // not suppress genuine device death for a non-paused track).
+  it("regression: a NON-paused track that idles out to null/204 after playing still emits exactly one trackEnded", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState(); // observed playing, not paused
+    h.connect.getPlaybackState.mockResolvedValue(null as any); // device idles out
+    await (h.backend as any).pollState();
+    await (h.backend as any).pollState(); // idempotent: no second emit
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  // R3-1: a GENUINE end where the item stays null across TWO consecutive
+  // non-paused polls still emits exactly one trackEnded. The null-item path now
+  // shares the external-stop two-poll confirmation, so a single transient null
+  // (Connect handoff / market relink) no longer skips a still-playing track —
+  // but a persistent null is still detected. (Previously this test asserted a
+  // single null poll ended the track; that encoded the R3-1 corner-case bug.)
+  it("emits trackEnded once when the track uri stays null across TWO consecutive polls after playing", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 1000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValue({ isPlaying: true, progressMs: 0, trackUri: null, durationMs: 0 });
+    await (h.backend as any).pollState(); // observed playing our uri
+    await (h.backend as any).pollState(); // FIRST null item -> unconfirmed (could be transient)
+    expect(ended).not.toHaveBeenCalled();
+    await (h.backend as any).pollState(); // SECOND consecutive null -> confirmed end
+    await (h.backend as any).pollState(); // idempotent: no second emit for same track
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  // R3-1: Spotify legitimately returns item:null transiently at a track/Connect
+  // handoff boundary, and getPlaybackState omits the `market` param so a
+  // region-relinked/restricted item can momentarily map to uri:null. A SINGLE
+  // {isPlaying:true, trackUri:null} poll mid-track must NOT skip a still-playing
+  // track; a following poll showing our real uri again confirms it kept playing.
+  it("a transient single null-item poll (isPlaying:true, trackUri:null) followed by our uri again does NOT emit trackEnded", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 0, trackUri: null, durationMs: 0 })
+      .mockResolvedValue({ isPlaying: true, progressMs: 6000, trackUri: "spotify:track:A", durationMs: 200000 });
+    await (h.backend as any).pollState(); // playing our uri
+    await (h.backend as any).pollState(); // momentary null item (handoff / market relink)
+    await (h.backend as any).pollState(); // our uri again -> confirmation reset, still playing
+    expect(ended).not.toHaveBeenCalled();
+  });
+
+  // R3-1 (pause invariant): a self-paused track must NEVER skip. A {trackUri:null}
+  // poll while WE hold a pause must not be read as a track end (same invariant
+  // the finishedByStop / null-204 paths already enforce with a `!paused` guard).
+  it("does NOT emit trackEnded on a null-item poll while the track is self-paused", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState(); // observed playing
+    await h.backend.pause();
+    // While paused, a null item appears across multiple polls (state present, no item).
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: false, progressMs: 5000, trackUri: null, durationMs: 0,
+    });
+    await (h.backend as any).pollState();
+    await (h.backend as any).pollState();
+    expect(ended).not.toHaveBeenCalled();
+  });
+
+  it("ignores a null playback state (no active device) without emitting", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.connect.getPlaybackState.mockResolvedValue(null);
+    await (h.backend as any).pollState();
+    expect(ended).not.toHaveBeenCalled();
+  });
+
+  it("C3.4: a startup poll before playTrack never emits (foreign track near its end)", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    const meta = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.backend.on("metadata", meta);
+    // Backend started, but playTrack has NOT been called => detection disarmed.
+    await h.backend.start();
+    // First poll observes a FOREIGN track that is actively playing near its end.
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: true,
+      progressMs: 199000,
+      trackUri: "spotify:foreign",
+      durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    // No spurious end-of-track and no bogus metadata before the bot ever plays.
+    expect(ended).not.toHaveBeenCalled();
+    expect(meta).not.toHaveBeenCalled();
+    expect(h.backend.getPositionMs()).toBe(0);
+    h.backend.stop();
+  });
+
+  it("after playTrack, a normal finish emits trackEnded exactly once for our uri", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    // Arm detection via our own play, then confirm-then-finish our uri.
+    await h.backend.playTrack("spotify:track:ours");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 1000, trackUri: "spotify:track:ours", durationMs: 200000 })
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 199000, trackUri: "spotify:track:ours", durationMs: 200000 });
+    await (h.backend as any).pollState(); // confirms our uri playing
+    expect(ended).not.toHaveBeenCalled();
+    await (h.backend as any).pollState(); // finishes
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:ours", reason: "ended" });
+  });
+});
+
+// R4-4 (multi-bot): config.spotify (and thus the Connect session) is shared by
+// every bot under one Premium account, which supports only ONE active playback
+// stream. GET /v1/me/player is account-wide, so once bot B steals the active
+// session our poll would see B's device + B's track. The backend now stores OUR
+// device id and, when the reported activeDeviceId differs, refuses to treat the
+// foreign playback as ours: no foreign metadata, no misattribution — the stolen
+// session instead advances OUR queue cleanly via the existing two-poll stop.
+describe("RustLibrespotBackend multi-bot device scoping (R4-4)", () => {
+  it("ignores foreign-device poll state: no foreign metadata, no misattribution, and a stolen session advances OUR queue once", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    const meta = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.backend.on("metadata", meta);
+    // Our track plays on OUR device (dev1 — the findDeviceByName mock id).
+    await h.backend.playTrack("spotify:track:ours");
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:ours",
+      durationMs: 200000, activeDeviceId: "dev1",
+    });
+    await (h.backend as any).pollState(); // our track observed playing on our device
+    expect(meta).toHaveBeenCalledTimes(1);
+    expect(meta).toHaveBeenCalledWith(expect.objectContaining({ uri: "spotify:track:ours" }));
+    expect(h.backend.getPositionMs()).toBe(5000);
+    meta.mockClear();
+    // Bot B steals the single active Connect session: /v1/me/player now reports
+    // THEIR device (dev2) + THEIR track, and would keep doing so every poll.
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: true, progressMs: 123000, trackUri: "spotify:track:foreign",
+      durationMs: 200000, activeDeviceId: "dev2",
+    });
+    await (h.backend as any).pollState(); // FIRST foreign poll -> unconfirmed stop, no side effects
+    expect(meta).not.toHaveBeenCalled();            // foreign metadata NOT surfaced as ours
+    expect(ended).not.toHaveBeenCalled();            // two-poll confirmation not met yet
+    expect(h.backend.getPositionMs()).toBe(5000);    // foreign progress NOT misattributed
+    await (h.backend as any).pollState(); // SECOND foreign poll -> confirmed -> OUR queue advances
+    await (h.backend as any).pollState(); // idempotent: no second emit for our track
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:ours", reason: "ended" });
+    expect(meta).not.toHaveBeenCalled();             // never emitted metadata for the foreign uri
+  });
+
+  it("when the active device IS ours (activeDeviceId === our id), end-detection behaves exactly as today", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 1000, trackUri: "spotify:track:A", durationMs: 200000, activeDeviceId: "dev1" })
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 199000, trackUri: "spotify:track:A", durationMs: 200000, activeDeviceId: "dev1" });
+    await (h.backend as any).pollState(); // confirms our uri playing on our device
+    expect(ended).not.toHaveBeenCalled();
+    await (h.backend as any).pollState(); // near-end -> finishes normally
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+});
+
+// R4-6: finishedByProgress is a SINGLE-poll near-end heuristic. If a user
+// deliberately SEEKS to within the final END_OF_TRACK_WINDOW_MS, the next poll
+// would see progressMs >= durationMs-1500 and emit trackEnded — skipping the ~1s
+// the user seeked into. A one-poll "just-seeked" grace suppresses that single
+// misfire; the track then plays its remaining <=1.5s and ends naturally on the
+// following poll via the stop/null detection. Natural near-end (reached by
+// PLAYING, no seek) must be UNCHANGED.
+describe("RustLibrespotBackend seek-into-end grace (R4-6)", () => {
+  it("does NOT skip when a seek lands within the final end-of-track window; ends naturally on the following poll", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    // Observe our track actually playing first (mid-track).
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    // User deliberately seeks to ~1s before the end (inside the 1.5s window).
+    await h.backend.seek(199000);
+    // First poll after the seek: progress is already inside the near-end window
+    // and still playing. The grace must suppress this single finishedByProgress.
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 199000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    expect(ended).not.toHaveBeenCalled();
+    // The remaining <=1.5s plays out and librespot goes idle (null/204) — the
+    // genuine end. It emits exactly one trackEnded.
+    h.connect.getPlaybackState.mockResolvedValue(null as any);
+    await (h.backend as any).pollState();
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  it("the seek grace only spans ONE poll: a second in-window playing poll still finishes by progress", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 5000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState(); // observed playing
+    await h.backend.seek(199000); // seek into the final window
+    // Grace poll: suppressed.
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 199000, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    expect(ended).not.toHaveBeenCalled();
+    // Second in-window playing poll: grace already consumed -> natural near-end
+    // detection fires exactly once (grace must not permanently disable it).
+    h.connect.getPlaybackState.mockResolvedValueOnce({
+      isPlaying: true, progressMs: 199500, trackUri: "spotify:track:A", durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+
+  it("regression: a track that reaches the final window by PLAYING (no seek) still emits trackEnded on the first in-window poll", async () => {
+    const h = makeHarness();
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    await h.backend.playTrack("spotify:track:A");
+    h.connect.getPlaybackState
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 1000, trackUri: "spotify:track:A", durationMs: 200000 })
+      .mockResolvedValueOnce({ isPlaying: true, progressMs: 199000, trackUri: "spotify:track:A", durationMs: 200000 });
+    await (h.backend as any).pollState(); // observed playing (no seek)
+    expect(ended).not.toHaveBeenCalled();
+    await (h.backend as any).pollState(); // reaches window by PLAYING -> natural end
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:A", reason: "ended" });
+  });
+});
+
+describe("RustLibrespotBackend playback-start watchdog (I4 degrade-to-skip)", () => {
+  /** A controllable timer seam matching the file's injected-deps style. */
+  function makeFakeTimer() {
+    const pending: Array<{ cb: () => void; ms: number; handle: object }> = [];
+    const setTimer = vi.fn((cb: () => void, ms: number) => {
+      const handle = {};
+      pending.push({ cb, ms, handle });
+      return handle;
+    });
+    const clearTimer = vi.fn((h: unknown) => {
+      const i = pending.findIndex((p) => p.handle === h);
+      if (i >= 0) pending.splice(i, 1);
+    });
+    // "advance fake timers": run (and drain) every armed callback.
+    const advance = () => pending.splice(0).forEach((p) => p.cb());
+    return { setTimer, clearTimer, advance, pending };
+  }
+
+  it("emits exactly ONE trackEnded{reason:'error'} when playback never starts", async () => {
+    const timer = makeFakeTimer();
+    const h = makeHarness({
+      deps: {
+        playbackStartTimeoutMs: 8000,
+        setTimeout: timer.setTimer as any,
+        clearTimeout: timer.clearTimer as any,
+      },
+    });
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+
+    // The connect layer NEVER reports our track playing (204 / idle).
+    h.connect.getPlaybackState.mockResolvedValue(null);
+    await h.backend.playTrack("spotify:track:stuck");
+    // Polls that never observe playback must NOT emit anything on their own.
+    await (h.backend as any).pollState();
+    await (h.backend as any).pollState();
+    expect(ended).not.toHaveBeenCalled();
+
+    // The watchdog is armed exactly once; advance past playbackStartTimeoutMs.
+    expect(timer.pending).toHaveLength(1);
+    expect(timer.pending[0].ms).toBe(8000);
+    timer.advance();
+
+    // Degraded-to-skip: one trackEnded with reason "error" for our uri.
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:stuck", reason: "error" });
+
+    // A late idle poll after the watchdog fired does not double-emit.
+    await (h.backend as any).pollState();
+    expect(ended).toHaveBeenCalledTimes(1);
+    h.backend.stop();
+  });
+
+  it("does NOT fire the watchdog when the device actually starts playing", async () => {
+    const timer = makeFakeTimer();
+    const h = makeHarness({
+      deps: {
+        playbackStartTimeoutMs: 8000,
+        setTimeout: timer.setTimer as any,
+        clearTimeout: timer.clearTimer as any,
+      },
+    });
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+
+    await h.backend.playTrack("spotify:track:ok");
+    // Our device reports the track actually playing -> watchdog is disarmed.
+    h.connect.getPlaybackState.mockResolvedValue({
+      isPlaying: true,
+      progressMs: 1000,
+      trackUri: "spotify:track:ok",
+      durationMs: 200000,
+    });
+    await (h.backend as any).pollState();
+
+    // Real playback observed -> the watchdog was cleared, not left armed.
+    expect(timer.clearTimer).toHaveBeenCalled();
+    expect(timer.pending).toHaveLength(0);
+    // Even if a stale timer somehow fired, no "error" end must be emitted.
+    timer.advance();
+    expect(ended).not.toHaveBeenCalledWith(
+      expect.objectContaining({ reason: "error" }),
+    );
+    h.backend.stop();
+  });
+
+  it("a new playTrack() cancels the previous track's watchdog (at-most-one per track)", async () => {
+    const timer = makeFakeTimer();
+    const h = makeHarness({
+      deps: {
+        playbackStartTimeoutMs: 8000,
+        setTimeout: timer.setTimer as any,
+        clearTimeout: timer.clearTimer as any,
+      },
+    });
+    const ended = vi.fn();
+    h.backend.on("trackEnded", ended);
+    h.connect.getPlaybackState.mockResolvedValue(null);
+
+    await h.backend.playTrack("spotify:track:one");
+    await h.backend.playTrack("spotify:track:two");
+    // The first track's watchdog was cleared; only the second remains armed.
+    expect(timer.clearTimer).toHaveBeenCalled();
+    expect(timer.pending).toHaveLength(1);
+    timer.advance();
+    expect(ended).toHaveBeenCalledTimes(1);
+    expect(ended).toHaveBeenCalledWith({ uri: "spotify:track:two", reason: "error" });
+    h.backend.stop();
+  });
+});
+
+describe("RustLibrespotBackend.stop", () => {
+  it("kills librespot + ffmpeg, clears ready, and is idempotent", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    h.backend.stop();
+    h.backend.stop(); // second call must not throw
+    expect(h.librespotChild.kill).toHaveBeenCalled();
+    expect(h.ffmpegChild.kill).toHaveBeenCalled();
+    expect(h.backend.isReady()).toBe(false);
+  });
+});
+
+describe("RustLibrespotBackend.start failure cleanup", () => {
+  it("tears down librespot + ffmpeg when the device never appears", async () => {
+    const h = makeHarness();
+    h.connect.getDevices.mockResolvedValue([]); // device never shows up -> waitForDevice times out
+    await expect(h.backend.start()).rejects.toThrow(/did not appear/i);
+    expect(h.librespotChild.kill).toHaveBeenCalled();
+    expect(h.ffmpegChild.kill).toHaveBeenCalled();
+    expect(h.backend.isReady()).toBe(false);
+  });
+});
+
+describe("RustLibrespotBackend child-process error handling", () => {
+  it("swallows+logs a child 'error' when no backend 'error' listener is attached", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.backend.listenerCount("error")).toBe(0);
+    expect(() => h.librespotChild.emit("error", new Error("boom"))).not.toThrow();
+    expect(() => h.ffmpegChild.emit("error", new Error("boom"))).not.toThrow();
+    h.backend.stop();
+  });
+
+  it("re-emits a child 'error' to an attached backend 'error' listener", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const onErr = vi.fn();
+    h.backend.on("error", onErr);
+    const err = new Error("ffmpeg boom");
+    h.ffmpegChild.emit("error", err);
+    expect(onErr).toHaveBeenCalledWith(err);
+    h.backend.stop();
+  });
+
+  // I(pipe): ffmpeg dying mid-track while librespot keeps producing PCM raises
+  // EPIPE on ffmpeg.stdin. With no stdin 'error' listener Node escalates it to
+  // process 'uncaughtException'. The backend must handle it in-band.
+  it("swallows an EPIPE 'error' on ffmpeg.stdin (ffmpeg died mid-track) without an unhandled throw", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    expect(h.backend.listenerCount("error")).toBe(0);
+    const epipe = Object.assign(new Error("write EPIPE"), { code: "EPIPE" });
+    expect(() => h.ffmpegChild.stdin.emit("error", epipe)).not.toThrow();
+    h.backend.stop(); // idempotent second teardown must not throw
+  });
+
+  it("routes an ffmpeg.stdin EPIPE to the backend 'error' listener and tears down cleanly", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const onErr = vi.fn();
+    h.backend.on("error", onErr);
+    const epipe = Object.assign(new Error("write EPIPE"), { code: "EPIPE" });
+    expect(() => h.ffmpegChild.stdin.emit("error", epipe)).not.toThrow();
+    expect(onErr).toHaveBeenCalledWith(epipe);
+    // Broken pipe -> clean teardown: children killed, not ready.
+    expect(h.librespotChild.kill).toHaveBeenCalled();
+    expect(h.ffmpegChild.kill).toHaveBeenCalled();
+    expect(h.backend.isReady()).toBe(false);
+    h.backend.stop(); // second teardown must not throw (no double-teardown crash)
+    expect(onErr).toHaveBeenCalledTimes(1); // single emit despite both pipe ends
+  });
+
+  it("does not throw when librespot proc.stdout emits an EPIPE on the broken pipe", async () => {
+    const h = makeHarness();
+    await h.backend.start();
+    const epipe = Object.assign(new Error("read/write EPIPE"), { code: "EPIPE" });
+    expect(() => h.librespotChild.stdout.emit("error", epipe)).not.toThrow();
+    h.backend.stop();
+  });
+});

src/music/spotify/rust-librespot.ts

@@ -0,0 +1,620 @@
+import { EventEmitter } from "node:events";
+import type { Readable } from "node:stream";
+import type { ChildProcess } from "node:child_process";
+import { spawn as realSpawn } from "node:child_process";
+import { mkdirSync as realMkdirSync } from "node:fs";
+import type { Logger } from "pino";
+import type {
+  SpotifyAudioBackend,
+  SpotifyTrackEndedEvent,
+  SpotifyNowPlaying,
+} from "./backend.js";
+import { findLibrespot } from "./binary.js";
+import { SpotifyConnectApi } from "./connect-api.js";
+import type { PlaybackState, SpotifyDevice } from "./connect-api.js";
+import type { SpotifyOAuth } from "./spotify-oauth.js";
+import { getFfmpegCommand } from "../../audio/player.js";
+
+export interface RustLibrespotBackendOptions {
+  deviceName: string;
+  bitrate: number;
+  cacheDir: string;
+  oauth: SpotifyOAuth;
+  connect?: SpotifyConnectApi;
+  logger: Logger;
+  deps?: RustLibrespotBackendDeps;
+}
+
+/** Injectable seams so the whole lifecycle is testable without a real binary/network. */
+export interface RustLibrespotBackendDeps {
+  spawn?: typeof realSpawn;
+  mkdirSync?: typeof realMkdirSync;
+  findBinary?: () => string;
+  /**
+   * C1: override the ffmpeg command. Production resolves it via
+   * getFfmpegCommand() (bundled ffmpeg-static fallback when `ffmpeg` isn't on
+   * PATH); tests pin it to "ffmpeg" for stable arg assertions.
+   */
+  ffmpegCommand?: string;
+  sleep?: (ms: number) => Promise<void>;
+  readyPollIntervalMs?: number;
+  readyTimeoutMs?: number;
+  statePollIntervalMs?: number;
+  /**
+   * I4 (§13): bounded window after playTrack() within which our own track must
+   * be observed playing, else we degrade-to-skipped (emit trackEnded
+   * reason:"error"). Injectable timer seams (default real setTimeout, unref'd)
+   * let tests advance it deterministically.
+   */
+  playbackStartTimeoutMs?: number;
+  setTimeout?: (cb: () => void, ms: number) => unknown;
+  clearTimeout?: (handle: unknown) => void;
+}
+
+const DEFAULT_READY_POLL_MS = 500;
+const DEFAULT_READY_TIMEOUT_MS = 20_000;
+const DEFAULT_STATE_POLL_MS = 2_000;
+/** How close to the end (ms) counts as "track finished" when polling player state. */
+const END_OF_TRACK_WINDOW_MS = 1_500;
+/**
+ * I4 (§13): if our own track has not been observed playing within this window
+ * after playTrack(), degrade-to-skipped so the queue advances instead of
+ * stalling on a persistently-failing play (403 non-Premium, 404 outliving
+ * retries). Bounded and injectable for tests.
+ */
+const DEFAULT_PLAYBACK_START_TIMEOUT_MS = 8_000;
+
+const defaultSleep = (ms: number) => new Promise<void>((r) => setTimeout(r, ms));
+
+export class RustLibrespotBackend extends EventEmitter implements SpotifyAudioBackend {
+  private readonly opts: RustLibrespotBackendOptions;
+  private readonly log: Logger;
+  private readonly deps: RustLibrespotBackendDeps;
+  private readonly oauth: SpotifyOAuth;
+  private readonly connect: SpotifyConnectApi;
+
+  private proc: ChildProcess | null = null;
+  private ffmpeg: ChildProcess | null = null;
+  private pollTimer: ReturnType<typeof setInterval> | null = null;
+  // I4 (§13): "did our track actually start playing?" watchdog handle (opaque —
+  // produced by the injectable setTimeout seam). null when disarmed.
+  private watchdogTimer: unknown = null;
+  private ready = false;
+  private positionMs = 0;
+
+  // R4-4 (multi-bot): OUR resolved Connect device id, captured from
+  // findDeviceByName() in playTrack(). config.spotify (hence this Connect
+  // session) is shared process-wide across every BotInstance under one Premium
+  // account, and both the control API (pause/resume/seek) and the state read
+  // (GET /v1/me/player) are ACCOUNT-wide by default. Persisting our device id
+  // lets us (a) device-scope our control commands so we only ever act on our own
+  // device, and (b) in pollState, ignore state reported for a foreign device
+  // another bot stole the single active session with. null until first playTrack.
+  private deviceId: string | null = null;
+
+  // track-end poll state machine
+  private currentUri: string | null = null;
+  private hasPlayed = false;
+  private endedForCurrent = false;
+  // C3.4: end-detection is DISARMED until our own playTrack() runs. A poll that
+  // fires before the bot ever asks to play (e.g. at startup) must produce no
+  // side effects at all, so a foreign track already near its end can't emit a
+  // spurious trackEnded/metadata and wrongly advance the queue.
+  private armed = false;
+  // C1(pause-skip): our own pause state. A self-initiated pause makes the
+  // Connect device report is_playing:false with the SAME uri; without tracking
+  // it we'd misread that as a track end and SKIP the paused track (breaking the
+  // pause command and occupancy auto-pause-when-alone on the Rust backend). Set
+  // by pause(), cleared by resume() and playTrack().
+  private paused = false;
+  // Robustness (finishedByStop + finishedByNull / R3-1): a "not clearly playing
+  // our track" signal — either a non-paused is_playing:false (external stop) OR
+  // a null item (trackUri===null) observed while our track was playing — must be
+  // confirmed across TWO consecutive non-paused polls before we emit trackEnded.
+  // Both signals can be momentary: is_playing:false from a buffering hiccup; a
+  // null item from a Connect/track handoff boundary or a region-relinked /
+  // restricted item that momentarily maps to uri:null (getPlaybackState omits
+  // the `market` param). Set on the first such poll; reset only when a poll
+  // clearly shows our track playing again (is_playing AND a real item), on track
+  // change, or on playTrack(). Sharing ONE flag keeps both sibling paths from
+  // false-skipping on a single transient poll.
+  private stopSeen = false;
+  // R4-6: one-poll "just-seeked" grace. finishedByProgress is a SINGLE-poll
+  // near-end heuristic; if a user deliberately seeks to within the final
+  // END_OF_TRACK_WINDOW_MS, the very next poll would see progressMs >=
+  // durationMs-window and emit trackEnded, SKIPPING the ~1s the user seeked
+  // into. Set by seek(); consumed on the next poll so it suppresses that single
+  // finishedByProgress misfire only — the track then plays its remaining <=1.5s
+  // and ends naturally on the following poll via the stop/null detection. It
+  // deliberately does NOT touch the paused/stop/null two-poll logic, and a track
+  // reaching the window by PLAYING (no preceding seek) still ends normally.
+  private seekGrace = false;
+  // I(pipe): one teardown per broken librespot->ffmpeg pipe. Both stream ends
+  // (ffmpeg.stdin write side, proc.stdout read side) can report the same EPIPE;
+  // this guard prevents a double teardown / double error-emit.
+  private pipeBroke = false;
+
+  constructor(o: RustLibrespotBackendOptions) {
+    super();
+    this.opts = o;
+    this.log = o.logger;
+    this.deps = o.deps ?? {};
+    this.oauth = o.oauth;
+    // The Connect API shares the backend's OAuth token source. Reuse the
+    // injected instance in tests; otherwise build one over oauth.getAccessToken().
+    this.connect = o.connect ?? new SpotifyConnectApi(() => this.oauth.getAccessToken());
+  }
+
+  async start(): Promise<void> {
+    const spawn = this.deps.spawn ?? realSpawn;
+    const mkdirSync = this.deps.mkdirSync ?? realMkdirSync;
+    const findBinary = this.deps.findBinary ?? findLibrespot;
+    // C1: resolve ffmpeg via getFfmpegCommand() unless injected for tests.
+    const ffmpegCommand = this.deps.ffmpegCommand ?? getFfmpegCommand();
+
+    // A valid USER control token is required before we spawn anything.
+    const token = await this.oauth.getAccessToken();
+    if (!token) {
+      throw new Error("Spotify not authorized (no access token) — sign in first");
+    }
+
+    mkdirSync(this.opts.cacheDir, { recursive: true });
+
+    // Everything past here spawns children / opens the state poll. On any
+    // failure (e.g. the device never appears), tear it all down via stop().
+    try {
+      this.pipeBroke = false; // fresh pipe for this start()
+      // 1. Spawn ffmpeg FIRST (the reader) so its stdin pipe is ready before
+      //    librespot starts pushing raw 44.1k s16le PCM into it.
+      this.ffmpeg = spawn(
+        ffmpegCommand,
+        [
+          "-f", "s16le", "-ar", "44100", "-ac", "2", "-i", "pipe:0",
+          "-f", "s16le", "-ar", "48000", "-ac", "2", "-acodec", "pcm_s16le", "pipe:1",
+        ],
+        { stdio: ["pipe", "pipe", "pipe"] },
+      );
+      this.ffmpeg.stderr?.on("data", (b: Buffer) =>
+        this.log.debug({ ffmpeg: b.toString().trim() }, "ffmpeg"),
+      );
+      this.ffmpeg.on("error", (err) => this.emitError(err));
+      // I(pipe): if ffmpeg dies mid-track while librespot keeps producing PCM,
+      // the next write into its stdin hits the now-closed pipe -> EPIPE 'error'
+      // on stdin. With no listener Node escalates that to process
+      // 'uncaughtException' (the global handler logs but leaves undefined
+      // state). Handle it in-band: tear down cleanly and surface via emitError.
+      this.ffmpeg.stdin?.on("error", (err) => this.onPipeError(err));
+
+      // 2. Spawn librespot: --backend pipe with NO --device => raw s16le/44100/2
+      //    on stdout, NO --passthrough (that would emit raw Ogg). --access-token
+      //    authenticates it as a Connect device controllable via the Web API.
+      const bin = findBinary();
+      this.proc = spawn(
+        bin,
+        [
+          "--name", this.opts.deviceName,
+          "--backend", "pipe",
+          "--bitrate", String(this.opts.bitrate),
+          "--format", "S16",
+          "--cache", this.opts.cacheDir,
+          "--device-type", "speaker",
+          // KNOWN LIMITATION (CWE-214): the live Spotify access token is passed in
+          // the child argv, so on a shared/multi-tenant host a co-located local
+          // process could read it via `ps` / /proc/<pid>/cmdline. Bounded (~1h token,
+          // needs local access) and `--access-token` is librespot's supported bootstrap.
+          "--access-token", token,
+        ],
+        { stdio: ["ignore", "pipe", "pipe"] },
+      );
+      // stdout carries PCM — pipe it, never attach a data listener that consumes it.
+      if (this.proc.stdout && this.ffmpeg.stdin) {
+        this.proc.stdout.pipe(this.ffmpeg.stdin);
+        // Guard the read side too: a broken pipe can surface as an 'error' on
+        // proc.stdout when ffmpeg's stdin closes underneath it. Same handler,
+        // guarded against a double teardown.
+        this.proc.stdout.on("error", (err) => this.onPipeError(err));
+      }
+      this.proc.stderr?.on("data", (b: Buffer) =>
+        this.log.info({ librespot: b.toString().trim() }, "librespot"),
+      );
+      this.proc.on("error", (err) => this.emitError(err));
+      this.proc.on("exit", (code, signal) => {
+        this.ready = false;
+        this.log.warn({ code, signal }, "librespot exited");
+      });
+
+      // 3. Poll the Connect device list until our device registers.
+      await this.waitForDevice();
+
+      // 4. Begin the player-state poll loop (track-end / position / metadata).
+      this.startPollLoop();
+
+      this.ready = true;
+      this.emit("ready");
+    } catch (e) {
+      this.stop();
+      throw e;
+    }
+  }
+
+  /**
+   * Re-emit a child "error" only when a consumer is listening; Node throws on an
+   * unhandled "error" event, so with no listener we log via the injected logger.
+   */
+  private emitError(err: unknown): void {
+    if (this.listenerCount("error") > 0) {
+      this.emit("error", err);
+    } else {
+      this.log.error({ err }, "rust-librespot backend error (no listener)");
+    }
+  }
+
+  /**
+   * I(pipe): the librespot->ffmpeg PCM pipe broke — typically ffmpeg died
+   * mid-track and librespot's next write hit the closed pipe (EPIPE), or the
+   * stream was destroyed (ERR_STREAM_DESTROYED). Both are expected teardown
+   * signals, not programming errors. Log, tear down cleanly (stop() is
+   * idempotent), then surface via emitError so a listening controller reacts.
+   * Guarded so both stream ends reporting the same break don't double-teardown.
+   */
+  private onPipeError(err: unknown): void {
+    if (this.pipeBroke) return;
+    this.pipeBroke = true;
+    this.log.warn(
+      { err },
+      "rust-librespot: librespot->ffmpeg pipe broke (ffmpeg died?) — tearing down",
+    );
+    this.stop();
+    this.emitError(err);
+  }
+
+  private async waitForDevice(): Promise<void> {
+    const sleep = this.deps.sleep ?? defaultSleep;
+    const interval = this.deps.readyPollIntervalMs ?? DEFAULT_READY_POLL_MS;
+    const timeout = this.deps.readyTimeoutMs ?? DEFAULT_READY_TIMEOUT_MS;
+    const deadline = Date.now() + timeout;
+    while (Date.now() < deadline) {
+      let devices: SpotifyDevice[] = [];
+      try {
+        devices = await this.connect.getDevices();
+      } catch (err) {
+        this.log.debug({ err }, "getDevices failed during readiness poll");
+      }
+      if (devices.some((d) => d.name === this.opts.deviceName)) return;
+      await sleep(interval);
+    }
+    throw new Error(`librespot device "${this.opts.deviceName}" did not appear within timeout`);
+  }
+
+  private startPollLoop(): void {
+    const interval = this.deps.statePollIntervalMs ?? DEFAULT_STATE_POLL_MS;
+    this.pollTimer = setInterval(() => {
+      void this.pollState();
+    }, interval);
+    // Don't keep the event loop / test process alive on account of the poll timer.
+    this.pollTimer.unref?.();
+  }
+
+  /** One player-state poll iteration: updates position/metadata and detects track end. */
+  private async pollState(): Promise<void> {
+    let state: PlaybackState | null;
+    try {
+      state = await this.connect.getPlaybackState();
+    } catch (err) {
+      this.log.debug({ err }, "getPlaybackState failed");
+      return;
+    }
+
+    // C3.4: detection is armed ONLY by our own playTrack(). Until then a poll
+    // must have NO side effects — no metadata/trackEnded emit and no mutation of
+    // currentUri/hasPlayed/positionMs. This single gate covers the null-state
+    // (C3.5 post-play-204) branch, the metadata-emit block, and every end
+    // heuristic below, so a foreign track sitting near its end at startup can
+    // never spuriously advance the queue. (Device-readiness polling via
+    // getDevices is separate and stays active regardless of this flag.)
+    if (!this.armed) return;
+
+    // R4-6: consume the one-poll "just-seeked" grace up front so it spans exactly
+    // ONE poll regardless of which branch this poll takes. `justSeeked` then
+    // suppresses a single finishedByProgress misfire below (a deliberate seek
+    // into the final END_OF_TRACK_WINDOW_MS must not be read as a natural end).
+    const justSeeked = this.seekGrace;
+    this.seekGrace = false;
+
+    if (!state) {
+      // C3.5: a 204 / no-active-device response. AFTER our own track has been
+      // seen playing, librespot going idle means the track ended — emit once so
+      // the queue advances instead of stalling. BEFORE any play (hasPlayed
+      // false), a null state is just "nothing active yet" and is ignored.
+      // C1(pause-skip) residual: while WE hold a self-initiated pause, a long
+      // pause can let the Connect device idle out to 204/null. That is still
+      // "paused", NOT a track end — do nothing this poll, or we'd skip the
+      // paused track. Once resume() clears `paused`, a genuinely-dead device
+      // (persistent null) is detected and skipped on the next poll as before.
+      if (this.hasPlayed && this.currentUri && !this.endedForCurrent && !this.paused) {
+        this.endedForCurrent = true;
+        const endedUri = this.currentUri;
+        this.currentUri = null;
+        this.positionMs = 0;
+        const e: SpotifyTrackEndedEvent = { uri: endedUri, reason: "ended" };
+        this.emit("trackEnded", e);
+      }
+      return;
+    }
+
+    // R4-4 (multi-bot): is the account's single ACTIVE Connect device ours? The
+    // Premium account allows one active playback stream, so if another bot stole
+    // the session, GET /v1/me/player now reports THAT device + its track. Only
+    // when the active device is foreign (we know our id AND the state names a
+    // DIFFERENT active id) do we refuse to treat this poll as our own track:
+    // skip metadata/track-change (so B's now-playing isn't surfaced as ours),
+    // don't advance our position/hasPlayed off foreign playback, and feed the
+    // stop/null two-poll detection so the stolen session cleanly ends OUR track
+    // and advances OUR queue instead of thrashing/misattributing. LENIENT: if
+    // our id is unknown or activeDeviceId is absent we can't tell, so we fall
+    // back to today's behavior byte-for-byte (single-bot: activeDeviceId === ours).
+    const foreignActive =
+      this.deviceId != null &&
+      state.activeDeviceId != null &&
+      state.activeDeviceId !== this.deviceId;
+
+    // Don't misattribute a foreign device's playback position as ours.
+    if (!foreignActive) this.positionMs = state.progressMs;
+
+    // Track change -> reset the end-detection state and surface best-effort
+    // metadata. Skipped entirely when a foreign device is active: its uri is NOT
+    // our track, so adopting it / emitting metadata would surface another bot's
+    // now-playing as ours and later misread that bot's stop as our track's end.
+    if (!foreignActive && state.trackUri && state.trackUri !== this.currentUri) {
+      this.currentUri = state.trackUri;
+      this.hasPlayed = false;
+      this.endedForCurrent = false;
+      this.stopSeen = false; // new track -> drop any pending stop confirmation
+      const np: SpotifyNowPlaying = {
+        uri: state.trackUri,
+        name: "",
+        artist: "",
+        album: "",
+        coverUrl: "",
+        durationMs: state.durationMs,
+      };
+      this.emit("metadata", np);
+    }
+
+    // R4-4: only OUR device actually playing counts as our track playing. When a
+    // foreign device is active, `state.isPlaying` reflects THAT bot's playback,
+    // not ours — so it must not mark hasPlayed or clear the stop confirmation.
+    const ourTrackPlaying = state.isPlaying && !foreignActive;
+    if (ourTrackPlaying) {
+      this.hasPlayed = true;
+      // Real playback observed -> the I4 degrade-to-skip watchdog is moot.
+      this.clearPlaybackWatchdog();
+    }
+    // Clearly playing our track again (is_playing AND a real item) -> any earlier
+    // transient is_playing:false (buffering) or null item (Connect handoff /
+    // market relink) was a hiccup; drop the pending two-poll end confirmation.
+    // A null item under is_playing:true is NOT "clearly playing" and must NOT
+    // reset the confirmation, or a genuine null-item end could never accumulate
+    // its second poll.
+    if (ourTrackPlaying && state.trackUri !== null) {
+      this.stopSeen = false;
+    }
+    if (!this.currentUri || this.endedForCurrent) return;
+
+    // C3.4: EVERY end condition is gated on hasPlayed so no end can fire until
+    // the bot's own track has actually been observed playing. Without this, the
+    // first poll (before playTrack) could observe the account's stale/paused
+    // track sitting near its end and spuriously emit "trackEnded".
+    // m(sub-window): only apply the near-end window when the track is LONGER
+    // than the window. For durationMs in [1, END_OF_TRACK_WINDOW_MS],
+    // `durationMs - window` is negative, so the old `> 0` guard made this
+    // unconditionally true and finished the track on its first poll. A
+    // sub-window track instead relies on normal stop/next-track detection.
+    // C1(pause-skip) residual: a self-initiated pause within the final window
+    // freezes progress at >= dur-window with is_playing:false and the SAME uri;
+    // without `!this.paused` this near-end heuristic would fire and skip the
+    // paused track. resume() clears `paused`, so a genuine natural end is still
+    // detected afterwards.
+    // R4-4: progress/duration under a foreign-active state belong to another
+    // bot's track, so the near-end heuristic must not fire off them. A stolen
+    // session ends OUR track through the stop/null two-poll path below instead.
+    const finishedByProgress =
+      this.hasPlayed &&
+      !this.paused &&
+      !foreignActive &&
+      state.durationMs > END_OF_TRACK_WINDOW_MS &&
+      state.progressMs >= state.durationMs - END_OF_TRACK_WINDOW_MS;
+    // C1(pause-skip) + R3-1(null-item): a self-initiated pause (this.paused)
+    // reports is_playing:false (and can idle to a null item) with the SAME uri —
+    // that is NOT a track end, so never finish while paused. Beyond pause, the
+    // two "not clearly playing our track" signals — an external stop
+    // (is_playing:false) and a null item (trackUri===null) — are BOTH momentary
+    // at the boundaries (buffering; Connect handoff / market relink), so they
+    // share ONE two-poll confirmation (stopSeen): require the signal to persist
+    // across two consecutive non-paused polls before ending. A single transient
+    // stop OR null is absorbed; a confirmed end still emits within ~one extra
+    // poll interval, and a genuine end (item stays null / stopped across two
+    // polls) still fires exactly once.
+    // R4-4: a foreign device becoming the active one means OUR device is no
+    // longer playing our track — the same signal as an external stop / null
+    // item, so it shares the two-poll confirmation: one foreign poll is
+    // unconfirmed (could be a transient handoff), two consecutive foreign polls
+    // cleanly end our track and advance our queue.
+    let finishedByStopOrNull = false;
+    if (
+      this.hasPlayed &&
+      !this.paused &&
+      (foreignActive || !state.isPlaying || state.trackUri === null)
+    ) {
+      if (this.stopSeen) {
+        finishedByStopOrNull = true;
+      } else {
+        this.stopSeen = true; // first such poll — await confirmation
+      }
+    }
+
+    // R4-6: a deliberate seek into the near-end window suppresses this single
+    // finishedByProgress. `justSeeked` was already consumed above, so the NEXT
+    // in-window poll finishes normally — the grace never permanently disables
+    // near-end detection, and it never touches the stop/null two-poll path.
+    if ((finishedByProgress && !justSeeked) || finishedByStopOrNull) {
+      this.endedForCurrent = true; // latch: emit at most once per track
+      const endedUri = this.currentUri;
+      this.currentUri = null;
+      this.positionMs = 0;
+      const e: SpotifyTrackEndedEvent = { uri: endedUri, reason: "ended" };
+      this.emit("trackEnded", e);
+    }
+  }
+
+  isReady(): boolean {
+    return this.ready;
+  }
+
+  async playTrack(uri: string): Promise<void> {
+    const deviceId = await this.connect.findDeviceByName(this.opts.deviceName);
+    if (!deviceId) throw new Error(`Connect device "${this.opts.deviceName}" not found`);
+    // R4-4: persist OUR device id so control (pause/resume/seek) is device-scoped
+    // and pollState can distinguish our device from a foreign one that stole the
+    // shared account's single active Connect session.
+    this.deviceId = deviceId;
+    // Reset the track-end state machine for the new track: clear the once-only
+    // latch and drop hasPlayed so no end can fire until a poll re-confirms this
+    // uri playing. currentUri is cleared so the next poll re-detects the track
+    // (fresh metadata) rather than treating it as unchanged. Arming here is the
+    // primary guarantee that no end/metadata can fire before the bot plays.
+    // Cancel the previous track's degrade-to-skip watchdog first (at-most-one
+    // trackEnded per track, I4).
+    this.clearPlaybackWatchdog();
+    this.currentUri = null;
+    this.hasPlayed = false;
+    this.endedForCurrent = false;
+    this.armed = true;
+    // A newly-started track is not paused, and carries no pending stop
+    // confirmation from the previous track. (C1 pause-skip.)
+    this.paused = false;
+    this.stopSeen = false;
+    // R4-6: a fresh track carries no pending seek grace from the previous one.
+    this.seekGrace = false;
+    // transfer(false) activates our device WITHOUT starting audio; play() then
+    // actually starts the uri. The two-step is required — transfer alone won't
+    // begin playback.
+    await this.connect.transfer(deviceId, false);
+    await this.connect.play(deviceId, uri);
+    // I4 (§13): arm the "did it actually start playing?" watchdog. If our own
+    // track is never seen playing within the window, degrade-to-skipped so the
+    // queue advances instead of hanging on a silent, persistently-failing play.
+    this.armPlaybackWatchdog(uri);
+  }
+
+  /**
+   * I4 (§13) degrade-to-skip watchdog. Arms a bounded timer; if our own track
+   * has not been observed playing by the time it fires, emit a single
+   * trackEnded{reason:"error"} so the controller re-emits it and BotInstance
+   * advances the queue. Cleared when real playback is observed, on stop(), and
+   * at the start of a new playTrack().
+   */
+  private armPlaybackWatchdog(uri: string): void {
+    this.clearPlaybackWatchdog();
+    const timeoutMs = this.deps.playbackStartTimeoutMs ?? DEFAULT_PLAYBACK_START_TIMEOUT_MS;
+    const setTimer =
+      this.deps.setTimeout ??
+      ((cb: () => void, ms: number) => {
+        const t = setTimeout(cb, ms);
+        (t as { unref?: () => void }).unref?.();
+        return t;
+      });
+    this.watchdogTimer = setTimer(() => {
+      this.watchdogTimer = null;
+      this.onPlaybackStartTimeout(uri);
+    }, timeoutMs);
+  }
+
+  private clearPlaybackWatchdog(): void {
+    if (this.watchdogTimer == null) return;
+    const clearTimer =
+      this.deps.clearTimeout ??
+      ((h: unknown) => clearTimeout(h as ReturnType<typeof setTimeout>));
+    clearTimer(this.watchdogTimer);
+    this.watchdogTimer = null;
+  }
+
+  /**
+   * Fired when the playback-start window elapses with no observed playback.
+   * C3.6: never throws up the queue path. Guarded by the same endedForCurrent
+   * latch as normal end-detection so at most one trackEnded per track (no
+   * double-emit with the poll-loop end detection).
+   */
+  private onPlaybackStartTimeout(uri: string): void {
+    // Real playback was seen (hasPlayed) or the track already ended -> moot.
+    if (this.hasPlayed || this.endedForCurrent) return;
+    this.endedForCurrent = true; // latch
+    this.currentUri = null;
+    this.positionMs = 0;
+    const e: SpotifyTrackEndedEvent = { uri, reason: "error" };
+    this.emit("trackEnded", e);
+  }
+
+  async pause(): Promise<void> {
+    // R4-4: device-scope to OUR device so bot A's pause / auto-pause-when-alone
+    // can't pause whatever device is currently active for the shared account
+    // (= bot B's playback). Falls back to account-wide only before first play.
+    await this.connect.pause(this.deviceId ?? undefined);
+    // C1(pause-skip): mark our own pause so the next poll's is_playing:false
+    // (same uri) is not misread as a track end and skipped.
+    this.paused = true;
+  }
+
+  async resume(): Promise<void> {
+    // R4-4: device-scope to OUR device (see pause()).
+    await this.connect.resume(this.deviceId ?? undefined);
+    // Resumed -> normal end-detection applies again.
+    this.paused = false;
+  }
+
+  async seek(ms: number): Promise<void> {
+    // R4-4: device-scope to OUR device (see pause()).
+    await this.connect.seek(ms, this.deviceId ?? undefined);
+    this.positionMs = ms;
+    // R4-6: arm the one-poll grace so a seek landing inside the final
+    // END_OF_TRACK_WINDOW_MS is not immediately treated as a natural end.
+    this.seekGrace = true;
+  }
+
+  getPcmStream(): Readable {
+    const out = this.ffmpeg?.stdout;
+    if (!out) throw new Error("PCM stream unavailable (rust-librespot backend not started)");
+    return out;
+  }
+
+  getPositionMs(): number {
+    return this.positionMs;
+  }
+
+  stop(): void {
+    this.ready = false;
+    // Disarm the I4 degrade-to-skip watchdog so a stopped backend never emits.
+    this.clearPlaybackWatchdog();
+    // Clear the state poll interval FIRST so no poll fires mid-teardown.
+    if (this.pollTimer) {
+      clearInterval(this.pollTimer);
+      this.pollTimer = null;
+    }
+    if (this.proc) {
+      try {
+        this.proc.kill();
+      } catch {
+        /* ignore */
+      }
+      this.proc = null;
+    }
+    if (this.ffmpeg) {
+      try {
+        this.ffmpeg.kill();
+      } catch {
+        /* ignore */
+      }
+      this.ffmpeg = null;
+    }
+  }
+}

src/music/spotify/spotify-oauth.test.ts

@@ -0,0 +1,587 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import {
+  mkdtempSync,
+  rmSync,
+  readdirSync,
+  readFileSync,
+  renameSync,
+  statSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import {
+  SpotifyOAuth,
+  SPOTIFY_CONTROL_SCOPES,
+  generateCodeVerifier,
+  codeChallengeS256,
+  createFileOAuthTokenStore,
+  type OAuthTokens,
+  type OAuthTokenStore,
+} from "./spotify-oauth.js";
+
+// Wrap the fs functions the token store uses in call-through spies so the
+// atomic-write path (temp file + rename) can be observed/forced. Everything else
+// (mkdtemp, rmSync, readdir, …) is the real implementation via `...actual`, so
+// all other tests keep real filesystem behavior. `vi.spyOn` can't be used here
+// because the node:fs ESM namespace is non-configurable in this setup.
+vi.mock("node:fs", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:fs")>();
+  return {
+    ...actual,
+    writeFileSync: vi.fn(actual.writeFileSync),
+    renameSync: vi.fn(actual.renameSync),
+  };
+});
+
+// Correction C3.2: the control OAuth REQUIRES a user-provided client_id (their
+// own Spotify Developer app) + caller-supplied loopback redirect. There is NO
+// librespot public-client / :5588 default.
+const CLIENT_ID = "test-client-id";
+const REDIRECT_URI = "http://127.0.0.1:8888/api/spotify/callback";
+
+/** In-memory store exposing `.value` so tests can assert persistence. */
+function memStore(
+  initial: OAuthTokens | null = null,
+): OAuthTokenStore & { value: OAuthTokens | null } {
+  const s = {
+    value: initial,
+    load() {
+      return s.value;
+    },
+    save(t: OAuthTokens) {
+      s.value = t;
+    },
+    clear() {
+      s.value = null;
+    },
+  };
+  return s;
+}
+
+/** A manually-settled promise, to hold a token POST "in flight" during a test. */
+function deferred<T>() {
+  let resolve!: (v: T) => void;
+  let reject!: (e: unknown) => void;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return { promise, resolve, reject };
+}
+
+describe("PKCE helpers", () => {
+  it("generateCodeVerifier returns 64 chars from the unreserved set", () => {
+    const v = generateCodeVerifier();
+    expect(v).toHaveLength(64);
+    expect(v).toMatch(/^[A-Za-z0-9\-._~]{64}$/);
+    expect(generateCodeVerifier()).not.toBe(v); // random
+  });
+
+  it("codeChallengeS256 is base64url(sha256) with no padding (43 chars)", () => {
+    const c = codeChallengeS256("abc123");
+    expect(c).toHaveLength(43); // 32-byte digest -> 43 base64url chars
+    expect(c).not.toContain("=");
+    expect(c).toMatch(/^[A-Za-z0-9_-]+$/);
+  });
+});
+
+describe("SpotifyOAuth.buildAuthorizeUrl", () => {
+  it("builds accounts.spotify.com/authorize with the caller's clientId + redirectUri + S256", () => {
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      redirectUri: REDIRECT_URI,
+      store: memStore(),
+    });
+    const { url, state } = oauth.buildAuthorizeUrl();
+    const u = new URL(url);
+    expect(u.origin + u.pathname).toBe("https://accounts.spotify.com/authorize");
+    const p = u.searchParams;
+    expect(p.get("client_id")).toBe(CLIENT_ID);
+    expect(p.get("response_type")).toBe("code");
+    expect(p.get("redirect_uri")).toBe(REDIRECT_URI);
+    expect(p.get("code_challenge_method")).toBe("S256");
+    expect(p.get("code_challenge")).toHaveLength(43);
+    expect(p.get("scope")).toBe(SPOTIFY_CONTROL_SCOPES);
+    expect(p.get("state")).toBe(state);
+    expect(state).toMatch(/^[0-9a-f]{32}$/);
+    expect(oauth.getClientId()).toBe(CLIENT_ID);
+    expect(oauth.getRedirectUri()).toBe(REDIRECT_URI);
+  });
+
+  // Correction C3.2: no clientId => cannot start OAuth; throw a clear message.
+  it("throws a clear error when clientId is empty", () => {
+    const oauth = new SpotifyOAuth({ redirectUri: REDIRECT_URI, store: memStore() });
+    expect(() => oauth.buildAuthorizeUrl()).toThrow(/Client ID/i);
+  });
+});
+
+describe("SpotifyOAuth.isAuthorized (C3.2)", () => {
+  it("is false without a clientId even if a refresh token is stored", () => {
+    const store = memStore({
+      accessToken: "a",
+      refreshToken: "r",
+      expiresAt: Date.now() + 60_000,
+      scope: "s",
+    });
+    const oauth = new SpotifyOAuth({ redirectUri: REDIRECT_URI, store });
+    expect(oauth.isAuthorized()).toBe(false);
+  });
+
+  it("is true with a clientId and a stored refresh token", () => {
+    const store = memStore({
+      accessToken: "a",
+      refreshToken: "r",
+      expiresAt: Date.now() + 60_000,
+      scope: "s",
+    });
+    const oauth = new SpotifyOAuth({ clientId: CLIENT_ID, store });
+    expect(oauth.isAuthorized()).toBe(true);
+  });
+});
+
+describe("SpotifyOAuth.handleCallback", () => {
+  it("exchanges the code (PKCE verifier matches the authorize challenge) and persists tokens", async () => {
+    const store = memStore();
+    const http = {
+      post: vi.fn().mockResolvedValue({
+        data: {
+          access_token: "a1",
+          refresh_token: "r1",
+          expires_in: 3600,
+          scope: SPOTIFY_CONTROL_SCOPES,
+        },
+      }),
+    } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      redirectUri: REDIRECT_URI,
+      store,
+      deps: { http },
+    });
+
+    const { url, state } = oauth.buildAuthorizeUrl();
+    const challenge = new URL(url).searchParams.get("code_challenge")!;
+
+    const ok = await oauth.handleCallback("CODE123", state);
+    expect(ok).toBe(true);
+
+    const [path, bodyStr, cfg] = http.post.mock.calls[0];
+    expect(path).toBe("/api/token");
+    expect(cfg.headers["Content-Type"]).toBe("application/x-www-form-urlencoded");
+    const body = new URLSearchParams(bodyStr as string);
+    expect(body.get("grant_type")).toBe("authorization_code");
+    expect(body.get("code")).toBe("CODE123");
+    expect(body.get("redirect_uri")).toBe(REDIRECT_URI);
+    expect(body.get("client_id")).toBe(CLIENT_ID);
+    // The verifier sent MUST hash to the challenge advertised in the authorize URL.
+    const verifier = body.get("code_verifier")!;
+    expect(codeChallengeS256(verifier)).toBe(challenge);
+
+    expect(store.value?.accessToken).toBe("a1");
+    expect(store.value?.refreshToken).toBe("r1");
+    expect(store.value?.expiresAt).toBeGreaterThan(Date.now());
+    expect(oauth.isAuthorized()).toBe(true);
+  });
+
+  it("rejects an unknown state without calling the token endpoint (CSRF guard)", async () => {
+    const http = { post: vi.fn() } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      redirectUri: REDIRECT_URI,
+      store: memStore(),
+      deps: { http },
+    });
+    expect(await oauth.handleCallback("CODE", "not-a-real-state")).toBe(false);
+    expect(http.post).not.toHaveBeenCalled();
+  });
+
+  // Correction C3.7: the state->verifier entry is deleted on EVERY terminal
+  // path (finally), so a failed login never leaks it and cannot be replayed.
+  it("deletes the pending verifier even when the token exchange fails", async () => {
+    const http = {
+      post: vi.fn().mockRejectedValue({
+        response: { status: 400, data: { error: "invalid_grant" } },
+      }),
+    } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      redirectUri: REDIRECT_URI,
+      store: memStore(),
+      deps: { http },
+    });
+    const { state } = oauth.buildAuthorizeUrl();
+
+    // First attempt fails at the network/token step.
+    expect(await oauth.handleCallback("CODE", state)).toBe(false);
+    expect(http.post).toHaveBeenCalledTimes(1);
+
+    // Replaying the same state now fails the CSRF guard (verifier was deleted),
+    // WITHOUT hitting the token endpoint again.
+    expect(await oauth.handleCallback("CODE", state)).toBe(false);
+    expect(http.post).toHaveBeenCalledTimes(1);
+  });
+});
+
+describe("SpotifyOAuth.getAccessToken", () => {
+  it("returns the cached token without refreshing when still valid", async () => {
+    const http = { post: vi.fn() } as any;
+    const store = memStore({
+      accessToken: "cached",
+      refreshToken: "r1",
+      expiresAt: Date.now() + 60_000,
+      scope: "s",
+    });
+    const oauth = new SpotifyOAuth({ clientId: CLIENT_ID, store, deps: { http } });
+    expect(await oauth.getAccessToken()).toBe("cached");
+    expect(http.post).not.toHaveBeenCalled();
+  });
+
+  it("refreshes when expired and persists the ROTATED refresh token", async () => {
+    const store = memStore({
+      accessToken: "old",
+      refreshToken: "r1",
+      expiresAt: Date.now() - 1000,
+      scope: "s",
+    });
+    const http = {
+      post: vi.fn().mockResolvedValue({
+        data: { access_token: "a2", refresh_token: "r2", expires_in: 3600 },
+      }),
+    } as any;
+    const oauth = new SpotifyOAuth({ clientId: CLIENT_ID, store, deps: { http } });
+
+    expect(await oauth.getAccessToken()).toBe("a2");
+    const body = new URLSearchParams(http.post.mock.calls[0][1] as string);
+    expect(body.get("grant_type")).toBe("refresh_token");
+    expect(body.get("refresh_token")).toBe("r1");
+    expect(body.get("client_id")).toBe(CLIENT_ID);
+    expect(store.value?.refreshToken).toBe("r2"); // rotated + persisted
+    expect(store.value?.accessToken).toBe("a2");
+  });
+
+  it("keeps the old refresh token when the refresh response omits a new one", async () => {
+    const store = memStore({
+      accessToken: "old",
+      refreshToken: "r1",
+      expiresAt: Date.now() - 1000,
+      scope: "s",
+    });
+    const http = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "a2", expires_in: 3600 } }),
+    } as any;
+    const oauth = new SpotifyOAuth({ clientId: CLIENT_ID, store, deps: { http } });
+    expect(await oauth.getAccessToken()).toBe("a2");
+    expect(store.value?.refreshToken).toBe("r1");
+  });
+
+  it("clears the store and returns null on invalid_grant (expired refresh token)", async () => {
+    const store = memStore({
+      accessToken: "old",
+      refreshToken: "r1",
+      expiresAt: Date.now() - 1000,
+      scope: "s",
+    });
+    const http = {
+      post: vi.fn().mockRejectedValue({
+        response: { status: 400, data: { error: "invalid_grant" } },
+      }),
+    } as any;
+    const oauth = new SpotifyOAuth({ clientId: CLIENT_ID, store, deps: { http } });
+    expect(await oauth.getAccessToken()).toBeNull();
+    expect(store.value).toBeNull();
+    expect(oauth.isAuthorized()).toBe(false);
+  });
+
+  it("returns null when unauthorized (no stored refresh token)", async () => {
+    const http = { post: vi.fn() } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      store: memStore(),
+      deps: { http },
+    });
+    expect(await oauth.getAccessToken()).toBeNull();
+    expect(oauth.isAuthorized()).toBe(false);
+    expect(http.post).not.toHaveBeenCalled();
+  });
+});
+
+// S4.3: refresh-token rotation makes two concurrent refreshes race — the second
+// would POST with a token the first already invalidated. Collapse them into one.
+describe("SpotifyOAuth.getAccessToken (in-flight refresh, S4.3)", () => {
+  it("collapses concurrent refreshes into a single token POST", async () => {
+    let t = 0;
+    const now = () => t;
+    const store = memStore({
+      accessToken: "old",
+      refreshToken: "r1",
+      expiresAt: 0, // now()=0 is not < 0 -> expired -> must refresh
+      scope: "s",
+    });
+    const d = deferred<any>();
+    const http = { post: vi.fn().mockReturnValue(d.promise) } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      store,
+      deps: { http, now },
+    });
+
+    // Fire two calls while the POST is still pending.
+    const p1 = oauth.getAccessToken();
+    const p2 = oauth.getAccessToken();
+    expect(http.post).toHaveBeenCalledTimes(1); // collapsed to ONE POST
+
+    d.resolve({
+      data: { access_token: "a2", refresh_token: "r2", expires_in: 3600 },
+    });
+    expect(await p1).toBe("a2");
+    expect(await p2).toBe("a2");
+    expect(http.post).toHaveBeenCalledTimes(1);
+    expect(store.value?.refreshToken).toBe("r2"); // rotated once, not twice
+  });
+
+  it("clears the in-flight refresh after it settles, allowing a later refresh", async () => {
+    let t = 0;
+    const now = () => t;
+    const store = memStore({
+      accessToken: "old",
+      refreshToken: "r1",
+      expiresAt: 0,
+      scope: "s",
+    });
+    const http = {
+      post: vi
+        .fn()
+        .mockResolvedValueOnce({
+          data: { access_token: "a2", refresh_token: "r2", expires_in: 3600 },
+        })
+        .mockResolvedValueOnce({
+          data: { access_token: "a3", refresh_token: "r3", expires_in: 3600 },
+        }),
+    } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      store,
+      deps: { http, now },
+    });
+
+    expect(await oauth.getAccessToken()).toBe("a2");
+    expect(http.post).toHaveBeenCalledTimes(1);
+
+    // toTokens now uses this.now(): saved expiresAt = 0 + 3600s - 30s skew.
+    // Still valid at t=0 -> cached, no new POST.
+    expect(await oauth.getAccessToken()).toBe("a2");
+    expect(http.post).toHaveBeenCalledTimes(1);
+
+    // Advance past the newly-saved expiry -> in-flight was cleared, so a fresh
+    // refresh fires (proves .finally() reset refreshInFlight).
+    t = 3600 * 1000;
+    expect(await oauth.getAccessToken()).toBe("a3");
+    expect(http.post).toHaveBeenCalledTimes(2);
+    expect(store.value?.refreshToken).toBe("r3");
+  });
+});
+
+// S4.3: bound the PKCE verifier map so abandoned logins can't accumulate.
+describe("SpotifyOAuth PKCE verifier TTL + cap (S4.3)", () => {
+  it("expires a pending verifier after the TTL (handleCallback returns false)", async () => {
+    let t = 0;
+    const now = () => t;
+    const http = { post: vi.fn() } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      redirectUri: REDIRECT_URI,
+      store: memStore(),
+      deps: { http, now },
+    });
+    const { state } = oauth.buildAuthorizeUrl();
+
+    t = 10 * 60 * 1000 + 1; // VERIFIER_TTL_MS + 1
+    expect(await oauth.handleCallback("CODE", state)).toBe(false);
+    expect(http.post).not.toHaveBeenCalled(); // never reached the token step
+  });
+
+  it("caps the verifier map, evicting the oldest state (behavioral)", async () => {
+    let t = 0;
+    const now = () => t;
+    const http = {
+      post: vi.fn().mockResolvedValue({
+        data: { access_token: "a1", refresh_token: "r1", expires_in: 3600 },
+      }),
+    } as any;
+    const oauth = new SpotifyOAuth({
+      clientId: CLIENT_ID,
+      redirectUri: REDIRECT_URI,
+      store: memStore(),
+      deps: { http, now },
+    });
+
+    // First (oldest) state, then enough more to exceed VERIFIER_MAX (32).
+    const first = oauth.buildAuthorizeUrl().state;
+    let last = first;
+    for (let i = 0; i < 32; i++) last = oauth.buildAuthorizeUrl().state;
+
+    // The oldest was evicted -> unknown state -> CSRF guard, no token POST.
+    expect(await oauth.handleCallback("CODE", first)).toBe(false);
+    expect(http.post).not.toHaveBeenCalled();
+
+    // A still-pending (newest) state DOES resolve -> only the oldest was dropped.
+    expect(await oauth.handleCallback("CODE", last)).toBe(true);
+    expect(http.post).toHaveBeenCalledTimes(1);
+  });
+});
+
+// Whole-branch I2: a UI-entered Client ID (saved in Settings) must reach the
+// single live SpotifyOAuth WITHOUT a process restart. configure() re-arms the
+// runtime credentials; an empty clientId re-disables OAuth.
+describe("SpotifyOAuth.configure (runtime credentials, whole-branch I2)", () => {
+  it("applies a UI-entered clientId + redirectUri so OAuth arms without a restart", () => {
+    // Fresh install: boot-time config had no clientId, so OAuth is disabled.
+    const store = memStore({
+      accessToken: "a",
+      refreshToken: "r",
+      expiresAt: Date.now() + 60_000,
+      scope: "s",
+    });
+    const oauth = new SpotifyOAuth({ clientId: "", store });
+    expect(oauth.isAuthorized()).toBe(false);
+    expect(() => oauth.buildAuthorizeUrl()).toThrow(/Client ID/i);
+
+    // Operator enters creds in Settings -> POST /settings calls configure().
+    const REDIRECT = "http://127.0.0.1:3000/api/spotify/callback";
+    oauth.configure("cid", REDIRECT);
+    expect(oauth.getClientId()).toBe("cid");
+    expect(oauth.getRedirectUri()).toBe(REDIRECT);
+    // Now authorize + isAuthorized work against the newly-supplied app.
+    expect(oauth.isAuthorized()).toBe(true);
+    const { url } = oauth.buildAuthorizeUrl();
+    const p = new URL(url).searchParams;
+    expect(p.get("client_id")).toBe("cid");
+    expect(p.get("redirect_uri")).toBe(REDIRECT);
+  });
+
+  it("trims the clientId and re-disables OAuth when cleared", () => {
+    const oauth = new SpotifyOAuth({
+      clientId: "old",
+      redirectUri: "http://old",
+      store: memStore(),
+    });
+    oauth.configure("  cid  ", "http://127.0.0.1:3000/api/spotify/callback");
+    expect(oauth.getClientId()).toBe("cid"); // trimmed
+
+    // Empty clientId disables OAuth again (gate on buildAuthorizeUrl/isAuthorized).
+    oauth.configure("");
+    expect(oauth.getClientId()).toBe("");
+    expect(oauth.getRedirectUri()).toBe("");
+    expect(oauth.isAuthorized()).toBe(false);
+    expect(() => oauth.buildAuthorizeUrl()).toThrow(/Client ID/i);
+  });
+});
+
+describe("createFileOAuthTokenStore", () => {
+  it("round-trips save/load and clear() removes it", () => {
+    const dir = mkdtempSync(join(tmpdir(), "sp-oauth-"));
+    const file = join(dir, "nested", "tokens.json");
+    try {
+      const store = createFileOAuthTokenStore(file);
+      expect(store.load()).toBeNull(); // missing file
+      const t: OAuthTokens = {
+        accessToken: "a",
+        refreshToken: "r",
+        expiresAt: 123,
+        scope: "s",
+      };
+      store.save(t);
+      expect(store.load()).toEqual(t);
+      store.clear();
+      expect(store.load()).toBeNull();
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+});
+
+// R3-5: the token store save() must be atomic (same-dir temp file + rename), so
+// a crash / power loss / ENOSPC while persisting a ROTATED refresh token (Spotify
+// invalidates the OLD one the instant it responds — the new one lives only in
+// memory until this write lands) can never truncate the file and silently
+// de-authenticate the operator. Mirrors the hardened config.ts saveConfig.
+describe("createFileOAuthTokenStore atomic write (R3-5)", () => {
+  const dirs: string[] = [];
+  function makeTmpDir(): string {
+    const dir = mkdtempSync(join(tmpdir(), "sp-oauth-atomic-"));
+    dirs.push(dir);
+    return dir;
+  }
+  beforeEach(() => {
+    vi.clearAllMocks(); // reset call history, keep the call-through implementations
+  });
+  afterEach(() => {
+    for (const d of dirs) rmSync(d, { recursive: true, force: true });
+    dirs.length = 0;
+  });
+
+  const TOK: OAuthTokens = {
+    accessToken: "a",
+    refreshToken: "r",
+    expiresAt: 123,
+    scope: "s",
+  };
+
+  it("round-trips save/load and leaves NO .tmp file behind", () => {
+    const dir = makeTmpDir();
+    const file = join(dir, "tokens.json");
+    const store = createFileOAuthTokenStore(file);
+
+    store.save(TOK);
+
+    expect(store.load()).toEqual(TOK);
+    // No temp remnants in the target directory.
+    expect(readdirSync(dir).filter((f) => f.includes(".tmp"))).toEqual([]);
+  });
+
+  it("writes via a same-dir temp file then renameSync onto the final path", () => {
+    const dir = makeTmpDir();
+    const file = join(dir, "tokens.json");
+
+    createFileOAuthTokenStore(file).save(TOK);
+
+    expect(vi.mocked(renameSync)).toHaveBeenCalled();
+    const [from, to] = vi.mocked(renameSync).mock.calls[0] as [string, string];
+    expect(to).toBe(file); // renamed ONTO the real path
+    expect(String(from)).not.toBe(file); // ...from a distinct temp file
+    expect(join(String(from), "..")).toBe(join(file, "..")); // ...in the SAME dir
+  });
+
+  it("persists the token file with 0600 permissions (POSIX)", () => {
+    if (process.platform === "win32") return; // mode bits aren't meaningful on Windows
+    const dir = makeTmpDir();
+    const file = join(dir, "tokens.json");
+    createFileOAuthTokenStore(file).save(TOK);
+    expect(statSync(file).mode & 0o777).toBe(0o600);
+  });
+
+  it("does NOT truncate/corrupt a pre-existing valid token file when the write fails mid-way", () => {
+    const dir = makeTmpDir();
+    const file = join(dir, "tokens.json");
+    const store = createFileOAuthTokenStore(file);
+
+    // A valid, previously-persisted token file (the live refresh token on disk).
+    store.save(TOK);
+    const before = readFileSync(file, "utf-8");
+
+    // Simulate a crash / ENOSPC at the atomic-replace step while persisting a
+    // ROTATED refresh token.
+    const rotated: OAuthTokens = { ...TOK, accessToken: "a2", refreshToken: "r2" };
+    vi.mocked(renameSync).mockImplementationOnce(() => {
+      throw new Error("rename boom");
+    });
+
+    expect(() => store.save(rotated)).toThrow(/rename boom/);
+
+    // The original file is untouched: present, byte-identical, still parseable.
+    expect(readFileSync(file, "utf-8")).toBe(before);
+    expect(store.load()).toEqual(TOK);
+    // ...and the failed write left no temp file lying around.
+    expect(readdirSync(dir).filter((f) => f.includes(".tmp"))).toEqual([]);
+  });
+});

src/music/spotify/spotify-oauth.ts

@@ -0,0 +1,298 @@
+import axios, { type AxiosInstance } from "axios";
+import { createHash, randomBytes } from "node:crypto";
+import {
+  existsSync,
+  mkdirSync,
+  readFileSync,
+  renameSync,
+  rmSync,
+  writeFileSync,
+} from "node:fs";
+import { dirname } from "node:path";
+
+/**
+ * Player-control scopes requested for the USER token: streaming (drives
+ * librespot as a Connect device) + read/modify playback + currently-playing +
+ * private-playlist reads.
+ */
+export const SPOTIFY_CONTROL_SCOPES =
+  "streaming user-read-playback-state user-modify-playback-state user-read-currently-playing playlist-read-private";
+
+const ACCOUNTS_BASE = "https://accounts.spotify.com";
+// Hand a token back only if it survives ~30s, matching webapi.ts's skew.
+const EXPIRY_SKEW_MS = 30_000;
+// RFC 7636 §4.1 unreserved set: [A-Za-z0-9-._~].
+const PKCE_CHARS =
+  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~";
+const FORM_HEADERS = { "Content-Type": "application/x-www-form-urlencoded" };
+
+export interface OAuthTokens {
+  accessToken: string;
+  refreshToken: string;
+  expiresAt: number;
+  scope: string;
+}
+
+export interface OAuthTokenStore {
+  load(): OAuthTokens | null;
+  save(t: OAuthTokens): void;
+  clear(): void;
+}
+
+export interface SpotifyOAuthOptions {
+  /**
+   * Correction C3.2: the caller's OWN Spotify Developer app client_id. There is
+   * no librespot-public-client fallback — an empty clientId disables OAuth.
+   */
+  clientId?: string;
+  /**
+   * Loopback redirect registered on the caller's Spotify app, supplied by the
+   * bot's web layer (e.g. its `/api/spotify/callback`). Must exactly match the
+   * value used at both the authorize and token steps.
+   */
+  redirectUri?: string;
+  store: OAuthTokenStore;
+  deps?: { http?: AxiosInstance; now?: () => number };
+}
+
+/** 64 random chars from the PKCE unreserved set (43-128 allowed by the spec). */
+export function generateCodeVerifier(): string {
+  const bytes = randomBytes(64);
+  let out = "";
+  for (let i = 0; i < 64; i++) out += PKCE_CHARS[bytes[i] % PKCE_CHARS.length];
+  return out;
+}
+
+/** base64url(SHA256(verifier)) with no padding — the S256 code challenge. */
+export function codeChallengeS256(verifier: string): string {
+  return createHash("sha256").update(verifier).digest("base64url");
+}
+
+/** Persist OAuth tokens as a 0600 JSON file (used by the controller). */
+export function createFileOAuthTokenStore(filePath: string): OAuthTokenStore {
+  return {
+    load() {
+      try {
+        if (!existsSync(filePath)) return null;
+        const parsed = JSON.parse(readFileSync(filePath, "utf8"));
+        return parsed?.refreshToken ? (parsed as OAuthTokens) : null;
+      } catch {
+        return null; // missing/corrupt -> treat as unauthorized
+      }
+    },
+    save(t: OAuthTokens) {
+      mkdirSync(dirname(filePath), { recursive: true });
+      // Atomic write: serialize to a sibling temp file in the SAME directory,
+      // then rename it onto the final path. rename is an atomic replace on POSIX
+      // and modern Windows, so a crash / power loss / ENOSPC mid-write can never
+      // leave the token file truncated — a reader always sees either the previous
+      // file or the fully-written new one, never a partial. This matters because
+      // refresh() persists a ROTATED refresh token here: Spotify invalidates the
+      // OLD one the instant it responds, so a torn write would silently
+      // de-authenticate the operator (next load() JSON.parse-fails -> null ->
+      // full PKCE re-login). Mirrors config.ts saveConfig. The temp lives in the
+      // same dir so the rename stays on one filesystem; pid + timestamp keep
+      // concurrent writers from colliding on the temp name. 0600 is preserved.
+      const tmp = `${filePath}.${process.pid}.${Date.now()}.tmp`;
+      try {
+        writeFileSync(tmp, JSON.stringify(t, null, 2), { mode: 0o600 });
+        renameSync(tmp, filePath);
+      } catch (err) {
+        // Never leave a partial temp file behind on failure.
+        try {
+          rmSync(tmp, { force: true });
+        } catch {
+          /* best-effort cleanup */
+        }
+        throw err;
+      }
+    },
+    clear() {
+      try {
+        rmSync(filePath, { force: true });
+      } catch {
+        /* already gone */
+      }
+    },
+  };
+}
+
+/**
+ * Authorization Code + PKCE flow for the USER player-control token. Public
+ * client (no secret).
+ *
+ * Correction C3.2: this REQUIRES the operator's own registered Spotify app —
+ * there is NO reuse of librespot's first-party keymaster client / fixed
+ * :5588 redirect. Without a clientId, `isAuthorized()` is false and
+ * `buildAuthorizeUrl()` throws.
+ *
+ * Refresh rotates the refresh token, so the newest is always persisted;
+ * invalid_grant clears the store (re-login required). Access/refresh tokens
+ * are never logged.
+ */
+export class SpotifyOAuth {
+  private clientId: string;
+  private redirectUri: string;
+  private store: OAuthTokenStore;
+  private http: AxiosInstance;
+  // Injectable clock (tests drive a mutable now); defaults to Date.now.
+  private now: () => number;
+  // Pending PKCE verifiers keyed by state, awaiting the loopback redirect back.
+  private pendingVerifiers = new Map<
+    string,
+    { verifier: string; expiresAt: number }
+  >();
+  // A verifier is abandoned if the redirect never returns; drop it after TTL and
+  // cap the map so parallel logins can't grow it without bound.
+  private static readonly VERIFIER_TTL_MS = 10 * 60 * 1000;
+  private static readonly VERIFIER_MAX = 32;
+  // Collapse concurrent refreshes into one POST (rotation invalidates the token
+  // a second in-flight refresh would send); cleared in .finally().
+  private refreshInFlight: Promise<string | null> | null = null;
+
+  constructor(o: SpotifyOAuthOptions) {
+    this.clientId = o.clientId ?? "";
+    this.redirectUri = o.redirectUri ?? "";
+    this.store = o.store;
+    this.http =
+      o.deps?.http ?? axios.create({ baseURL: ACCOUNTS_BASE, timeout: 15_000 });
+    this.now = o.deps?.now ?? (() => Date.now());
+  }
+
+  private evictStaleVerifiers(): void {
+    const t = this.now();
+    for (const [state, e] of this.pendingVerifiers) {
+      if (e.expiresAt < t) this.pendingVerifiers.delete(state);
+    }
+    // Bound memory even if all are unexpired: drop oldest (insertion order).
+    while (this.pendingVerifiers.size >= SpotifyOAuth.VERIFIER_MAX) {
+      const oldest = this.pendingVerifiers.keys().next().value;
+      if (oldest === undefined) break;
+      this.pendingVerifiers.delete(oldest);
+    }
+  }
+
+  /** Update the operator's app credentials at runtime (from Settings save) so
+   *  a UI-entered Client ID takes effect without a process restart. Empty
+   *  clientId disables OAuth (isAuthorized()/buildAuthorizeUrl() gate on it). */
+  configure(clientId?: string, redirectUri?: string): void {
+    this.clientId = clientId?.trim() || "";
+    this.redirectUri = redirectUri || "";
+  }
+
+  getClientId(): string {
+    return this.clientId;
+  }
+
+  getRedirectUri(): string {
+    return this.redirectUri;
+  }
+
+  isAuthorized(): boolean {
+    // C3.2: no client_id means we could never refresh, so treat as unauthorized.
+    return !!this.clientId && !!this.store.load()?.refreshToken;
+  }
+
+  buildAuthorizeUrl(): { url: string; state: string } {
+    if (!this.clientId) {
+      // C3.2: cannot start OAuth against nobody's app.
+      throw new Error("Set your Spotify Client ID in settings first");
+    }
+    const state = randomBytes(16).toString("hex");
+    const verifier = generateCodeVerifier();
+    this.evictStaleVerifiers();
+    this.pendingVerifiers.set(state, {
+      verifier,
+      expiresAt: this.now() + SpotifyOAuth.VERIFIER_TTL_MS,
+    });
+    const params = new URLSearchParams({
+      client_id: this.clientId,
+      response_type: "code",
+      redirect_uri: this.redirectUri,
+      code_challenge: codeChallengeS256(verifier),
+      code_challenge_method: "S256",
+      scope: SPOTIFY_CONTROL_SCOPES,
+      state,
+    });
+    return { url: `${ACCOUNTS_BASE}/authorize?${params.toString()}`, state };
+  }
+
+  async handleCallback(code: string, state: string): Promise<boolean> {
+    const entry = this.pendingVerifiers.get(state);
+    if (!entry || entry.expiresAt < this.now()) {
+      // unknown/expired state -> CSRF guard (drop any stale entry too)
+      this.pendingVerifiers.delete(state);
+      return false;
+    }
+    const verifier = entry.verifier;
+    // C3.7: drop the state->verifier entry on EVERY terminal path (success,
+    // rejected token exchange, or throw) so a failed login can't leak/replay it.
+    try {
+      const body = new URLSearchParams({
+        grant_type: "authorization_code",
+        code,
+        redirect_uri: this.redirectUri,
+        client_id: this.clientId,
+        code_verifier: verifier,
+      });
+      const { data } = await this.http.post("/api/token", body.toString(), {
+        headers: FORM_HEADERS,
+      });
+      if (!data?.access_token || !data?.refresh_token) return false;
+      this.store.save(this.toTokens(data, data.refresh_token, data.scope));
+      return true;
+    } catch {
+      return false;
+    } finally {
+      this.pendingVerifiers.delete(state);
+    }
+  }
+
+  async getAccessToken(): Promise<string | null> {
+    if (!this.clientId) return null; // C3.2: no app => nothing to mint against
+    const tokens = this.store.load();
+    if (!tokens?.refreshToken) return null; // unauthorized
+    if (tokens.accessToken && this.now() < tokens.expiresAt) {
+      return tokens.accessToken;
+    }
+    // Collapse concurrent refreshes: rotation makes a second in-flight refresh
+    // use a refresh token the first one already invalidated.
+    if (this.refreshInFlight) return this.refreshInFlight;
+    this.refreshInFlight = this.refresh(tokens).finally(() => {
+      this.refreshInFlight = null;
+    });
+    return this.refreshInFlight;
+  }
+
+  private async refresh(current: OAuthTokens): Promise<string | null> {
+    const body = new URLSearchParams({
+      grant_type: "refresh_token",
+      refresh_token: current.refreshToken,
+      client_id: this.clientId,
+    });
+    try {
+      const { data } = await this.http.post("/api/token", body.toString(), {
+        headers: FORM_HEADERS,
+      });
+      if (!data?.access_token) return null;
+      // PKCE rotates the refresh token; fall back to the current one if omitted.
+      const rotated = data.refresh_token || current.refreshToken;
+      const saved = this.toTokens(data, rotated, data.scope ?? current.scope);
+      this.store.save(saved);
+      return saved.accessToken;
+    } catch (err: any) {
+      // invalid_grant => refresh token revoked/expired: discard, force re-login.
+      if (err?.response?.data?.error === "invalid_grant") this.store.clear();
+      return null;
+    }
+  }
+
+  private toTokens(data: any, refreshToken: string, scope: string): OAuthTokens {
+    return {
+      accessToken: data.access_token,
+      refreshToken,
+      expiresAt: this.now() + (data.expires_in ?? 3600) * 1000 - EXPIRY_SKEW_MS,
+      scope: scope ?? SPOTIFY_CONTROL_SCOPES,
+    };
+  }
+}

src/music/spotify/webapi.test.ts

@@ -118,8 +118,460 @@ describe("SpotifyWebApi rate-limit handling", () => {
     expect(out.songs[0].id).toBe("t1");
   });
 
+  // I5: the retry is BOUNDED — get() passes `false` on the recursive call so a
+  // second 429 is NOT retried. Without that bound arg this recurses forever;
+  // this test must FAIL (time out) if the `false` in `this.get(path, params, false)`
+  // is removed. retry-after "0" keeps the single permitted wait instant.
+  it("gives up after exactly ONE 429 retry when every call 429s (bounded, no infinite loop)", async () => {
+    const auth = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+    } as any;
+    const http = {
+      get: vi.fn().mockRejectedValue({
+        response: { status: 429, headers: { "retry-after": "0" } },
+      }),
+    } as any;
+    const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+    const out = await api.search("queen");
+    // Exactly two attempts: the original + one bounded retry, then it stops.
+    expect(http.get).toHaveBeenCalledTimes(2);
+    // Giving up returns null from get() → search yields an empty (non-throwing) result.
+    expect(out).toEqual({ songs: [], playlists: [], albums: [] });
+  });
+
+  // I5: the advised wait is capped by Math.min(retryAfter, 10). A large Retry-After
+  // (999s) must still wait only 10s, proving the cap. Fake timers keep it instant
+  // while letting us assert the retry fires at 10s, not before.
+  it("caps the Retry-After wait at 10s (Math.min(retryAfter, 10))", async () => {
+    vi.useFakeTimers();
+    try {
+      const auth = {
+        post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+      } as any;
+      let call = 0;
+      const http = {
+        get: vi.fn().mockImplementation(() => {
+          call += 1;
+          if (call === 1) {
+            return Promise.reject({ response: { status: 429, headers: { "retry-after": "999" } } });
+          }
+          return Promise.resolve({
+            data: { tracks: { items: [{ id: "t1", name: "n", artists: [], duration_ms: 1000 }] } },
+          });
+        }),
+      } as any;
+      const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+      const p = api.search("queen");
+
+      // Let the token fetch + first (429) call settle and schedule the wait.
+      await vi.advanceTimersByTimeAsync(9_000); // 9s < cap → retry has NOT fired yet
+      expect(http.get).toHaveBeenCalledTimes(1);
+
+      await vi.advanceTimersByTimeAsync(1_000); // now 10s total → cap reached, retry fires
+      const out = await p;
+      expect(http.get).toHaveBeenCalledTimes(2);
+      expect(out.songs[0].id).toBe("t1");
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  // Corner-case: a non-numeric Retry-After (HTTP-date) must NOT coerce to NaN and
+  // fire the retry at 0ms. `Number("Wed, 21 Oct 2025 07:28:00 GMT")` is NaN, so the
+  // pre-fix `Math.min(NaN,10)*1000` schedules an immediate (or never-firing) retry
+  // that ignores the advised backoff. Post-fix falls back to a finite 1s wait: the
+  // retry stays scheduled at exactly the fallback, still bounded to ONE retry.
+  it("falls back to a finite 1s wait when Retry-After is an HTTP-date (not NaN/immediate)", async () => {
+    vi.useFakeTimers();
+    try {
+      const auth = {
+        post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+      } as any;
+      let call = 0;
+      const http = {
+        get: vi.fn().mockImplementation(() => {
+          call += 1;
+          if (call === 1) {
+            return Promise.reject({
+              response: {
+                status: 429,
+                headers: { "retry-after": "Wed, 21 Oct 2025 07:28:00 GMT" },
+              },
+            });
+          }
+          return Promise.resolve({
+            data: { tracks: { items: [{ id: "t1", name: "n", artists: [], duration_ms: 1000 }] } },
+          });
+        }),
+      } as any;
+      const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+      const p = api.search("queen");
+
+      // Let the token fetch + first (429) call settle and schedule the wait.
+      await vi.advanceTimersByTimeAsync(999); // < 1s fallback → retry has NOT fired yet
+      expect(http.get).toHaveBeenCalledTimes(1);
+
+      await vi.advanceTimersByTimeAsync(1); // now 1s total → finite fallback reached, retry fires
+      const out = await p;
+      expect(http.get).toHaveBeenCalledTimes(2); // exactly one bounded retry
+      expect(out.songs[0].id).toBe("t1");
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  // Corner-case sibling: an empty Retry-After coerces to 0 (`Number("")` === 0), so
+  // pre-fix `Math.min(0,10)*1000` fires the retry immediately at 0ms. Post-fix guards
+  // `raw > 0`, so it falls back to the same finite 1s wait rather than firing at 0.
+  it("falls back to a finite 1s wait when Retry-After is empty (not 0/immediate)", async () => {
+    vi.useFakeTimers();
+    try {
+      const auth = {
+        post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+      } as any;
+      let call = 0;
+      const http = {
+        get: vi.fn().mockImplementation(() => {
+          call += 1;
+          if (call === 1) {
+            return Promise.reject({ response: { status: 429, headers: { "retry-after": "" } } });
+          }
+          return Promise.resolve({
+            data: { tracks: { items: [{ id: "t1", name: "n", artists: [], duration_ms: 1000 }] } },
+          });
+        }),
+      } as any;
+      const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+      const p = api.search("queen");
+
+      await vi.advanceTimersByTimeAsync(999); // < 1s fallback → retry must NOT have fired at 0ms
+      expect(http.get).toHaveBeenCalledTimes(1);
+
+      await vi.advanceTimersByTimeAsync(1); // 1s total → fallback reached, retry fires
+      const out = await p;
+      expect(http.get).toHaveBeenCalledTimes(2);
+      expect(out.songs[0].id).toBe("t1");
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  // Regression guard: a NORMAL numeric Retry-After ("3") is finite/positive, so the
+  // guard is transparent — the advised ~3s wait (below the 10s cap) is honored intact.
+  it("still honors a normal numeric Retry-After (~3s, below the 10s cap)", async () => {
+    vi.useFakeTimers();
+    try {
+      const auth = {
+        post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+      } as any;
+      let call = 0;
+      const http = {
+        get: vi.fn().mockImplementation(() => {
+          call += 1;
+          if (call === 1) {
+            return Promise.reject({ response: { status: 429, headers: { "retry-after": "3" } } });
+          }
+          return Promise.resolve({
+            data: { tracks: { items: [{ id: "t1", name: "n", artists: [], duration_ms: 1000 }] } },
+          });
+        }),
+      } as any;
+      const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+      const p = api.search("queen");
+
+      await vi.advanceTimersByTimeAsync(2_999); // < 3s advised → retry has NOT fired yet
+      expect(http.get).toHaveBeenCalledTimes(1);
+
+      await vi.advanceTimersByTimeAsync(1); // 3s total → advised wait reached, retry fires
+      const out = await p;
+      expect(http.get).toHaveBeenCalledTimes(2);
+      expect(out.songs[0].id).toBe("t1");
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
   it("returns empty results when unconfigured (no creds → no token)", async () => {
     const api = new SpotifyWebApi(() => ({ clientId: "", clientSecret: "" }));
     expect(await api.search("queen")).toEqual({ songs: [], playlists: [], albums: [] });
   });
 });
+
+// m4: the catalog fetch mappers (getTrack / getAlbumTracks / getPlaylistTracks)
+// were untested. They shape raw Spotify payloads into Songs, inject album cover
+// context, and drop malformed playlist rows.
+describe("SpotifyWebApi catalog mappers", () => {
+  /** Builds an api whose single http.get resolves the supplied payload. */
+  function makeApi(payload: unknown) {
+    const auth = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+    } as any;
+    const http = { get: vi.fn().mockResolvedValue({ data: payload }) } as any;
+    const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+    return { api, http };
+  }
+
+  describe("getTrack", () => {
+    it("maps a single track payload to a Song", async () => {
+      const { api } = makeApi({
+        id: "trk1",
+        name: "Song One",
+        artists: [{ name: "Alice" }, { name: "Bob" }],
+        album: { name: "Album X", images: [{ url: "https://i.scdn.co/t.jpg" }] },
+        duration_ms: 210000,
+      });
+      const s = await api.getTrack("trk1");
+      expect(s).toEqual({
+        id: "trk1",
+        name: "Song One",
+        artist: "Alice, Bob",
+        album: "Album X",
+        duration: 210,
+        coverUrl: "https://i.scdn.co/t.jpg",
+        platform: "spotify",
+      });
+    });
+
+    it("returns null when the track fetch yields no data", async () => {
+      const auth = {
+        post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+      } as any;
+      const http = { get: vi.fn().mockResolvedValue({ data: null }) } as any;
+      const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+      expect(await api.getTrack("nope")).toBeNull();
+    });
+  });
+
+  describe("getAlbumTracks", () => {
+    it("injects the album name + cover into every returned track", async () => {
+      // Album-track objects omit their own album block; the album cover/name must
+      // be back-filled from the album payload.
+      const { api } = makeApi({
+        name: "A Night at the Opera",
+        images: [{ url: "https://i.scdn.co/album.jpg" }],
+        tracks: {
+          items: [
+            { id: "a1", name: "Death on Two Legs", artists: [{ name: "Queen" }], duration_ms: 223000 },
+            { id: "a2", name: "Lazing on a Sunday", artists: [{ name: "Queen" }], duration_ms: 68000 },
+          ],
+        },
+      });
+      const out = await api.getAlbumTracks("alb1");
+      expect(out).toHaveLength(2);
+      for (const t of out) {
+        expect(t.album).toBe("A Night at the Opera");
+        expect(t.coverUrl).toBe("https://i.scdn.co/album.jpg");
+        expect(t.platform).toBe("spotify");
+      }
+      expect(out[0].id).toBe("a1");
+      expect(out[0].name).toBe("Death on Two Legs");
+      expect(out[1].id).toBe("a2");
+    });
+
+    it("drops null track entries and returns [] when tracks.items is absent", async () => {
+      const { api: withNull } = makeApi({
+        name: "Alb",
+        images: [{ url: "https://i.scdn.co/c.jpg" }],
+        tracks: { items: [null, { id: "ok", name: "Keep", artists: [], duration_ms: 1000 }] },
+      });
+      const out = await withNull.getAlbumTracks("alb1");
+      expect(out).toHaveLength(1);
+      expect(out[0].id).toBe("ok");
+      expect(out[0].coverUrl).toBe("https://i.scdn.co/c.jpg");
+
+      const { api: noItems } = makeApi({ name: "Alb", images: [], tracks: {} });
+      expect(await noItems.getAlbumTracks("alb1")).toEqual([]);
+    });
+  });
+
+  describe("getPlaylistTracks", () => {
+    it("filters null items, {track:null}, and id-less tracks (never emits an id:'' Song)", async () => {
+      const { api } = makeApi({
+        items: [
+          null,
+          { track: null },
+          { track: { name: "No Id Here", artists: [], duration_ms: 1000 } }, // track present but no id
+          { track: { id: "p1", name: "Real Song", artists: [{ name: "X" }], duration_ms: 1000 } },
+        ],
+      });
+      const out = await api.getPlaylistTracks("pl1");
+      expect(out).toHaveLength(1);
+      expect(out[0].id).toBe("p1");
+      expect(out[0].name).toBe("Real Song");
+      // Guard the specific defect: no malformed empty-id Song leaks through.
+      expect(out.every((s) => s.id !== "")).toBe(true);
+    });
+
+    it("returns [] when items is absent", async () => {
+      const { api } = makeApi({});
+      expect(await api.getPlaylistTracks("pl1")).toEqual([]);
+    });
+  });
+});
+
+// R2-3: getPlaylistTracks must paginate (follow data.next) rather than silently
+// truncating to the first 100 tracks, and must stop at a bounded cap so a
+// pathological 10k-track playlist can't fan out into dozens of API calls.
+describe("getPlaylistTracks pagination (R2-3)", () => {
+  const trackItem = (i: number) => ({
+    track: { id: `p${i}`, name: `Song ${i}`, artists: [{ name: "X" }], duration_ms: 1000 },
+  });
+
+  it("follows data.next across pages, returning all tracks in order with nulls filtered", async () => {
+    const auth = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+    } as any;
+    const page1 = {
+      items: Array.from({ length: 100 }, (_, i) => trackItem(i)),
+      next: "https://api.spotify.com/v1/playlists/pl1/tracks?offset=100&limit=100",
+    };
+    // Nulls / {track:null} / id-less rows on page 2 prove cross-page filtering.
+    const page2 = {
+      items: [null, { track: null }, { track: { name: "NoId", artists: [], duration_ms: 1 } }, trackItem(100), trackItem(101)],
+      next: null,
+    };
+    const http = {
+      get: vi.fn().mockResolvedValueOnce({ data: page1 }).mockResolvedValueOnce({ data: page2 }),
+    } as any;
+    const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+    const out = await api.getPlaylistTracks("pl1");
+
+    expect(http.get).toHaveBeenCalledTimes(2);
+    expect(out).toHaveLength(102); // 100 (page 1) + 2 real (page 2, three junk rows dropped)
+    expect(out[0].id).toBe("p0");
+    expect(out[99].id).toBe("p99");
+    expect(out[100].id).toBe("p100"); // page 2 appended in order
+    expect(out[101].id).toBe("p101");
+    expect(out.every((s) => s.id !== "")).toBe(true);
+  });
+
+  it("stops at the bounded cap when pages never end (call count bounded)", async () => {
+    const auth = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+    } as any;
+    // Every page is full (100) and always advertises a further next page.
+    const http = {
+      get: vi.fn().mockImplementation((_path: string, cfg: any) => {
+        const offset = Number(cfg?.params?.offset ?? 0);
+        return Promise.resolve({
+          data: {
+            items: Array.from({ length: 100 }, (_, i) => trackItem(offset + i)),
+            next: `https://api.spotify.com/v1/playlists/pl1/tracks?offset=${offset + 100}`,
+          },
+        });
+      }),
+    } as any;
+    const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+    const out = await api.getPlaylistTracks("pl1");
+
+    expect(out.length).toBeLessThanOrEqual(500); // never exceeds the cap
+    expect(out).toHaveLength(500); // and reaches it exactly
+    expect(http.get.mock.calls.length).toBeLessThanOrEqual(5); // 500 cap / 100 per page
+  });
+});
+
+// R2-6: getAlbumTracks must page beyond the 50-track embedded cap via the
+// dedicated /albums/{id}/tracks endpoint, still injecting album name + cover.
+describe("getAlbumTracks pagination (R2-6)", () => {
+  const albTrack = (i: number) => ({ id: `a${i}`, name: `T${i}`, artists: [{ name: "Queen" }], duration_ms: 1000 });
+
+  it("pages beyond the embedded 50-track cap, injecting album name/cover", async () => {
+    const auth = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+    } as any;
+    const albumPayload = {
+      name: "Big Compilation",
+      images: [{ url: "https://i.scdn.co/big.jpg" }],
+      tracks: {
+        items: Array.from({ length: 50 }, (_, i) => albTrack(i)),
+        next: "https://api.spotify.com/v1/albums/alb1/tracks?offset=50&limit=50",
+      },
+    };
+    const page2 = { items: [albTrack(50), albTrack(51), null, albTrack(52)], next: null };
+    const http = {
+      get: vi
+        .fn()
+        .mockResolvedValueOnce({ data: albumPayload }) // GET /v1/albums/alb1
+        .mockResolvedValueOnce({ data: page2 }), // GET /v1/albums/alb1/tracks
+    } as any;
+    const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+    const out = await api.getAlbumTracks("alb1");
+
+    expect(http.get).toHaveBeenCalledTimes(2);
+    expect(out).toHaveLength(53); // 50 + 3 real (one null dropped)
+    for (const t of out) {
+      expect(t.album).toBe("Big Compilation");
+      expect(t.coverUrl).toBe("https://i.scdn.co/big.jpg");
+      expect(t.platform).toBe("spotify");
+    }
+    expect(out[0].id).toBe("a0");
+    expect(out[49].id).toBe("a49");
+    expect(out[50].id).toBe("a50"); // page 2 appended in order
+    expect(out[52].id).toBe("a52");
+  });
+
+  // Robustness: a malformed/proxy response of { items: [], next: <non-null> } never
+  // grows songs.length nor nulls `next`, so a while-loop bounded ONLY by
+  // songs.length >= cap spins forever. getAlbumTracks must have a hard offset/page
+  // bound (mirroring the playlist loop) so it TERMINATES regardless of items/next.
+  it("terminates on a { items: [], next: <non-null> } response (bounded call count, no hang)", async () => {
+    const auth = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+    } as any;
+    const http = {
+      get: vi.fn().mockImplementation((path: string) => {
+        if (path === "/v1/albums/alb1") {
+          // Even the embedded first page is malformed: empty items, non-null next.
+          return Promise.resolve({
+            data: {
+              name: "Broken",
+              images: [{ url: "https://i.scdn.co/broken.jpg" }],
+              tracks: { items: [], next: "http://x/next" },
+            },
+          });
+        }
+        // Every /albums/{id}/tracks page keeps advertising a further page forever.
+        return Promise.resolve({ data: { items: [], next: "http://x/next" } });
+      }),
+    } as any;
+    const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+    const out = await api.getAlbumTracks("alb1");
+
+    // Returns what it has (nothing) rather than hanging.
+    expect(out).toEqual([]);
+    // Bounded: 1 album GET + at most MAX_ALBUM_TRACKS/ALBUM_PAGE_SIZE (=10) page fetches.
+    expect(http.get.mock.calls.length).toBeLessThanOrEqual(12);
+  });
+});
+
+// R2-7: search() must null-filter tracks.items like its album/playlist siblings so
+// an unavailable/relinked null track never becomes a bogus id:'' "Unknown" Song.
+describe("search track null-filtering (R2-7)", () => {
+  it("drops null and id-less track entries (never emits an empty-id 'Unknown' song)", async () => {
+    const auth = {
+      post: vi.fn().mockResolvedValue({ data: { access_token: "t", expires_in: 3600 } }),
+    } as any;
+    const http = {
+      get: vi.fn().mockResolvedValue({
+        data: {
+          tracks: {
+            items: [
+              null, // unavailable/relinked track
+              { id: "t1", name: "Real", artists: [{ name: "X" }], duration_ms: 1000 },
+              {}, // id-less → maps to {id:'', name:'Unknown'}, must also be dropped
+            ],
+          },
+          albums: { items: [] },
+          playlists: { items: [] },
+        },
+      }),
+    } as any;
+    const api = new SpotifyWebApi(() => ({ clientId: "a", clientSecret: "b" }), { http, auth });
+    const out = await api.search("x");
+
+    expect(out.songs).toHaveLength(1);
+    expect(out.songs[0].id).toBe("t1");
+    expect(out.songs.some((s) => s.id === "")).toBe(false);
+    expect(out.songs.some((s) => s.name === "Unknown")).toBe(false);
+  });
+});

src/music/spotify/webapi.ts

@@ -9,6 +9,14 @@ export interface SpotifyCreds {
 const ACCOUNTS_BASE = "https://accounts.spotify.com";
 const API_BASE = "https://api.spotify.com";
 
+// Spotify pages collection tracks at 100 (playlists) / 50 (albums). Follow the
+// paging links, but bound the fan-out so a pathological 10k-track collection
+// can't explode into dozens of API calls; stop once the cap is reached.
+const PLAYLIST_PAGE_SIZE = 100;
+const ALBUM_PAGE_SIZE = 50;
+const MAX_PLAYLIST_TRACKS = 500;
+const MAX_ALBUM_TRACKS = 500;
+
 function artistsToString(artists: unknown): string {
   return Array.isArray(artists)
     ? artists.map((a: any) => a?.name).filter(Boolean).join(", ")
@@ -127,7 +135,11 @@ export class SpotifyWebApi {
       // Spotify rate-limits on a rolling 30s window (429 + Retry-After seconds).
       // Retry once after the advised delay before giving up.
       if (retryOn429 && err?.response?.status === 429) {
-        const retryAfter = Number(err.response.headers?.["retry-after"] ?? 1);
+        // Retry-After may be a non-numeric HTTP-date or empty string; Number(...)
+        // then yields NaN/0 and fires the single retry at 0ms, ignoring the advised
+        // backoff. Guard for a finite positive value (mirrors connect-api.ts).
+        const raw = Number(err.response?.headers?.["retry-after"]);
+        const retryAfter = Number.isFinite(raw) && raw > 0 ? raw : 1;
         await new Promise((r) => setTimeout(r, Math.min(retryAfter, 10) * 1000));
         return this.get(path, params, false);
       }
@@ -143,7 +155,14 @@ export class SpotifyWebApi {
     });
     if (!data) return { songs: [], playlists: [], albums: [] };
     return {
-      songs: mapSpotifyTracks(data?.tracks?.items),
+      // Mirror the album/playlist filtering: a null entry (unavailable/relinked
+      // track) must not become a bogus id:'' "Unknown" Song. Drop empty ids too.
+      songs: Array.isArray(data?.tracks?.items)
+        ? data.tracks.items
+            .filter(Boolean)
+            .map(mapSpotifyTrack)
+            .filter((s: Song) => s.id !== "")
+        : [],
       albums: Array.isArray(data?.albums?.items)
         ? data.albums.items.filter(Boolean).map(mapSpotifyAlbum)
         : [],
@@ -163,22 +182,51 @@ export class SpotifyWebApi {
     const album = await this.get(`/v1/albums/${albumId}`);
     const cover = album?.images?.[0]?.url ?? "";
     const albumName = album?.name ?? "";
-    const items = album?.tracks?.items;
-    if (!Array.isArray(items)) return [];
-    return items.filter(Boolean).map((t: any) => ({
-      ...mapSpotifyTrack(t),
-      album: albumName,
-      coverUrl: cover,
-    }));
+
+    // Page 1 (up to 50 tracks) is embedded in the album payload; the embedded
+    // paging object caps at 50, so follow its `next` via the dedicated
+    // /albums/{id}/tracks endpoint until exhausted or the cap is reached. The
+    // offset bound is a HARD termination guarantee (mirrors the playlist loop):
+    // a malformed/proxy response of { items: [], next: <non-null> } never grows
+    // songs.length nor nulls `next`, so a loop bounded only by songs.length would
+    // spin forever — the offset cap stops it regardless of items/next.
+    const songs: Song[] = [];
+    let page = album?.tracks;
+    let offset = ALBUM_PAGE_SIZE;
+    while (page && Array.isArray(page.items)) {
+      for (const t of page.items.filter(Boolean)) {
+        songs.push({ ...mapSpotifyTrack(t), album: albumName, coverUrl: cover });
+      }
+      if (!page.next || songs.length >= MAX_ALBUM_TRACKS || offset >= MAX_ALBUM_TRACKS) break;
+      page = await this.get(`/v1/albums/${albumId}/tracks`, {
+        limit: ALBUM_PAGE_SIZE,
+        offset,
+      });
+      offset += ALBUM_PAGE_SIZE;
+    }
+    return songs.slice(0, MAX_ALBUM_TRACKS);
   }
 
   async getPlaylistTracks(playlistId: string): Promise<Song[]> {
-    const data = await this.get(`/v1/playlists/${playlistId}/tracks`, { limit: 100 });
-    const items = data?.items;
-    if (!Array.isArray(items)) return [];
-    return items
-      .map((it: any) => it?.track)
-      .filter((t: any) => t && t.id)
-      .map(mapSpotifyTrack);
+    // A single limit:100 page silently truncates 100+ track playlists. Follow
+    // `data.next` (advancing offset) until exhausted or the cap is reached, so a
+    // 10k-track playlist can't fan out into 100 API calls.
+    const songs: Song[] = [];
+    for (let offset = 0; offset < MAX_PLAYLIST_TRACKS; offset += PLAYLIST_PAGE_SIZE) {
+      const data = await this.get(`/v1/playlists/${playlistId}/tracks`, {
+        limit: PLAYLIST_PAGE_SIZE,
+        offset,
+      });
+      const items = data?.items;
+      if (!Array.isArray(items)) break;
+      // Keep the null / {track:null} / id-less filtering across ALL pages.
+      const mapped = items
+        .map((it: any) => it?.track)
+        .filter((t: any) => t && t.id)
+        .map(mapSpotifyTrack);
+      songs.push(...mapped);
+      if (!data.next) break;
+    }
+    return songs.slice(0, MAX_PLAYLIST_TRACKS);
   }
 }

src/web/api/bot.test.ts

@@ -3,7 +3,7 @@ import express from "express";
 import cookieParser from "cookie-parser";
 import request from "supertest";
 import pino from "pino";
-import { mkdtempSync, rmSync } from "node:fs";
+import { mkdtempSync, rmSync, readFileSync, existsSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { createDatabase, type BotDatabase } from "../../data/database.js";
@@ -196,6 +196,304 @@ describe("bot router /settings", () => {
     expect(res.status).toBe(200);
     expect(config.adminGroups).toEqual([6]);
   });
+
+  it("GET /settings includes a masked spotify block (hasClientSecret, never a raw secret)", async () => {
+    config.spotify.enabled = true;
+    config.spotify.backend = "librespot";
+    config.spotify.clientId = "cid-1";
+    config.spotify.deviceName = "MyDevice";
+    config.spotify.bitrate = 160;
+    config.spotify.clientSecret = "supersecret";
+
+    const withSecret = await request(app).get("/api/bot/settings").set("Cookie", cookie);
+    expect(withSecret.status).toBe(200);
+    expect(withSecret.body.spotify).toEqual({
+      enabled: true,
+      backend: "librespot",
+      clientId: "cid-1",
+      deviceName: "MyDevice",
+      bitrate: 160,
+      hasClientSecret: true,
+    });
+    // The raw secret is never serialized to the client.
+    expect(withSecret.body.spotify).not.toHaveProperty("clientSecret");
+
+    config.spotify.clientSecret = "";
+    const noSecret = await request(app).get("/api/bot/settings").set("Cookie", cookie);
+    expect(noSecret.body.spotify.hasClientSecret).toBe(false);
+    expect(noSecret.body.spotify).not.toHaveProperty("clientSecret");
+  });
+
+  it("POST /settings updates the spotify block, echoes the masked view, and persists", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { enabled: true, backend: "librespot", clientId: "cid", deviceName: "Dev", bitrate: 160 } });
+    expect(res.status).toBe(200);
+    expect(config.spotify.enabled).toBe(true);
+    expect(config.spotify.backend).toBe("librespot");
+    expect(config.spotify.clientId).toBe("cid");
+    expect(config.spotify.deviceName).toBe("Dev");
+    expect(config.spotify.bitrate).toBe(160);
+
+    expect(res.body.spotify).toEqual({
+      enabled: true,
+      backend: "librespot",
+      clientId: "cid",
+      deviceName: "Dev",
+      bitrate: 160,
+      hasClientSecret: false,
+    });
+    expect(res.body.spotify).not.toHaveProperty("clientSecret");
+
+    // saveConfig persisted the block to disk.
+    expect(existsSync(configPath)).toBe(true);
+    const persisted = JSON.parse(readFileSync(configPath, "utf-8"));
+    expect(persisted.spotify.clientId).toBe("cid");
+    expect(persisted.spotify.backend).toBe("librespot");
+  });
+
+  it("POST /settings ignores an invalid spotify backend/bitrate (partial-merge, no 400)", async () => {
+    config.spotify.backend = "auto";
+    config.spotify.bitrate = 320;
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { backend: "bogus", bitrate: 999 } });
+    expect(res.status).toBe(200);
+    expect(config.spotify.backend).toBe("auto");
+    expect(config.spotify.bitrate).toBe(320);
+  });
+
+  it("POST /settings sets a non-empty clientSecret but a blank one never wipes it", async () => {
+    const set = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { clientSecret: "newsecret" } });
+    expect(set.status).toBe(200);
+    expect(config.spotify.clientSecret).toBe("newsecret");
+    expect(set.body.spotify.hasClientSecret).toBe(true);
+    expect(set.body.spotify).not.toHaveProperty("clientSecret");
+
+    const blank = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { clientSecret: "" } });
+    expect(blank.status).toBe(200);
+    expect(config.spotify.clientSecret).toBe("newsecret");
+    expect(blank.body.spotify.hasClientSecret).toBe(true);
+  });
+
+  it("POST /settings ignores a blank/whitespace deviceName but stores a trimmed non-empty one", async () => {
+    config.spotify.deviceName = "OldDevice";
+
+    // Empty string leaves the prior deviceName untouched.
+    const empty = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { deviceName: "" } });
+    expect(empty.status).toBe(200);
+    expect(config.spotify.deviceName).toBe("OldDevice");
+    expect(empty.body.spotify.deviceName).toBe("OldDevice");
+
+    // Whitespace-only is likewise ignored (trim().length === 0).
+    const ws = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { deviceName: "   " } });
+    expect(ws.status).toBe(200);
+    expect(config.spotify.deviceName).toBe("OldDevice");
+
+    // A non-empty value is stored TRIMMED.
+    const set = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { deviceName: "  Dev  " } });
+    expect(set.status).toBe(200);
+    expect(config.spotify.deviceName).toBe("Dev");
+    expect(set.body.spotify.deviceName).toBe("Dev");
+  });
+
+  it("POST /settings that omits spotify leaves config.spotify untouched (no regression)", async () => {
+    config.spotify.clientId = "keep-me";
+    config.spotify.clientSecret = "keep-secret";
+    const before = { ...config.spotify };
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ autoPauseOnEmpty: false });
+    expect(res.status).toBe(200);
+    expect(config.spotify).toEqual(before);
+  });
+});
+
+// Whole-branch I2: saving a Client ID in Settings must re-configure the single
+// live SpotifyOAuth so the operator can Connect without a process restart.
+describe("bot router /settings applies spotify creds to the live OAuth (I2)", () => {
+  let botDb: BotDatabase;
+  let app: express.Express;
+  let cookie: string;
+  let config: BotConfig;
+  let configPath: string;
+  let tmpDir: string;
+  let configureCalls: Array<[string | undefined, string | undefined]>;
+
+  beforeEach(async () => {
+    botDb = createDatabase(":memory:");
+    const users = createUserStore(botDb.db);
+    const sessions = createSessionStore(botDb.db);
+    const alice = await users.createUser("alice", "pw-alice", "admin");
+    cookie = `${SESSION_COOKIE_NAME}=${sessions.createSession(alice.id).token}`;
+
+    tmpDir = mkdtempSync(join(tmpdir(), "botsettings-oauth-"));
+    configPath = join(tmpDir, "config.json");
+    config = getDefaultConfig(); // webPort defaults to 3000
+
+    const fakeManager = { getAllBots: () => [] } as unknown as BotManager;
+    const avatarStore = createAvatarStore(tmpDir);
+
+    // Fake OAuth recording every configure(clientId, redirectUri) call.
+    configureCalls = [];
+    const fakeOAuth = {
+      configure(clientId?: string, redirectUri?: string) {
+        configureCalls.push([clientId, redirectUri]);
+      },
+    };
+
+    app = express();
+    app.use(express.json());
+    app.use(cookieParser());
+    app.use("/api", createRequireAuth(sessions, createPermissionStore(botDb.db), () => getDefaultConfig().guestMode));
+    app.use(
+      "/api/bot",
+      createBotRouter(fakeManager, config, configPath, pino({ level: "silent" }), botDb, avatarStore, undefined, fakeOAuth),
+    );
+  });
+
+  afterEach(() => {
+    botDb.close();
+    rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  it("configures the live OAuth once with the derived callback redirectUri when a Client ID is saved", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { clientId: "cid", enabled: true } });
+    expect(res.status).toBe(200);
+    expect(configureCalls).toEqual([
+      ["cid", `http://127.0.0.1:${config.webPort}/api/spotify/callback`],
+    ]);
+  });
+
+  it("does NOT touch the OAuth when the request has no spotify block", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ autoPauseOnEmpty: false });
+    expect(res.status).toBe(200);
+    expect(configureCalls).toEqual([]);
+  });
+
+  it("configures with ('', undefined) when a spotify block clears the Client ID (disables OAuth)", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { clientId: "" } });
+    expect(res.status).toBe(200);
+    expect(configureCalls).toEqual([["", undefined]]);
+  });
+});
+
+// R2-4: saving Spotify creds in Settings must also refresh the Web API SEARCH
+// provider (spotifyProvider.setCreds), not only the OAuth playback path. Without
+// this, a fresh install (enabled defaults false) keeps empty search creds until a
+// full process restart even after an admin enters Client ID + Secret.
+describe("bot router /settings refreshes the Web API search provider creds (R2-4)", () => {
+  let botDb: BotDatabase;
+  let app: express.Express;
+  let cookie: string;
+  let config: BotConfig;
+  let configPath: string;
+  let tmpDir: string;
+  let setCredsCalls: Array<[string, string]>;
+  let configureCalls: Array<[string | undefined, string | undefined]>;
+
+  beforeEach(async () => {
+    botDb = createDatabase(":memory:");
+    const users = createUserStore(botDb.db);
+    const sessions = createSessionStore(botDb.db);
+    const alice = await users.createUser("alice", "pw-alice", "admin");
+    cookie = `${SESSION_COOKIE_NAME}=${sessions.createSession(alice.id).token}`;
+
+    tmpDir = mkdtempSync(join(tmpdir(), "botsettings-provider-"));
+    configPath = join(tmpDir, "config.json");
+    config = getDefaultConfig();
+
+    const fakeManager = { getAllBots: () => [] } as unknown as BotManager;
+    const avatarStore = createAvatarStore(tmpDir);
+
+    // Fake search provider recording every setCreds(clientId, clientSecret) call.
+    setCredsCalls = [];
+    const fakeProvider = {
+      setCreds(clientId: string, clientSecret: string) {
+        setCredsCalls.push([clientId, clientSecret]);
+      },
+    };
+    // Fake OAuth so we can assert the playback path is still wired alongside.
+    configureCalls = [];
+    const fakeOAuth = {
+      configure(clientId?: string, redirectUri?: string) {
+        configureCalls.push([clientId, redirectUri]);
+      },
+    };
+
+    app = express();
+    app.use(express.json());
+    app.use(cookieParser());
+    app.use("/api", createRequireAuth(sessions, createPermissionStore(botDb.db), () => getDefaultConfig().guestMode));
+    app.use(
+      "/api/bot",
+      createBotRouter(fakeManager, config, configPath, pino({ level: "silent" }), botDb, avatarStore, undefined, fakeOAuth, fakeProvider),
+    );
+  });
+
+  afterEach(() => {
+    botDb.close();
+    rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  it("calls setCreds once with (clientId, clientSecret) when a spotify block is saved (and OAuth is still configured)", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { clientId: "cid", clientSecret: "sec", enabled: true } });
+    expect(res.status).toBe(200);
+    expect(setCredsCalls).toEqual([["cid", "sec"]]);
+    // The OAuth playback path is still wired on the same save.
+    expect(configureCalls.length).toBe(1);
+  });
+
+  it("does NOT call setCreds when the request has no spotify block", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ autoPauseOnEmpty: false });
+    expect(res.status).toBe(200);
+    expect(setCredsCalls).toEqual([]);
+  });
+
+  it("calls setCreds with the PRESERVED stored secret when the spotify block omits/blanks clientSecret", async () => {
+    config.spotify.clientId = "cid0";
+    config.spotify.clientSecret = "stored-secret";
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ spotify: { clientId: "cid0", clientSecret: "", enabled: true } });
+    expect(res.status).toBe(200);
+    // Post-merge values: masked/blank secret must keep the stored one, never "".
+    expect(setCredsCalls).toEqual([["cid0", "stored-secret"]]);
+  });
 });
 
 describe("bot router /settings guest-mode gating + persistence", () => {
@@ -251,4 +549,15 @@ describe("bot router /settings guest-mode gating + persistence", () => {
     expect(res.body.guestMode.permissions.playNext).toBe(true);
     expect(res.body.guestMode.permissions.addToQueue).toBe(true); // untouched default
   });
+
+  it("POST /settings spotify write is 403 for a member lacking bot.manage", async () => {
+    const memberApp = mountBot(() => ({ role: "member", capabilities: new Set([]) }));
+    const res = await request(memberApp)
+      .post("/api/bot/settings")
+      .send({ spotify: { enabled: true, clientId: "cid" } });
+    expect(res.status).toBe(403);
+    // Gate rejected before any mutation.
+    expect(config.spotify.enabled).toBe(false);
+    expect(config.spotify.clientId).toBe("");
+  });
 });

src/web/api/bot.ts

@@ -1,6 +1,6 @@
 import { Router } from "express";
 import type { BotManager } from "../../bot/manager.js";
-import type { BotConfig, GuestModeConfig } from "../../data/config.js";
+import type { BotConfig, GuestModeConfig, SpotifyConfig } from "../../data/config.js";
 import { saveConfig } from "../../data/config.js";
 import type { Logger } from "../../logger.js";
 import type { BotDatabase } from "../../data/database.js";
@@ -17,9 +17,28 @@ export function createBotRouter(
   botDb: BotDatabase,
   avatarStore: AvatarStore,
   onGuestPolicyChanged?: (cfg: GuestModeConfig) => void,
+  // I2: the single process-wide OAuth, so a UI-entered Client ID reaches the
+  // live instance on save (no restart). Structural type = SpotifyOAuth.configure.
+  spotifyOAuth?: { configure(clientId?: string, redirectUri?: string): void },
+  // R2-4: the process-wide Web API SEARCH provider. Boot only wires creds when
+  // spotify.enabled is already true, so a fresh install that enables Spotify via
+  // Settings must push creds here too, or search stays empty until a restart.
+  // Structural type = SpotifyProvider.setCreds.
+  spotifyProvider?: { setCreds(clientId: string, clientSecret: string): void },
 ): Router {
   const router = Router();
 
+  // Masked spotify view shared by the GET response and the POST echo. The raw
+  // clientSecret is write-only and NEVER serialized — only whether one is stored.
+  const maskedSpotify = () => ({
+    enabled: config.spotify.enabled,
+    backend: config.spotify.backend,
+    clientId: config.spotify.clientId,
+    deviceName: config.spotify.deviceName,
+    bitrate: config.spotify.bitrate,
+    hasClientSecret: config.spotify.clientSecret.length > 0,
+  });
+
   router.get("/", (req, res) => {
     const all = botManager.getAllBots().map((b) => b.getStatus());
     const u = req.user!;
@@ -39,6 +58,7 @@ export function createBotRouter(
       localAudioEnabled: config.localAudioEnabled,
       adminGroups: config.adminGroups ?? [],
       guestMode: config.guestMode,
+      spotify: maskedSpotify(),
     });
   });
 
@@ -85,8 +105,47 @@ export function createBotRouter(
       );
     }
 
+    // Partial-merge the spotify block (mirrors config.ts validation). Invalid
+    // sub-fields are ignored rather than 400-ing the whole request. Omitting
+    // `spotify` entirely leaves config.spotify untouched.
+    const VALID_BACKENDS = ["auto", "go-librespot", "librespot"] as const;
+    const VALID_BITRATES = [96, 160, 320];
+    const sp = req.body?.spotify;
+    if (sp && typeof sp === "object") {
+      const t = config.spotify;
+      if (typeof sp.enabled === "boolean") t.enabled = sp.enabled;
+      if (typeof sp.backend === "string" && (VALID_BACKENDS as readonly string[]).includes(sp.backend)) {
+        t.backend = sp.backend as SpotifyConfig["backend"];
+      }
+      if (typeof sp.clientId === "string") t.clientId = sp.clientId;
+      // Secret is write-only + set-on-non-empty so a blank field never wipes it.
+      if (typeof sp.clientSecret === "string" && sp.clientSecret.length > 0) {
+        t.clientSecret = sp.clientSecret;
+      }
+      if (typeof sp.deviceName === "string" && sp.deviceName.trim().length > 0) {
+        t.deviceName = sp.deviceName.trim();
+      }
+      if (typeof sp.bitrate === "number" && VALID_BITRATES.includes(sp.bitrate)) {
+        t.bitrate = sp.bitrate;
+      }
+    }
+
     saveConfig(configPath, config);
 
+    // I2: only when the spotify block was present, push the (possibly UI-entered)
+    // Client ID into the live process-wide OAuth so Connect works without a
+    // restart. Empty clientId => undefined redirect => configure() disables OAuth.
+    if (sp && typeof sp === "object") {
+      const redirectUri = config.spotify.clientId
+        ? `http://127.0.0.1:${config.webPort}/api/spotify/callback`
+        : undefined;
+      spotifyOAuth?.configure(config.spotify.clientId, redirectUri);
+      // R2-4: also refresh the live Web API search provider so search/getAuthStatus
+      // work without a restart. Uses the post-merge values so a masked/omitted
+      // secret keeps the stored one. The secret is never logged.
+      spotifyProvider?.setCreds(config.spotify.clientId, config.spotify.clientSecret);
+    }
+
     // Guest-mode changed: tear down / re-scope in-flight guest WS sockets so a
     // disabled or narrowed scope takes effect immediately (matches requireAuth's
     // "disabling immediately invalidates in-flight guest sessions" invariant).
@@ -106,6 +165,7 @@ export function createBotRouter(
       localAudioEnabled: config.localAudioEnabled,
       adminGroups: config.adminGroups ?? [],
       guestMode: config.guestMode,
+      spotify: maskedSpotify(),
     });
   });
 

src/web/api/player.ts

@@ -198,7 +198,7 @@ export function createPlayerRouter(
           .json({ error: "position must be a finite non-negative number" });
         return;
       }
-      bot.getPlayer().seek(position);
+      bot.seek(position);
       res.json({ message: `Seeked to ${Math.floor(position)}s`, seekOffset: position });
     } catch (err) {
       res.status(500).json({ error: (err as Error).message });

src/web/api/spotify.test.ts

@@ -0,0 +1,95 @@
+import { describe, it, expect, vi } from "vitest";
+import express from "express";
+import request from "supertest";
+import pino from "pino";
+import { createSpotifyRouter, type SpotifyOAuthLike } from "./spotify.js";
+
+type Role = "admin" | "member" | "guest";
+function makeApp(oauth: SpotifyOAuthLike, role: Role = "admin", caps: string[] = []) {
+  const app = express();
+  app.use(express.json());
+  // Stand in for the global requireAuth that populates req.user.
+  app.use((req, _res, next) => {
+    (req as any).user = { role, capabilities: new Set(caps) };
+    next();
+  });
+  app.use(
+    "/api/spotify",
+    createSpotifyRouter({
+      oauth,
+      logger: pino({ level: "silent" }),
+      getBackendInfo: () => ({ backend: "librespot", deviceName: "TS-Bot", binaryAvailable: true }),
+      webUiRedirect: "/",
+    }),
+  );
+  return app;
+}
+
+function fakeOauth(over: Partial<SpotifyOAuthLike> = {}): SpotifyOAuthLike {
+  return {
+    buildAuthorizeUrl: () => ({ url: "https://accounts.spotify.com/authorize?x=1", state: "st" }),
+    handleCallback: async () => true,
+    isAuthorized: () => false,
+    ...over,
+  };
+}
+
+describe("spotify OAuth router", () => {
+  it("GET /login returns the authorize url for a permitted user", async () => {
+    const app = makeApp(fakeOauth());
+    const res = await request(app).get("/api/spotify/login");
+    expect(res.status).toBe(200);
+    expect(res.body.url).toContain("accounts.spotify.com/authorize");
+  });
+
+  it("GET /login is 403 for a member lacking platform.auth", async () => {
+    const app = makeApp(fakeOauth(), "member", []);
+    const res = await request(app).get("/api/spotify/login");
+    expect(res.status).toBe(403);
+  });
+
+  it("GET /callback with a good code+state redirects to success", async () => {
+    const handleCallback = vi.fn(async () => true);
+    const app = makeApp(fakeOauth({ handleCallback }));
+    const res = await request(app).get("/api/spotify/callback?code=abc&state=st");
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toBe("/?spotify=success");
+    expect(handleCallback).toHaveBeenCalledWith("abc", "st");
+  });
+
+  it("GET /callback with a bad state (handleCallback false) redirects to error", async () => {
+    const app = makeApp(fakeOauth({ handleCallback: async () => false }));
+    const res = await request(app).get("/api/spotify/callback?code=abc&state=WRONG");
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toBe("/?spotify=error");
+  });
+
+  it("GET /callback with missing code does not call oauth and redirects to error", async () => {
+    const handleCallback = vi.fn(async () => true);
+    const app = makeApp(fakeOauth({ handleCallback }));
+    const res = await request(app).get("/api/spotify/callback?state=st");
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toBe("/?spotify=error");
+    expect(handleCallback).not.toHaveBeenCalled();
+  });
+
+  it("GET /callback swallows a throwing handleCallback and redirects to error", async () => {
+    const app = makeApp(fakeOauth({ handleCallback: async () => { throw new Error("boom"); } }));
+    const res = await request(app).get("/api/spotify/callback?code=abc&state=st");
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toBe("/?spotify=error");
+  });
+
+  it("GET /status reflects authorized + backend + deviceName", async () => {
+    const app = makeApp(fakeOauth({ isAuthorized: () => true }));
+    const res = await request(app).get("/api/spotify/status");
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual({ authorized: true, backend: "librespot", deviceName: "TS-Bot", binaryAvailable: true });
+  });
+
+  it("GET /status is 403 for a guest", async () => {
+    const app = makeApp(fakeOauth(), "guest");
+    const res = await request(app).get("/api/spotify/status");
+    expect(res.status).toBe(403);
+  });
+});

src/web/api/spotify.ts

@@ -0,0 +1,79 @@
+import { Router } from "express";
+import type { Logger } from "pino";
+import { requirePermission } from "../middleware/requirePermission.js";
+import { requireNotGuest } from "../middleware/requireNotGuest.js";
+
+/** Minimal structural seam over SpotifyOAuth so this router needs no real
+ *  network/crypto in tests. The concrete SpotifyOAuth satisfies it verbatim. */
+export interface SpotifyOAuthLike {
+  buildAuthorizeUrl(): { url: string; state: string };
+  handleCallback(code: string, state: string): Promise<boolean>;
+  isAuthorized(): boolean;
+}
+
+export interface SpotifyRouterOptions {
+  oauth: SpotifyOAuthLike;
+  logger: Logger;
+  /** Process-wide backend info for /status (single Premium account, Stage 3). */
+  getBackendInfo: () => {
+    backend: string;
+    deviceName: string;
+    binaryAvailable: boolean;
+  };
+  /** Web UI page to bounce the browser back to after the OAuth callback. */
+  webUiRedirect?: string;
+}
+
+export function createSpotifyRouter(opts: SpotifyRouterOptions): Router {
+  const { oauth, logger } = opts;
+  const redirectBase = opts.webUiRedirect ?? "/";
+  const sep = redirectBase.includes("?") ? "&" : "?";
+  const router = Router();
+
+  // Start the Authorization Code + PKCE flow: hand the WebUI the accounts.spotify.com
+  // authorize URL (verifier is stashed by state inside SpotifyOAuth). Gated like the
+  // other platform logins in auth.ts.
+  router.get("/login", requirePermission("platform.auth"), (_req, res) => {
+    try {
+      const { url } = oauth.buildAuthorizeUrl();
+      res.json({ url });
+    } catch (err) {
+      logger.error({ err }, "Spotify authorize URL build failed");
+      res.status(500).json({ error: (err as Error).message });
+    }
+  });
+
+  // OAuth redirect target (own-app clientId => redirect_uri points here). This is a
+  // top-level browser navigation carrying the SameSite=Lax session cookie, so the
+  // global requireAuth passes; state is the CSRF guard for the flow itself. Always
+  // redirect (never JSON) so the user lands back in the UI.
+  router.get("/callback", async (req, res) => {
+    const code = typeof req.query.code === "string" ? req.query.code : "";
+    const state = typeof req.query.state === "string" ? req.query.state : "";
+    if (!code || !state) {
+      res.redirect(`${redirectBase}${sep}spotify=error`);
+      return;
+    }
+    try {
+      const ok = await oauth.handleCallback(code, state);
+      res.redirect(`${redirectBase}${sep}spotify=${ok ? "success" : "error"}`);
+    } catch (err) {
+      logger.error({ err }, "Spotify OAuth callback failed");
+      res.redirect(`${redirectBase}${sep}spotify=error`);
+    }
+  });
+
+  // Whether the (single, process-wide) account is authorized, plus which backend
+  // + device name are configured — used by the WebUI to show login-needed state.
+  router.get("/status", requireNotGuest, (_req, res) => {
+    const info = opts.getBackendInfo();
+    res.json({
+      authorized: oauth.isAuthorized(),
+      backend: info.backend,
+      deviceName: info.deviceName,
+      binaryAvailable: info.binaryAvailable,
+    });
+  });
+
+  return router;
+}

src/web/server.ts

@@ -19,6 +19,14 @@ import { createUsersRouter } from "./api/users.js";
 import { createAuditStore } from "../data/audit.js";
 import { createAuditRouter } from "./api/audit.js";
 import { createFavoritesRouter } from "./api/favorites.js";
+import { createSpotifyRouter } from "./api/spotify.js";
+import type { SpotifyOAuth } from "../music/spotify/spotify-oauth.js";
+import type { SpotifyProvider } from "../music/spotify/provider.js";
+import { resolveSpotifyBackendKind } from "../music/spotify/backend-select.js";
+import {
+  isGoLibrespotPresent,
+  isLibrespotPresent,
+} from "../music/spotify/binary.js";
 import { setupWebSocket } from "./websocket.js";
 import { createUserStore } from "../data/users.js";
 import { createSessionStore } from "../data/sessions.js";
@@ -48,6 +56,9 @@ export interface WebServerOptions {
   cookieStore?: CookieStore;
   avatarStore: AvatarStore;
   staticDir?: string;
+  /** Process-wide shared Spotify OAuth (single account, Stage 3). When set, the
+   *  /api/spotify {login,callback,status} router is mounted. */
+  spotifyOAuth?: SpotifyOAuth;
 }
 
 export interface WebServer {
@@ -122,6 +133,14 @@ export function createWebServer(options: WebServerOptions): WebServer {
       options.database,
       options.avatarStore,
       (cfg) => onGuestPolicyChanged(cfg),
+      // I2: so saving a Client ID in Settings re-configures the live OAuth
+      // (no restart needed for the UI-entered-creds -> Connect flow).
+      options.spotifyOAuth,
+      // R2-4: so saving Spotify creds in Settings also refreshes the live Web API
+      // search provider (search + getAuthStatus) without a process restart. The
+      // runtime object is a SpotifyProvider (see index.ts); WebServerOptions types
+      // it as the wider MusicProvider, so narrow it here for the setCreds contract.
+      options.spotifyProvider as SpotifyProvider,
     )
   );
   app.use(
@@ -136,6 +155,32 @@ export function createWebServer(options: WebServerOptions): WebServer {
     "/api/auth",
     createAuthRouter(options.neteaseProvider, options.qqProvider, options.bilibiliProvider, logger, options.cookieStore, options.kugouProvider, options.spotifyProvider)
   );
+  if (options.spotifyOAuth) {
+    app.use(
+      "/api/spotify",
+      createSpotifyRouter({
+        oauth: options.spotifyOAuth,
+        logger,
+        getBackendInfo: () => {
+          // PATH-aware presence (Bug m1): a PATH-installed binary (bare name)
+          // is resolved against $PATH, not existsSync()'d against cwd.
+          const goPresent = isGoLibrespotPresent();
+          const rustPresent = isLibrespotPresent();
+          const resolved = resolveSpotifyBackendKind(
+            options.config.spotify.backend,
+            goPresent,
+            rustPresent,
+          );
+          return {
+            backend: resolved ?? "none",
+            deviceName: options.config.spotify.deviceName,
+            binaryAvailable: resolved !== null,
+          };
+        },
+        webUiRedirect: "/",
+      }),
+    );
+  }
   app.use("/api/favorites", requireNotGuest, createFavoritesRouter(options.database, logger));
 
   // admin-only routes

web/src/composables/useSpotifySettings.test.ts

@@ -0,0 +1,119 @@
+import { describe, it, expect } from "vitest";
+import {
+  buildSpotifyPayload,
+  parseSpotifyRedirect,
+  statusSummary,
+  SPOTIFY_DISCLAIMER,
+  type SpotifyConfigForm,
+  type SpotifyStatus,
+} from "./useSpotifySettings.js";
+
+function form(overrides: Partial<SpotifyConfigForm> = {}): SpotifyConfigForm {
+  return {
+    enabled: true,
+    backend: "auto",
+    clientId: "abc",
+    clientSecret: "",
+    deviceName: "TS-Bot",
+    bitrate: 320,
+    ...overrides,
+  };
+}
+
+describe("buildSpotifyPayload", () => {
+  it("always includes enabled/backend/clientId/deviceName/bitrate under a spotify key", () => {
+    const { spotify } = buildSpotifyPayload(form());
+    expect(spotify).toMatchObject({
+      enabled: true,
+      backend: "auto",
+      clientId: "abc",
+      deviceName: "TS-Bot",
+      bitrate: 320,
+    });
+  });
+
+  it("omits clientSecret when blank (means unchanged, never wipes)", () => {
+    const { spotify } = buildSpotifyPayload(form({ clientSecret: "" }));
+    expect("clientSecret" in spotify).toBe(false);
+  });
+
+  it("includes clientSecret only when non-blank", () => {
+    const { spotify } = buildSpotifyPayload(form({ clientSecret: "s3cr3t" }));
+    expect(spotify.clientSecret).toBe("s3cr3t");
+  });
+
+  it("carries a non-default backend through unchanged", () => {
+    const { spotify } = buildSpotifyPayload(form({ backend: "librespot" }));
+    expect(spotify.backend).toBe("librespot");
+  });
+});
+
+describe("parseSpotifyRedirect", () => {
+  it("maps ?spotify=success to 'success'", () => {
+    expect(parseSpotifyRedirect("?spotify=success")).toBe("success");
+  });
+
+  it("maps ?spotify=error to 'error'", () => {
+    expect(parseSpotifyRedirect("?spotify=error")).toBe("error");
+  });
+
+  it("returns null for unrelated params", () => {
+    expect(parseSpotifyRedirect("?x=1")).toBeNull();
+  });
+
+  it("returns null for an empty search string", () => {
+    expect(parseSpotifyRedirect("")).toBeNull();
+  });
+
+  it("returns null for an unexpected spotify value", () => {
+    expect(parseSpotifyRedirect("?spotify=maybe")).toBeNull();
+  });
+});
+
+describe("statusSummary", () => {
+  const ok: SpotifyStatus = { authorized: true, backend: "go-librespot", deviceName: "d", binaryAvailable: true };
+
+  it("is 'off' tone when disabled, regardless of status", () => {
+    expect(statusSummary(null, false).tone).toBe("off");
+    expect(statusSummary(ok, false).tone).toBe("off");
+  });
+
+  it("is 'warn' tone when enabled but status is unknown (null)", () => {
+    expect(statusSummary(null, true).tone).toBe("warn");
+  });
+
+  it("is 'warn' tone when the binary is unavailable", () => {
+    const s: SpotifyStatus = { authorized: false, backend: "auto", deviceName: "d", binaryAvailable: false };
+    expect(statusSummary(s, true).tone).toBe("warn");
+  });
+
+  it("is 'warn' tone when the binary exists but not authorized", () => {
+    const s: SpotifyStatus = { authorized: false, backend: "auto", deviceName: "d", binaryAvailable: true };
+    expect(statusSummary(s, true).tone).toBe("warn");
+  });
+
+  it("is 'ok' tone when enabled, authorized and the binary is available", () => {
+    const summary = statusSummary(ok, true);
+    expect(summary.tone).toBe("ok");
+    expect(summary.label).toContain("go-librespot");
+  });
+});
+
+describe("SPOTIFY_DISCLAIMER", () => {
+  it("mentions the Premium + grey-area risk copy", () => {
+    expect(SPOTIFY_DISCLAIMER).toContain("Premium");
+    expect(SPOTIFY_DISCLAIMER.length).toBeGreaterThan(20);
+  });
+
+  // The disclaimer is compliance-critical: it must keep the Premium requirement,
+  // the ToS grey-area / at-your-own-risk warning, the default-off promise, and the
+  // "use your own developer credentials" notion. A refactor that drops any of these
+  // must fail here rather than silently shipping weakened copy.
+  it("retains every compliance-critical element (Premium, ToS/risk, default-off, own credentials)", () => {
+    expect(SPOTIFY_DISCLAIMER).toContain("Premium");
+    expect(SPOTIFY_DISCLAIMER).toContain("灰色地带"); // grey area of Spotify's ToS
+    expect(SPOTIFY_DISCLAIMER).toContain("风险自负"); // at your own risk
+    expect(SPOTIFY_DISCLAIMER).toContain("默认关闭"); // disabled by default
+    expect(SPOTIFY_DISCLAIMER).toContain("凭据"); // your own developer app credentials
+  });
+});

web/src/composables/useSpotifySettings.ts

@@ -0,0 +1,48 @@
+export interface SpotifyConfigForm {
+  enabled: boolean;
+  backend: "auto" | "go-librespot" | "librespot";
+  clientId: string;
+  clientSecret: string; // blank means "unchanged"
+  deviceName: string;
+  bitrate: number;
+}
+
+export interface SpotifyStatus {
+  authorized: boolean;
+  backend: string;
+  deviceName: string;
+  binaryAvailable: boolean;
+}
+
+// 实验性 · 灰色地带 · 需要 Premium · 使用你自己的开发者应用凭据
+export const SPOTIFY_DISCLAIMER =
+  "实验性功能：通过 librespot 播放 Spotify 需要 Spotify Premium 账号，并使用你自己注册的 Spotify 开发者应用凭据。" +
+  "该方式处于 Spotify 服务条款的灰色地带，风险自负；默认关闭，不会内置任何共享凭据。";
+
+export function buildSpotifyPayload(f: SpotifyConfigForm): { spotify: Record<string, unknown> } {
+  const spotify: Record<string, unknown> = {
+    enabled: f.enabled,
+    backend: f.backend,
+    clientId: f.clientId,
+    deviceName: f.deviceName,
+    bitrate: f.bitrate,
+  };
+  if (f.clientSecret && f.clientSecret.length > 0) spotify.clientSecret = f.clientSecret;
+  return { spotify };
+}
+
+export function parseSpotifyRedirect(search: string): "success" | "error" | null {
+  const v = new URLSearchParams(search).get("spotify");
+  return v === "success" || v === "error" ? v : null;
+}
+
+export function statusSummary(
+  s: SpotifyStatus | null,
+  enabled: boolean,
+): { label: string; tone: "ok" | "warn" | "off" } {
+  if (!enabled) return { label: "已关闭", tone: "off" };
+  if (!s) return { label: "未知", tone: "warn" };
+  if (!s.binaryAvailable) return { label: "未检测到 librespot 可执行文件", tone: "warn" };
+  if (!s.authorized) return { label: "未授权（点击“连接 Spotify”登录）", tone: "warn" };
+  return { label: `已就绪 · 后端 ${s.backend}`, tone: "ok" };
+}

web/src/views/Settings.vue

@@ -449,6 +449,106 @@
       </div>
     </section>
 
+    <!-- Spotify (Connect) playback via librespot — requires platform.auth -->
+    <section v-if="can('platform.auth')" class="settings-section">
+      <h2 class="section-title">Spotify 播放（实验性）</h2>
+      <p class="spotify-disclaimer">{{ SPOTIFY_DISCLAIMER }}</p>
+
+      <div class="account-card">
+        <div class="account-header">
+          <Icon icon="mdi:spotify" class="account-icon spotify-icon" />
+          <div class="account-info">
+            <div class="account-name">Spotify (librespot)</div>
+            <div class="account-status" :class="`tone-${spotifyStatusSummary.tone}`">
+              {{ spotifyStatusSummary.label }}
+            </div>
+          </div>
+        </div>
+
+        <!-- Enable -->
+        <label class="profile-toggle behavior-toggle spotify-toggle">
+          <div class="profile-toggle-text">
+            <div class="profile-toggle-label">启用 Spotify 播放</div>
+            <div class="profile-toggle-hint">开启后可通过 librespot 后端播放 Spotify（需 Premium 账号 + 你自己注册的开发者应用凭据）。默认关闭。</div>
+          </div>
+          <input v-model="spotifyForm.enabled" type="checkbox" class="profile-toggle-switch" />
+        </label>
+
+        <!-- Backend -->
+        <div class="setting-row spotify-row">
+          <div class="setting-label">播放后端</div>
+          <div class="spotify-btn-group">
+            <button
+              v-for="b in SPOTIFY_BACKENDS"
+              :key="b.value"
+              class="spotify-choice"
+              :class="{ active: spotifyForm.backend === b.value }"
+              @click="spotifyForm.backend = b.value"
+            >
+              {{ b.label }}
+            </button>
+          </div>
+        </div>
+
+        <!-- Bitrate -->
+        <div class="setting-row spotify-row">
+          <div class="setting-label">比特率 (kbps)</div>
+          <div class="spotify-btn-group">
+            <button
+              v-for="rate in SPOTIFY_BITRATES"
+              :key="rate"
+              class="spotify-choice"
+              :class="{ active: spotifyForm.bitrate === rate }"
+              @click="spotifyForm.bitrate = rate"
+            >
+              {{ rate }}
+            </button>
+          </div>
+        </div>
+
+        <!-- Client ID -->
+        <div class="form-group">
+          <label>Client ID</label>
+          <input v-model="spotifyForm.clientId" class="input" autocomplete="off" placeholder="Spotify 开发者应用 Client ID" />
+        </div>
+
+        <!-- Client Secret (write-only) -->
+        <div class="form-group">
+          <label>
+            Client Secret
+            <span class="spotify-secret-hint" :class="{ set: spotifyHasSecret }">
+              {{ spotifyHasSecret ? '已设置' : '未设置' }}
+            </span>
+          </label>
+          <input
+            v-model="spotifyForm.clientSecret"
+            class="input"
+            type="password"
+            autocomplete="off"
+            placeholder="留空表示不修改已保存的 Secret"
+          />
+        </div>
+
+        <!-- Device name -->
+        <div class="form-group">
+          <label>设备名称</label>
+          <input v-model="spotifyForm.deviceName" class="input" autocomplete="off" placeholder="TSMusicBot" />
+        </div>
+
+        <div class="spotify-actions">
+          <button class="btn-primary" :disabled="spotifySaving" @click="saveSpotify">
+            {{ spotifySaving ? '保存中…' : '保存' }}
+          </button>
+          <button class="btn-secondary spotify-connect" :disabled="spotifyConnecting" @click="connectSpotify">
+            <Icon icon="mdi:spotify" />
+            {{ spotifyConnecting ? '跳转中…' : '连接 Spotify' }}
+          </button>
+        </div>
+
+        <p v-if="spotifyMessage" class="spotify-message" :class="`tone-${spotifyMessageTone}`">{{ spotifyMessage }}</p>
+      </div>
+    </section>
+
     <!-- Audio Quality requires quality -->
     <section v-if="can('quality')" class="settings-section">
       <h2 class="section-title">音质设置</h2>
@@ -812,7 +912,7 @@
 </template>
 
 <script setup lang="ts">
-import { ref, reactive, onMounted, onUnmounted } from 'vue';
+import { ref, reactive, computed, onMounted, onUnmounted } from 'vue';
 import { Icon } from '@iconify/vue';
 import axios from 'axios';
 import AvatarUpload from '../components/AvatarUpload.vue';
@@ -820,6 +920,14 @@ import CustomAvatarRow from '../components/CustomAvatarRow.vue';
 import QRCode from 'qrcode';
 import { usePlayerStore } from '../stores/player.js';
 import { useSession } from '../composables/useSession.js';
+import {
+  buildSpotifyPayload,
+  parseSpotifyRedirect,
+  statusSummary,
+  SPOTIFY_DISCLAIMER,
+  type SpotifyConfigForm,
+  type SpotifyStatus,
+} from '../composables/useSpotifySettings.js';
 
 const store = usePlayerStore();
 
@@ -1144,6 +1252,7 @@ async function loadIdleTimeout() {
     localAudioEnabled.value = res.data.localAudioEnabled ?? true;
     applyGuestModeFromServer(res.data.guestMode);
     applyAdminGroupsFromServer(res.data.adminGroups);
+    applySpotifyConfig(res.data.spotify);
   } catch { /* ignore */ }
 }
 
@@ -1166,6 +1275,97 @@ async function saveLocalAudioEnabled() {
   } catch { /* ignore */ }
 }
 
+// --- Spotify (Connect) settings (§8) ---
+// NOTE: This card's status comes ONLY from /api/spotify/status (playback OAuth
+// `authorized`). It is deliberately NOT wired to the player store's
+// authStatus.spotify (which reflects /api/auth/status?platform=spotify — the
+// separate metadata Web-API login). Keep the two concepts distinct.
+const spotifyForm = reactive<SpotifyConfigForm>({
+  enabled: false,
+  backend: 'auto',
+  clientId: '',
+  clientSecret: '',
+  deviceName: 'TSMusicBot',
+  bitrate: 320,
+});
+const spotifyHasSecret = ref(false);
+const spotifyStatus = ref<SpotifyStatus | null>(null);
+const spotifySaving = ref(false);
+const spotifyConnecting = ref(false);
+const spotifyMessage = ref('');
+const spotifyMessageTone = ref<'ok' | 'warn'>('ok');
+
+const SPOTIFY_BACKENDS: { value: SpotifyConfigForm['backend']; label: string }[] = [
+  { value: 'auto', label: '自动' },
+  { value: 'go-librespot', label: 'go-librespot' },
+  { value: 'librespot', label: 'librespot' },
+];
+const SPOTIFY_BITRATES = [96, 160, 320];
+
+const spotifyStatusSummary = computed(() => statusSummary(spotifyStatus.value, spotifyForm.enabled));
+
+// Populate the form from the masked GET /api/bot/settings response. The raw
+// clientSecret is write-only and never returned — only `hasClientSecret`.
+function applySpotifyConfig(sp: any) {
+  if (!sp || typeof sp !== 'object') return;
+  spotifyForm.enabled = Boolean(sp.enabled);
+  if (sp.backend === 'auto' || sp.backend === 'go-librespot' || sp.backend === 'librespot') {
+    spotifyForm.backend = sp.backend;
+  }
+  spotifyForm.clientId = typeof sp.clientId === 'string' ? sp.clientId : '';
+  spotifyForm.deviceName = typeof sp.deviceName === 'string' ? sp.deviceName : '';
+  if (typeof sp.bitrate === 'number') spotifyForm.bitrate = sp.bitrate;
+  spotifyForm.clientSecret = ''; // always blank on load; blank on save = unchanged
+  spotifyHasSecret.value = Boolean(sp.hasClientSecret);
+}
+
+async function loadSpotifyStatus() {
+  try {
+    const res = await axios.get('/api/spotify/status');
+    spotifyStatus.value = res.data;
+  } catch {
+    spotifyStatus.value = null;
+  }
+}
+
+async function saveSpotify() {
+  spotifySaving.value = true;
+  try {
+    const res = await axios.post('/api/bot/settings', buildSpotifyPayload(spotifyForm));
+    applySpotifyConfig(res.data?.spotify);
+    await loadSpotifyStatus();
+  } catch { /* ignore */ } finally {
+    spotifySaving.value = false;
+  }
+}
+
+async function connectSpotify() {
+  spotifyConnecting.value = true;
+  spotifyMessage.value = '';
+  try {
+    const { data } = await axios.get('/api/spotify/login');
+    window.location.href = data.url;
+  } catch (err: any) {
+    spotifyConnecting.value = false;
+    spotifyMessageTone.value = 'warn';
+    spotifyMessage.value = err?.response?.status === 403
+      ? '没有权限执行平台登录（需要 platform.auth）'
+      : '无法开始 Spotify 授权，请稍后重试';
+  }
+}
+
+// On mount, surface the ?spotify=success|error redirect result, then strip the
+// param so a refresh doesn't re-show it.
+function handleSpotifyRedirect() {
+  const r = parseSpotifyRedirect(window.location.search);
+  if (!r) return;
+  spotifyMessageTone.value = r === 'success' ? 'ok' : 'warn';
+  spotifyMessage.value = r === 'success' ? 'Spotify 授权成功' : 'Spotify 授权失败或被取消';
+  const url = new URL(window.location.href);
+  url.searchParams.delete('spotify');
+  history.replaceState(null, '', url.pathname + url.search + url.hash);
+}
+
 // --- Guest mode (admin only) ---
 const GUEST_FLAGS: { token: string; label: string }[] = [
   { token: 'addToQueue', label: '添加到队列末尾' },
@@ -1630,7 +1830,9 @@ onMounted(() => {
   store.fetchBots(); // Refresh bot status on page visit
   checkAuthStatus();
   loadQuality();
-  loadIdleTimeout();
+  loadIdleTimeout(); // also populates the Spotify config form (same endpoint)
+  loadSpotifyStatus();
+  handleSpotifyRedirect();
   if (session.isAdmin.value) {
     loadUsers();
     loadAudit();
@@ -1891,6 +2093,79 @@ onUnmounted(() => {
   align-self: flex-end;
 }
 
+// --- Spotify (Connect) card ---
+.spotify-disclaimer {
+  font-size: 13px;
+  line-height: 1.6;
+  color: var(--color-paused);
+  background: var(--color-paused-15);
+  border-radius: var(--radius-sm);
+  padding: 12px 14px;
+  margin: -4px 0 16px;
+}
+
+.spotify-icon { color: #1db954; }
+
+.account-status {
+  &.tone-ok { color: var(--color-online); }
+  &.tone-warn { color: var(--color-paused); }
+  &.tone-off { color: var(--text-tertiary); }
+}
+
+.spotify-toggle { padding-top: 0; margin-bottom: 4px; }
+.spotify-row { margin-top: 4px; }
+
+.spotify-btn-group {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+}
+
+.spotify-choice {
+  padding: 8px 16px;
+  background: var(--bg-card);
+  border: 1px solid var(--border-color);
+  border-radius: var(--radius-sm);
+  font-size: 13px;
+  font-weight: 500;
+  cursor: pointer;
+  transition: all var(--transition-fast);
+
+  &:hover { border-color: var(--color-primary); color: var(--color-primary); }
+  &.active {
+    background: var(--color-primary-10);
+    border-color: var(--color-primary);
+    color: var(--color-primary);
+  }
+}
+
+.spotify-secret-hint {
+  margin-left: 8px;
+  font-size: 11px;
+  font-weight: 500;
+  color: var(--text-tertiary);
+  &.set { color: var(--color-online); }
+}
+
+.spotify-actions {
+  display: flex;
+  gap: 10px;
+  margin-top: 8px;
+}
+
+.spotify-connect {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+}
+
+.spotify-message {
+  margin: 10px 0 0;
+  font-size: 13px;
+  &.tone-ok { color: var(--color-online); }
+  &.tone-warn { color: #e26a6a; }
+}
+
 // Shared
 .form-row {
   display: flex;