diff --git a/src/data/users.test.ts b/src/data/users.test.ts
index dc6294b..2576238 100644
--- a/src/data/users.test.ts
+++ b/src/data/users.test.ts
@@ -208,4 +208,19 @@ describe("guest row exclusion", () => {
     expect(users.countUsers()).toBe(1); // alice only
     expect(users.listUsers().some((u) => u.id === GUEST_USER_ID)).toBe(false);
   });
+
+  it("setRoleIfNotLastAdmin refuses to re-role the reserved guest principal", () => {
+    // The guest row is seeded by createDatabase via ensureGuestUser.
+    expect(users.findById(GUEST_USER_ID)!.role).toBe("guest"); // sanity
+    expect(users.setRoleIfNotLastAdmin(GUEST_USER_ID, "admin")).toBe("not_found");
+    // The guest row's role is unchanged.
+    expect(users.findById(GUEST_USER_ID)!.role).toBe("guest");
+  });
+
+  it("deleteUserIfNotLastAdmin refuses to delete the reserved guest principal", () => {
+    expect(users.findById(GUEST_USER_ID)).not.toBeNull(); // sanity
+    expect(users.deleteUserIfNotLastAdmin(GUEST_USER_ID)).toBe("not_found");
+    // The guest row still exists.
+    expect(users.findById(GUEST_USER_ID)).not.toBeNull();
+  });
 });
diff --git a/src/data/users.ts b/src/data/users.ts
index 4a6d3b9..dbb530b 100644
--- a/src/data/users.ts
+++ b/src/data/users.ts
@@ -137,6 +137,7 @@ export function createUserStore(db: Database.Database): UserStore {
       const tx = db.transaction(() => {
         const row = findByIdStmt.get(id) as UserRow | undefined;
         if (!row) return "not_found" as const;
+        if (row.role === "guest") return "not_found" as const; // reserved synthetic principal
         if (row.role === newRole) return "ok" as const; // no-op
         if (row.role === "admin" && newRole === "member") {
           const adminCount = (countAdminsStmt.get() as { n: number }).n;
@@ -161,6 +162,7 @@ export function createUserStore(db: Database.Database): UserStore {
       const tx = db.transaction(() => {
         const row = findByIdStmt.get(id) as UserRow | undefined;
         if (!row) return "not_found" as const;
+        if (row.role === "guest") return "not_found" as const; // reserved synthetic principal
         if (row.role === "admin") {
           const adminCount = (countAdminsStmt.get() as { n: number }).n;
           if (adminCount <= 1) return "would_orphan" as const;
diff --git a/src/web/api/session.ts b/src/web/api/session.ts
index beaef21..4f4d5a0 100644
--- a/src/web/api/session.ts
+++ b/src/web/api/session.ts
@@ -141,7 +141,16 @@ export function createSessionRouter(
       res.status(403).json({ error: "guest mode disabled" });
       return;
     }
-    const { token } = sessions.createSession(GUEST_USER_ID, { ttlMs: GUEST_SESSION_TTL_MS, skipCap: true });
+    let token: string;
+    try {
+      // If the reserved guest row is somehow missing, the session FK would
+      // throw; surface a clean 503 rather than letting it become a 500.
+      ({ token } = sessions.createSession(GUEST_USER_ID, { ttlMs: GUEST_SESSION_TTL_MS, skipCap: true }));
+    } catch (err) {
+      logger.error({ err }, "guest session creation failed");
+      res.status(503).json({ error: "guest unavailable" });
+      return;
+    }
     setSessionCookie(res, token);
     res.json({ id: GUEST_USER_ID, username: GUEST_USERNAME, role: "guest" });
   });
diff --git a/src/web/api/users.test.ts b/src/web/api/users.test.ts
index c422907..1920fe4 100644
--- a/src/web/api/users.test.ts
+++ b/src/web/api/users.test.ts
@@ -4,7 +4,7 @@ import cookieParser from "cookie-parser";
 import request from "supertest";
 import pino from "pino";
 import { createDatabase, type BotDatabase } from "../../data/database.js";
-import { createUserStore, type UserStore } from "../../data/users.js";
+import { createUserStore, GUEST_USER_ID, type UserStore } from "../../data/users.js";
 import { createSessionStore, type SessionStore } from "../../data/sessions.js";
 import { createAuditStore, type AuditStore } from "../../data/audit.js";
 import { createPermissionStore, type PermissionStore } from "../../data/permissions.js";
@@ -342,4 +342,49 @@ describe("users router", () => {
     expect(perms.status).toBe(200);
     expect(perms.body).toEqual({ capabilities: [], bots: [] });
   });
+
+  // --- reserved guest principal is never mutable/visible via user mgmt -------
+  // The synthetic __guest__ row is seeded by createDatabase. findById has no
+  // role filter, so without the 404-guard these by-id handlers would operate
+  // on it (privilege-escalation / DoS / cred-login holes).
+  describe("reserved __guest__ principal is 404 on every by-id handler", () => {
+    it("DELETE /:id → 404 and the guest row survives", async () => {
+      const res = await request(app).delete(`/api/users/${GUEST_USER_ID}`).set("Cookie", aliceCookie);
+      expect(res.status).toBe(404);
+      expect(users.findById(GUEST_USER_ID)).not.toBeNull();
+      expect(users.findById(GUEST_USER_ID)!.role).toBe("guest");
+    });
+
+    it("PATCH /:id/role {role:'admin'} → 404 and the guest role is unchanged", async () => {
+      const res = await request(app)
+        .patch(`/api/users/${GUEST_USER_ID}/role`)
+        .set("Cookie", aliceCookie)
+        .send({ role: "admin" });
+      expect(res.status).toBe(404);
+      expect(users.findById(GUEST_USER_ID)!.role).toBe("guest");
+    });
+
+    it("POST /:id/reset-password → 404 (cannot give the guest a login)", async () => {
+      const res = await request(app)
+        .post(`/api/users/${GUEST_USER_ID}/reset-password`)
+        .set("Cookie", aliceCookie)
+        .send({ newPassword: "guest-new-pw" });
+      expect(res.status).toBe(404);
+    });
+
+    it("GET /:id/permissions → 404", async () => {
+      const res = await request(app)
+        .get(`/api/users/${GUEST_USER_ID}/permissions`)
+        .set("Cookie", aliceCookie);
+      expect(res.status).toBe(404);
+    });
+
+    it("PUT /:id/permissions → 404", async () => {
+      const res = await request(app)
+        .put(`/api/users/${GUEST_USER_ID}/permissions`)
+        .set("Cookie", aliceCookie)
+        .send({ capabilities: ["player.control"], bots: "all" });
+      expect(res.status).toBe(404);
+    });
+  });
 });
diff --git a/src/web/api/users.ts b/src/web/api/users.ts
index 7d38236..0e736fc 100644
--- a/src/web/api/users.ts
+++ b/src/web/api/users.ts
@@ -1,7 +1,7 @@
 import { Router } from "express";
 import type { Logger } from "../../logger.js";
 import type { UserStore } from "../../data/users.js";
-import { UsernameTakenError } from "../../data/users.js";
+import { UsernameTakenError, GUEST_USER_ID } from "../../data/users.js";
 import type { SessionStore } from "../../data/sessions.js";
 import type { AuditStore } from "../../data/audit.js";
 import { isCapability, BASIC_TIER_CAPABILITIES, type PermissionStore } from "../../data/permissions.js";
@@ -63,6 +63,7 @@ export function createUsersRouter(
 
   router.delete("/:id", (req, res) => {
     const targetId = req.params.id;
+    if (targetId === GUEST_USER_ID) { res.status(404).json({ error: "not found" }); return; }
     // Snapshot target's username BEFORE deletion for audit
     const target = users.findById(targetId);
     if (!target) {
@@ -104,6 +105,7 @@ export function createUsersRouter(
       return;
     }
     const targetId = req.params.id;
+    if (targetId === GUEST_USER_ID) { res.status(404).json({ error: "not found" }); return; }
     const target = users.findById(targetId);
     if (!target) {
       res.status(404).json({ error: "not found" });
@@ -130,6 +132,7 @@ export function createUsersRouter(
 
   router.patch("/:id/role", (req, res) => {
     const targetId = req.params.id;
+    if (targetId === GUEST_USER_ID) { res.status(404).json({ error: "not found" }); return; }
     const { role: newRole } = req.body ?? {};
     if (newRole !== "admin" && newRole !== "member") {
       res.status(400).json({ error: "invalid role" });
@@ -168,6 +171,7 @@ export function createUsersRouter(
   });
 
   router.get("/:id/permissions", (req, res) => {
+    if (req.params.id === GUEST_USER_ID) { res.status(404).json({ error: "not found" }); return; }
     const user = users.findById(req.params.id);
     if (!user) {
       res.status(404).json({ error: "not_found" });
@@ -180,6 +184,7 @@ export function createUsersRouter(
   });
 
   router.put("/:id/permissions", (req, res) => {
+    if (req.params.id === GUEST_USER_ID) { res.status(404).json({ error: "not found" }); return; }
     const user = users.findById(req.params.id);
     if (!user) {
       res.status(404).json({ error: "not_found" });