feat(KB-147): auto-pause engine on API usage limit errors

- Add UsageLimitPauser class and isUsageLimitError detector for rate limits, overloaded, and quota errors
- Integrate usage limit detection into executor, triage, and merger error handlers
- Wire shared UsageLimitPauser instance in dashboard startup across all agents
- Export UsageLimitPauser and isUsageLimitError from @kb/engine public API
- Add comprehensive tests for detector patterns and agent integration

packages/engine/src/executor.test.ts

@@ -69,6 +69,7 @@ function createMockStore() {
     moveTask: vi.fn().mockResolvedValue({}),
     logEntry: vi.fn().mockResolvedValue(undefined),
     parseStepsFromPrompt: vi.fn().mockResolvedValue([]),
+    updateSettings: vi.fn().mockResolvedValue({}),
     getSettings: vi.fn().mockResolvedValue({
       maxConcurrent: 2,
       maxWorktrees: 4,
@@ -2331,3 +2332,139 @@ describe("task_add_dep tool", () => {
     expect(store.updateTask).not.toHaveBeenCalledWith("KB-DEP", { status: "failed" });
   });
 });
+
+// ── Usage limit detection in executor ────────────────────────────────
+
+import { UsageLimitPauser } from "./usage-limit-detector.js";
+
+describe("TaskExecutor usage limit detection", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockedExistsSync.mockReturnValue(true);
+  });
+
+  it("triggers global pause when executor catches a usage-limit error", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockRejectedValue(new Error("rate_limit_error: Rate limit exceeded"));
+
+    const onError = vi.fn();
+    const executor = new TaskExecutor(store, "/tmp/test", {
+      onError,
+      usageLimitPauser: pauser,
+    });
+
+    await executor.execute({
+      id: "KB-001",
+      title: "Test",
+      description: "Test",
+      column: "in-progress",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    expect(onUsageLimitHitSpy).toHaveBeenCalledWith(
+      "executor",
+      "KB-001",
+      "rate_limit_error: Rate limit exceeded",
+    );
+    expect(store.updateSettings).toHaveBeenCalledWith({ globalPause: true });
+    // Task should still be marked as failed
+    expect(store.updateTask).toHaveBeenCalledWith("KB-001", { status: "failed" });
+    expect(onError).toHaveBeenCalled();
+  });
+
+  it("does NOT trigger global pause for non-usage-limit errors", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockRejectedValue(new Error("connection refused"));
+
+    const onError = vi.fn();
+    const executor = new TaskExecutor(store, "/tmp/test", {
+      onError,
+      usageLimitPauser: pauser,
+    });
+
+    await executor.execute({
+      id: "KB-001",
+      title: "Test",
+      description: "Test",
+      column: "in-progress",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    expect(onUsageLimitHitSpy).not.toHaveBeenCalled();
+    // Task should still be marked as failed
+    expect(store.updateTask).toHaveBeenCalledWith("KB-001", { status: "failed" });
+  });
+
+  it("works without usageLimitPauser (backward compatible)", async () => {
+    const store = createMockStore();
+
+    mockedCreateHaiAgent.mockRejectedValue(new Error("rate_limit_error: Rate limit exceeded"));
+
+    const onError = vi.fn();
+    const executor = new TaskExecutor(store, "/tmp/test", { onError });
+
+    await executor.execute({
+      id: "KB-001",
+      title: "Test",
+      description: "Test",
+      column: "in-progress",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    // Should not crash — just mark as failed
+    expect(store.updateTask).toHaveBeenCalledWith("KB-001", { status: "failed" });
+    expect(onError).toHaveBeenCalled();
+  });
+
+  it("triggers global pause for overloaded error", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockRejectedValue(new Error("overloaded_error: Overloaded"));
+
+    const executor = new TaskExecutor(store, "/tmp/test", {
+      usageLimitPauser: pauser,
+    });
+
+    await executor.execute({
+      id: "KB-002",
+      title: "Test",
+      description: "Test",
+      column: "in-progress",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    expect(onUsageLimitHitSpy).toHaveBeenCalledWith(
+      "executor",
+      "KB-002",
+      "overloaded_error: Overloaded",
+    );
+  });
+});

packages/engine/src/executor.ts

@@ -12,6 +12,7 @@ import { PRIORITY_EXECUTE, type AgentSemaphore } from "./concurrency.js";
 import type { WorktreePool } from "./worktree-pool.js";
 import { AgentLogger } from "./agent-logger.js";
 import { executorLog, reviewerLog } from "./logger.js";
+import { isUsageLimitError, type UsageLimitPauser } from "./usage-limit-detector.js";
 
 // Re-export for backward compatibility (tests import from executor.ts)
 export { summarizeToolArgs } from "./agent-logger.js";
@@ -144,6 +145,8 @@ export interface TaskExecutorOptions {
   semaphore?: AgentSemaphore;
   /** Worktree pool for recycling idle worktrees across tasks. */
   pool?: WorktreePool;
+  /** Usage limit pauser — triggers global pause when API limits are detected. */
+  usageLimitPauser?: UsageLimitPauser;
   onStart?: (task: Task, worktreePath: string) => void;
   onComplete?: (task: Task) => void;
   onError?: (task: Task, error: Error) => void;
@@ -452,6 +455,10 @@ export class TaskExecutor {
         await this.store.logEntry(task.id, "Execution paused — agent terminated, moved to todo");
         await this.store.moveTask(task.id, "todo");
       } else {
+        // Check if the error is a usage-limit error and trigger global pause
+        if (this.options.usageLimitPauser && isUsageLimitError(err.message)) {
+          await this.options.usageLimitPauser.onUsageLimitHit("executor", task.id, err.message);
+        }
         executorLog.error(`✗ ${task.id} execution failed:`, err.message);
         await this.store.logEntry(task.id, `Execution failed: ${err.message}`);
         await this.store.updateTask(task.id, { status: "failed" });

packages/engine/src/index.ts

@@ -8,3 +8,4 @@ export { reviewStep, type ReviewType, type ReviewVerdict, type ReviewResult, typ
 export { createKbAgent, type AgentOptions, type AgentResult } from "./pi.js";
 export { WorktreePool } from "./worktree-pool.js";
 export { createLogger, type Logger } from "./logger.js";
+export { isUsageLimitError, UsageLimitPauser } from "./usage-limit-detector.js";

packages/engine/src/merger.test.ts

@@ -46,6 +46,7 @@ function createMockStore(taskOverrides: Partial<Task> = {}, allTasks: Task[] = [
     moveTask: vi.fn().mockResolvedValue(baseTask),
     logEntry: vi.fn().mockResolvedValue(undefined),
     appendAgentLog: vi.fn().mockResolvedValue(undefined),
+    updateSettings: vi.fn().mockResolvedValue({}),
     getSettings: vi.fn().mockResolvedValue({ ...DEFAULT_SETTINGS }),
     emit: vi.fn(),
     on: vi.fn(),
@@ -426,3 +427,109 @@ describe("aiMergeTask — agent log persistence", () => {
     expect(store.appendAgentLog).toHaveBeenCalledWith("KB-050", "hi", "text", undefined, "merger");
   });
 });
+
+// ── Usage limit detection in merger ──────────────────────────────────
+
+import { UsageLimitPauser } from "./usage-limit-detector.js";
+
+describe("aiMergeTask — usage limit detection", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockedExistsSync.mockReturnValue(true);
+    setupHappyPathExecSync();
+  });
+
+  it("triggers global pause when merger catches a usage-limit error", async () => {
+    const store = createMockStore(
+      { id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050" },
+      [{ id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050", column: "in-review" } as Task],
+    );
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockResolvedValue({
+      session: {
+        prompt: vi.fn().mockRejectedValue(new Error("rate_limit_error: Rate limit exceeded")),
+        dispose: vi.fn(),
+      },
+    } as any);
+
+    await expect(
+      aiMergeTask(store, "/tmp/root", "KB-050", { usageLimitPauser: pauser }),
+    ).rejects.toThrow("AI merge failed");
+
+    expect(onUsageLimitHitSpy).toHaveBeenCalledWith(
+      "merger",
+      "KB-050",
+      "rate_limit_error: Rate limit exceeded",
+    );
+    expect(store.updateSettings).toHaveBeenCalledWith({ globalPause: true });
+  });
+
+  it("does NOT trigger global pause for non-usage-limit errors", async () => {
+    const store = createMockStore(
+      { id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050" },
+      [{ id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050", column: "in-review" } as Task],
+    );
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockResolvedValue({
+      session: {
+        prompt: vi.fn().mockRejectedValue(new Error("connection refused")),
+        dispose: vi.fn(),
+      },
+    } as any);
+
+    await expect(
+      aiMergeTask(store, "/tmp/root", "KB-050", { usageLimitPauser: pauser }),
+    ).rejects.toThrow("AI merge failed");
+
+    expect(onUsageLimitHitSpy).not.toHaveBeenCalled();
+  });
+
+  it("works without usageLimitPauser (backward compatible)", async () => {
+    const store = createMockStore(
+      { id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050" },
+      [{ id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050", column: "in-review" } as Task],
+    );
+
+    mockedCreateHaiAgent.mockResolvedValue({
+      session: {
+        prompt: vi.fn().mockRejectedValue(new Error("rate_limit_error: Rate limit exceeded")),
+        dispose: vi.fn(),
+      },
+    } as any);
+
+    // Should not crash — just re-throw
+    await expect(
+      aiMergeTask(store, "/tmp/root", "KB-050"),
+    ).rejects.toThrow("AI merge failed");
+  });
+
+  it("triggers global pause for overloaded error", async () => {
+    const store = createMockStore(
+      { id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050" },
+      [{ id: "KB-050", worktree: "/tmp/root/.worktrees/KB-050", column: "in-review" } as Task],
+    );
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockResolvedValue({
+      session: {
+        prompt: vi.fn().mockRejectedValue(new Error("overloaded_error: Overloaded")),
+        dispose: vi.fn(),
+      },
+    } as any);
+
+    await expect(
+      aiMergeTask(store, "/tmp/root", "KB-050", { usageLimitPauser: pauser }),
+    ).rejects.toThrow("AI merge failed");
+
+    expect(onUsageLimitHitSpy).toHaveBeenCalledWith(
+      "merger",
+      "KB-050",
+      "overloaded_error: Overloaded",
+    );
+  });
+});

packages/engine/src/merger.ts

@@ -5,6 +5,7 @@ import { createKbAgent } from "./pi.js";
 import type { WorktreePool } from "./worktree-pool.js";
 import { AgentLogger } from "./agent-logger.js";
 import { mergerLog } from "./logger.js";
+import { isUsageLimitError, type UsageLimitPauser } from "./usage-limit-detector.js";
 
 /**
  * Build the merge system prompt. When `includeTaskId` is true (default),
@@ -104,6 +105,8 @@ export interface MergerOptions {
   /** Worktree pool — when provided and `recycleWorktrees` is enabled,
    *  worktrees are released to the pool instead of being removed. */
   pool?: WorktreePool;
+  /** Usage limit pauser — triggers global pause when API limits are detected. */
+  usageLimitPauser?: UsageLimitPauser;
 }
 
 /**
@@ -269,6 +272,10 @@ export async function aiMergeTask(
   } catch (err: any) {
     // Agent failed — try to abort the merge
     mergerLog.error(`Agent failed: ${err.message}`);
+    // Check if the error is a usage-limit error and trigger global pause
+    if (options.usageLimitPauser && isUsageLimitError(err.message)) {
+      await options.usageLimitPauser.onUsageLimitHit("merger", taskId, err.message);
+    }
     try {
       execSync("git reset --merge", { cwd: rootDir, stdio: "pipe" });
     } catch { /* */ }

packages/engine/src/triage.test.ts

@@ -38,6 +38,7 @@ function createMockStore(tasks: any[] = []) {
     parseDependenciesFromPrompt: vi.fn().mockResolvedValue([]),
     logEntry: vi.fn().mockResolvedValue({}),
     deleteTask: vi.fn().mockResolvedValue({}),
+    updateSettings: vi.fn().mockResolvedValue({}),
     getSettings: vi.fn().mockResolvedValue({
       maxConcurrent: 2,
       maxWorktrees: 4,
@@ -952,3 +953,141 @@ describe("TriageProcessor dependency parsing", () => {
     });
   });
 });
+
+// ── Usage limit detection in triage ──────────────────────────────────
+
+import { UsageLimitPauser } from "./usage-limit-detector.js";
+
+describe("TriageProcessor usage limit detection", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("triggers global pause when triage catches a usage-limit error", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockRejectedValue(new Error("rate_limit_error: Rate limit exceeded"));
+
+    const onError = vi.fn();
+    const triage = new TriageProcessor(store, "/tmp/test", {
+      onSpecifyError: onError,
+      usageLimitPauser: pauser,
+    });
+
+    await triage.specifyTask({
+      id: "KB-001",
+      title: "Test",
+      description: "Test",
+      column: "triage",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    expect(onUsageLimitHitSpy).toHaveBeenCalledWith(
+      "triage",
+      "KB-001",
+      "rate_limit_error: Rate limit exceeded",
+    );
+    expect(store.updateSettings).toHaveBeenCalledWith({ globalPause: true });
+    // Error callback should still fire
+    expect(onError).toHaveBeenCalled();
+  });
+
+  it("does NOT trigger global pause for non-usage-limit errors", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    mockedCreateHaiAgent.mockRejectedValue(new Error("connection refused"));
+
+    const onError = vi.fn();
+    const triage = new TriageProcessor(store, "/tmp/test", {
+      onSpecifyError: onError,
+      usageLimitPauser: pauser,
+    });
+
+    await triage.specifyTask({
+      id: "KB-001",
+      title: "Test",
+      description: "Test",
+      column: "triage",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    expect(onUsageLimitHitSpy).not.toHaveBeenCalled();
+    expect(onError).toHaveBeenCalled();
+  });
+
+  it("does NOT trigger global pause for ENOENT errors (deleted tasks)", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+    const onUsageLimitHitSpy = vi.spyOn(pauser, "onUsageLimitHit");
+
+    const enoentError = Object.assign(
+      new Error("ENOENT: no such file or directory"),
+      { code: "ENOENT" },
+    );
+    store.updateTask.mockRejectedValue(enoentError);
+
+    const onError = vi.fn();
+    const triage = new TriageProcessor(store, "/tmp/test", {
+      onSpecifyError: onError,
+      usageLimitPauser: pauser,
+    });
+
+    await triage.specifyTask({
+      id: "KB-001",
+      title: "Test",
+      description: "Test",
+      column: "triage",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    expect(onUsageLimitHitSpy).not.toHaveBeenCalled();
+    // ENOENT errors don't call onSpecifyError
+    expect(onError).not.toHaveBeenCalled();
+  });
+
+  it("works without usageLimitPauser (backward compatible)", async () => {
+    const store = createMockStore();
+
+    mockedCreateHaiAgent.mockRejectedValue(new Error("rate_limit_error: Rate limit exceeded"));
+
+    const onError = vi.fn();
+    const triage = new TriageProcessor(store, "/tmp/test", {
+      onSpecifyError: onError,
+    });
+
+    await triage.specifyTask({
+      id: "KB-001",
+      title: "Test",
+      description: "Test",
+      column: "triage",
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    });
+
+    // Should not crash — just call onError
+    expect(onError).toHaveBeenCalled();
+  });
+});

packages/engine/src/triage.ts

@@ -6,6 +6,7 @@ import { createKbAgent } from "./pi.js";
 import { PRIORITY_SPECIFY, type AgentSemaphore } from "./concurrency.js";
 import { AgentLogger } from "./agent-logger.js";
 import { triageLog } from "./logger.js";
+import { isUsageLimitError, type UsageLimitPauser } from "./usage-limit-detector.js";
 
 const TRIAGE_SYSTEM_PROMPT = `You are a task specification agent for "kb", an AI-orchestrated task board.
 
@@ -157,6 +158,8 @@ Write the PROMPT.md directly using the write tool. Nothing else.`;
 export interface TriageProcessorOptions {
   pollIntervalMs?: number;
   semaphore?: AgentSemaphore;
+  /** Usage limit pauser — triggers global pause when API limits are detected. */
+  usageLimitPauser?: UsageLimitPauser;
   onSpecifyStart?: (task: Task) => void;
   onSpecifyComplete?: (task: Task) => void;
   onSpecifyError?: (task: Task, error: Error) => void;
@@ -361,6 +364,10 @@ export class TriageProcessor {
       if (err.code === "ENOENT") {
         triageLog.log(`${task.id} no longer exists — skipping`);
       } else {
+        // Check if the error is a usage-limit error and trigger global pause
+        if (this.options.usageLimitPauser && isUsageLimitError(err.message)) {
+          await this.options.usageLimitPauser.onUsageLimitHit("triage", task.id, err.message);
+        }
         await this.store.updateTask(task.id, { status: null }).catch(() => {});
         triageLog.error(`✗ ${task.id} specification failed:`, err.message);
         this.options.onSpecifyError?.(task, err);

packages/engine/src/usage-limit-detector.test.ts

@@ -0,0 +1,155 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { isUsageLimitError, UsageLimitPauser } from "./usage-limit-detector.js";
+
+// ── isUsageLimitError classification tests ───────────────────────────
+
+describe("isUsageLimitError", () => {
+  describe("should match usage-limit errors", () => {
+    const usageLimitMessages = [
+      // Anthropic overloaded
+      "overloaded_error: Overloaded",
+      "API is overloaded",
+      // Rate limiting
+      "rate_limit_error: Rate limit exceeded",
+      "rate limit exceeded",
+      "Rate Limit Reached",
+      "Too many requests",
+      "too many requests, please retry after 60s",
+      // HTTP status codes
+      "Request failed with status 429",
+      "HTTP 429: Too Many Requests",
+      "529 overloaded",
+      "Status 529",
+      // Quota / billing
+      "quota exceeded for this billing period",
+      "Quota limit reached",
+      "billing account is inactive",
+      "Billing issue detected",
+      "insufficient credit balance",
+      "Insufficient credits",
+      "credit balance too low",
+    ];
+
+    for (const msg of usageLimitMessages) {
+      it(`matches: "${msg}"`, () => {
+        expect(isUsageLimitError(msg)).toBe(true);
+      });
+    }
+  });
+
+  describe("should NOT match transient server errors", () => {
+    const transientMessages = [
+      "Internal Server Error",
+      "Request failed with status 500",
+      "HTTP 502: Bad Gateway",
+      "503 Service Unavailable",
+      "504 Gateway Timeout",
+      "connection refused",
+      "Connection reset by peer",
+      "ECONNREFUSED",
+      "timeout exceeded",
+      "request timed out",
+      "socket hang up",
+      "network error",
+      "ETIMEDOUT",
+      "DNS lookup failed",
+      "getaddrinfo ENOTFOUND",
+    ];
+
+    for (const msg of transientMessages) {
+      it(`does not match: "${msg}"`, () => {
+        expect(isUsageLimitError(msg)).toBe(false);
+      });
+    }
+  });
+
+  it("returns false for empty string", () => {
+    expect(isUsageLimitError("")).toBe(false);
+  });
+
+  it("returns false for generic error messages", () => {
+    expect(isUsageLimitError("Something went wrong")).toBe(false);
+    expect(isUsageLimitError("Unexpected token in JSON")).toBe(false);
+  });
+});
+
+// ── UsageLimitPauser tests ───────────────────────────────────────────
+
+function createMockStore(globalPause = false) {
+  return {
+    getSettings: vi.fn().mockResolvedValue({ globalPause }),
+    updateSettings: vi.fn().mockResolvedValue({ globalPause: true }),
+    logEntry: vi.fn().mockResolvedValue(undefined),
+  } as any;
+}
+
+describe("UsageLimitPauser", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("calls store.updateSettings({ globalPause: true }) on usage limit hit", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+
+    await pauser.onUsageLimitHit("executor", "KB-001", "rate_limit_error: Rate limit exceeded");
+
+    expect(store.updateSettings).toHaveBeenCalledWith({ globalPause: true });
+  });
+
+  it("logs the triggering error on the task via store.logEntry", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+
+    await pauser.onUsageLimitHit("triage", "KB-002", "overloaded_error");
+
+    expect(store.logEntry).toHaveBeenCalledWith(
+      "KB-002",
+      "Usage limit detected (triage): overloaded_error",
+    );
+  });
+
+  it("is idempotent — calling multiple times only triggers one pause", async () => {
+    const store = createMockStore();
+    // After first call, globalPause will be true
+    store.getSettings.mockResolvedValue({ globalPause: true });
+
+    const pauser = new UsageLimitPauser(store);
+
+    await pauser.onUsageLimitHit("executor", "KB-001", "rate limit");
+    await pauser.onUsageLimitHit("triage", "KB-002", "rate limit");
+    await pauser.onUsageLimitHit("merger", "KB-003", "rate limit");
+
+    // updateSettings should only be called once
+    expect(store.updateSettings).toHaveBeenCalledTimes(1);
+  });
+
+  it("re-triggers pause if globalPause was externally reset to false", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+
+    // First hit — triggers pause
+    store.getSettings.mockResolvedValue({ globalPause: true });
+    await pauser.onUsageLimitHit("executor", "KB-001", "rate limit");
+    expect(store.updateSettings).toHaveBeenCalledTimes(1);
+
+    // External reset: globalPause set to false
+    store.getSettings.mockResolvedValue({ globalPause: false });
+
+    // Second hit — should trigger again since it was reset
+    await pauser.onUsageLimitHit("executor", "KB-004", "rate limit again");
+    expect(store.updateSettings).toHaveBeenCalledTimes(2);
+  });
+
+  it("includes agent type in the log entry", async () => {
+    const store = createMockStore();
+    const pauser = new UsageLimitPauser(store);
+
+    await pauser.onUsageLimitHit("merger", "KB-005", "quota exceeded");
+
+    expect(store.logEntry).toHaveBeenCalledWith(
+      "KB-005",
+      expect.stringContaining("merger"),
+    );
+  });
+});

packages/engine/src/usage-limit-detector.ts

@@ -0,0 +1,93 @@
+/**
+ * Usage Limit Detector — classifies API errors as usage-limit-related
+ * and triggers the global pause mechanism when detected.
+ *
+ * Usage-limit errors indicate systemic conditions (rate limits, quota exceeded,
+ * billing issues, overloaded APIs) where continued retrying across multiple
+ * agents is wasteful. Transient server errors (500, timeout, connection refused)
+ * are NOT classified as usage-limit errors — they are temporary and may resolve
+ * on their own via per-session retry.
+ */
+
+import type { TaskStore } from "@kb/core";
+import { createLogger } from "./logger.js";
+
+const log = createLogger("usage-limit");
+
+/**
+ * Patterns that indicate API usage/capacity/billing limits.
+ * These are checked case-insensitively against error messages.
+ */
+const USAGE_LIMIT_PATTERNS: RegExp[] = [
+  /overloaded/i,
+  /rate[_\s]?limit/i,
+  /too many requests/i,
+  /\b429\b/,
+  /\b529\b/,
+  /quota/i,
+  /billing/i,
+  /\bcredit/i,
+  /insufficient/i,
+];
+
+/**
+ * Classify whether an error message indicates a usage-limit condition.
+ *
+ * Returns `true` for rate limits, overloaded errors, quota/billing issues —
+ * conditions where all agents should stop. Returns `false` for transient
+ * server errors (500/502/503/504, timeout, connection refused) that may
+ * resolve on their own.
+ */
+export function isUsageLimitError(errorMessage: string): boolean {
+  return USAGE_LIMIT_PATTERNS.some((pattern) => pattern.test(errorMessage));
+}
+
+/**
+ * Lightweight coordinator that agents call when they detect usage-limit errors.
+ * Triggers the global pause mechanism by calling `store.updateSettings({ globalPause: true })`.
+ *
+ * **Idempotency:** Tracks an internal `paused` flag so that multiple concurrent
+ * agents hitting limits only trigger one pause. The flag resets when `globalPause`
+ * is externally set back to `false` (detected by reading settings before pausing).
+ */
+export class UsageLimitPauser {
+  private paused = false;
+
+  constructor(private store: TaskStore) {}
+
+  /**
+   * Called by agents when a usage-limit error is detected after retries are exhausted.
+   * Triggers global pause if not already paused.
+   *
+   * @param agentType - The type of agent that hit the limit (e.g., "executor", "triage", "merger")
+   * @param taskId - The task that was being processed when the limit was hit
+   * @param errorMessage - The error message from the API
+   */
+  async onUsageLimitHit(agentType: string, taskId: string, errorMessage: string): Promise<void> {
+    // If we already triggered a pause, check if it was externally reset
+    if (this.paused) {
+      const settings = await this.store.getSettings();
+      if (settings.globalPause) {
+        // Still paused — no need to trigger again
+        return;
+      }
+      // External reset detected — allow re-triggering
+      this.paused = false;
+    }
+
+    this.paused = true;
+
+    log.warn(`${agentType} hit usage limit on ${taskId}: ${errorMessage}`);
+
+    // Log the triggering error on the task
+    await this.store.logEntry(
+      taskId,
+      `Usage limit detected (${agentType}): ${errorMessage}`,
+    );
+
+    // Activate global pause
+    await this.store.updateSettings({ globalPause: true });
+
+    log.warn("⚠ Global pause activated — all automated activity will halt");
+  }
+}