packages/daemon/src/__tests__/daemon-ipc.test.ts

@@ -31,6 +31,7 @@ function makeMockWorkflowManager() {
     stop: vi.fn(async () => {}),
     totalActiveCount: vi.fn(() => 0),
     drainAndRespawn: vi.fn(async () => {}),
+    drainWhenIdle: vi.fn(),
     updateConfig: vi.fn(),
     getActiveWorkflowRuns: vi.fn(() => []),
   };

packages/daemon/src/__tests__/hot-reload.test.ts

@@ -3,6 +3,7 @@
  *
  * Verifies that:
  * - drainAndRespawn() sends shutdown, waits for exit, then respawns the worker
+ * - drainWhenIdle() defers drain+respawn until in-flight threads finish
  * - Kernel dispatches handleWorkflowFileChange when file-watcher emits a workflow change
  * - Kernel logs a workflow_reload system event on hot reload
  * - drainAndRespawn on a non-existent worker is a no-op
@@ -238,6 +239,199 @@ describe("WorkflowManager — drainAndRespawn (Phase 3 hot reload)", () => {
   });
 });
 
+describe("WorkflowManager — drainWhenIdle (hot reload without interrupting in-flight)", () => {
+  beforeEach(() => {
+    mockChildren.length = 0;
+    vi.useFakeTimers({ shouldAdvanceTime: true });
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+    vi.clearAllMocks();
+  });
+
+  it("does not send shutdown while a thread is still active", () => {
+    const logStore = makeLogStore();
+    const config = makeWfConfig({ "my-wf": { concurrency: 1, overflow: "drop" } });
+    const mgr = createWorkflowManager("/nerve-root", config, logStore);
+
+    mgr.startWorkflow("my-wf", { prompt: "test", maxRounds: 10, dryRun: false });
+    const child = mockChildren[0];
+
+    mgr.drainWhenIdle("my-wf");
+
+    const shutdownCalls = (child.send as ReturnType<typeof vi.fn>).mock.calls.filter(
+      (args: unknown[]) =>
+        args[0] !== null &&
+        typeof args[0] === "object" &&
+        (args[0] as Record<string, unknown>).type === "shutdown",
+    );
+    expect(shutdownCalls).toHaveLength(0);
+  });
+
+  it("sends shutdown after the last active thread completes (deferred drain)", async () => {
+    const logStore = makeLogStore();
+    const config = makeWfConfig({ "my-wf": { concurrency: 1, overflow: "drop" } });
+    const mgr = createWorkflowManager("/nerve-root", config, logStore);
+
+    mgr.startWorkflow("my-wf", { prompt: "test", maxRounds: 10, dryRun: false });
+    const child = mockChildren[0];
+    const runId = (child.send as ReturnType<typeof vi.fn>).mock.calls[0][0] as { runId: string };
+
+    mgr.drainWhenIdle("my-wf");
+
+    child.emit("message", {
+      type: "thread-event",
+      runId: runId.runId,
+      eventType: "completed",
+      payload: null,
+    });
+
+    await vi.runAllTimersAsync();
+
+    expect(child.send).toHaveBeenCalledWith(expect.objectContaining({ type: "shutdown" }));
+    expect(mockChildren).toHaveLength(2);
+
+    const stopPromise = mgr.stop();
+    await vi.runAllTimersAsync();
+    await stopPromise;
+  });
+
+  it("waits for all concurrent threads before draining", async () => {
+    const logStore = makeLogStore();
+    const config = makeWfConfig({ "my-wf": { concurrency: 2, overflow: "drop" } });
+    const mgr = createWorkflowManager("/nerve-root", config, logStore);
+
+    mgr.startWorkflow("my-wf", { prompt: "a", maxRounds: 10, dryRun: false });
+    mgr.startWorkflow("my-wf", { prompt: "b", maxRounds: 10, dryRun: false });
+    const child = mockChildren[0];
+    const sendMock = child.send as ReturnType<typeof vi.fn>;
+    const runIdA = (sendMock.mock.calls[0][0] as { runId: string }).runId;
+    const runIdB = (sendMock.mock.calls[1][0] as { runId: string }).runId;
+
+    mgr.drainWhenIdle("my-wf");
+
+    child.emit("message", {
+      type: "thread-event",
+      runId: runIdA,
+      eventType: "completed",
+      payload: null,
+    });
+    await vi.runAllTimersAsync();
+
+    const shutdownBefore = sendMock.mock.calls.filter(
+      (args: unknown[]) =>
+        args[0] !== null &&
+        typeof args[0] === "object" &&
+        (args[0] as Record<string, unknown>).type === "shutdown",
+    );
+    expect(shutdownBefore).toHaveLength(0);
+
+    child.emit("message", {
+      type: "thread-event",
+      runId: runIdB,
+      eventType: "completed",
+      payload: null,
+    });
+    await vi.runAllTimersAsync();
+
+    expect(child.send).toHaveBeenCalledWith(expect.objectContaining({ type: "shutdown" }));
+
+    const stopPromise = mgr.stop();
+    await vi.runAllTimersAsync();
+    await stopPromise;
+  });
+
+  it("duplicate drainWhenIdle while busy only schedules one deferred drain", async () => {
+    const logStore = makeLogStore();
+    const config = makeWfConfig({ "my-wf": { concurrency: 1, overflow: "drop" } });
+    const mgr = createWorkflowManager("/nerve-root", config, logStore);
+
+    mgr.startWorkflow("my-wf", { prompt: "test", maxRounds: 10, dryRun: false });
+    const child = mockChildren[0];
+    const runId = (child.send as ReturnType<typeof vi.fn>).mock.calls[0][0] as { runId: string };
+
+    mgr.drainWhenIdle("my-wf");
+    mgr.drainWhenIdle("my-wf");
+
+    child.emit("message", {
+      type: "thread-event",
+      runId: runId.runId,
+      eventType: "completed",
+      payload: null,
+    });
+    await vi.runAllTimersAsync();
+
+    const shutdownCalls = (child.send as ReturnType<typeof vi.fn>).mock.calls.filter(
+      (args: unknown[]) =>
+        args[0] !== null &&
+        typeof args[0] === "object" &&
+        (args[0] as Record<string, unknown>).type === "shutdown",
+    );
+    expect(shutdownCalls).toHaveLength(1);
+
+    const stopPromise = mgr.stop();
+    await vi.runAllTimersAsync();
+    await stopPromise;
+  });
+
+  it("deferred drain runs after workflow-error clears the active thread", async () => {
+    const logStore = makeLogStore();
+    const config = makeWfConfig({ "my-wf": { concurrency: 1, overflow: "drop" } });
+    const mgr = createWorkflowManager("/nerve-root", config, logStore);
+
+    mgr.startWorkflow("my-wf", { prompt: "test", maxRounds: 10, dryRun: false });
+    const child = mockChildren[0];
+    const runId = (child.send as ReturnType<typeof vi.fn>).mock.calls[0][0] as { runId: string };
+
+    mgr.drainWhenIdle("my-wf");
+
+    child.emit("message", {
+      type: "workflow-error",
+      runId: runId.runId,
+      error: "role failed",
+      exitCode: 1,
+    });
+    await vi.runAllTimersAsync();
+
+    expect(child.send).toHaveBeenCalledWith(expect.objectContaining({ type: "shutdown" }));
+
+    const stopPromise = mgr.stop();
+    await vi.runAllTimersAsync();
+    await stopPromise;
+  });
+
+  it("drains immediately when there is no in-flight thread", async () => {
+    const logStore = makeLogStore();
+    const config = makeWfConfig({ "my-wf": { concurrency: 1, overflow: "drop" } });
+    const mgr = createWorkflowManager("/nerve-root", config, logStore);
+
+    mgr.startWorkflow("my-wf", { prompt: "once", maxRounds: 10, dryRun: false });
+    const firstChild = mockChildren[0];
+    const runId = (firstChild.send as ReturnType<typeof vi.fn>).mock.calls[0][0] as {
+      runId: string;
+    };
+
+    firstChild.emit("message", {
+      type: "thread-event",
+      runId: runId.runId,
+      eventType: "completed",
+      payload: null,
+    });
+    await vi.runAllTimersAsync();
+
+    mgr.drainWhenIdle("my-wf");
+    await vi.runAllTimersAsync();
+
+    expect(firstChild.send).toHaveBeenCalledWith(expect.objectContaining({ type: "shutdown" }));
+    expect(mockChildren).toHaveLength(2);
+
+    const stopPromise = mgr.stop();
+    await vi.runAllTimersAsync();
+    await stopPromise;
+  });
+});
+
 describe("Kernel — workflow hot reload via file-watcher (Phase 3)", () => {
   let nerveRoot: string;
 

packages/daemon/src/kernel-file-watch.ts

@@ -57,10 +57,7 @@ export function createKernelFileWatchHandlers(deps: KernelFileWatchDeps): Kernel
       payload: null,
       timestamp: Date.now(),
     });
-    deps.workflowManager.drainAndRespawn(workflowName).catch((e) => {
-      const msg = e instanceof Error ? e.message : String(e);
-      process.stderr.write(`[kernel] drainAndRespawn error for "${workflowName}": ${msg}\n`);
-    });
+    deps.workflowManager.drainWhenIdle(workflowName);
   }
 
   function onConfigFileChange(): void {

packages/daemon/src/workflow-manager.ts

@@ -57,6 +57,12 @@ export type WorkflowManager = {
    * Waits up to `drainTimeoutMs` for threads to complete before force-killing.
    */
   drainAndRespawn: (workflowName: string, drainTimeoutMs?: number) => Promise<void>;
+  /**
+   * Schedule a drain+respawn that waits for in-flight runs to finish first.
+   * If no runs are active, drains immediately. Otherwise marks a pending reload
+   * and drains automatically when the last active run completes.
+   */
+  drainWhenIdle: (workflowName: string) => void;
   /** Gracefully shut down all workflow workers. */
   stop: () => Promise<void>;
 };
@@ -222,6 +228,7 @@ export function createWorkflowManager(
   const crashTimestamps = new Map<string, number[]>();
   let stopped = false;
   let config = initialConfig;
+  const pendingDrains = new Set<string>();
 
   function getOrCreateState(workflowName: string): WorkflowState {
     let state = states.get(workflowName);
@@ -328,6 +335,24 @@ export function createWorkflowManager(
     }
   }
 
+  /** If a hot-reload was deferred, run drain+respawn once there are no active threads. */
+  function maybeDeferredHotReloadDrain(workflowName: string): void {
+    if (!pendingDrains.has(workflowName)) return;
+    const state = states.get(workflowName);
+    if (state === undefined || state.active.size !== 0) return;
+
+    pendingDrains.delete(workflowName);
+    process.stderr.write(
+      `[workflow-manager] all runs complete for "${workflowName}", executing deferred hot-reload drain\n`,
+    );
+    drainAndRespawn(workflowName).catch((e) => {
+      const errMsg = e instanceof Error ? e.message : String(e);
+      process.stderr.write(
+        `[workflow-manager] deferred drainAndRespawn error for "${workflowName}": ${errMsg}\n`,
+      );
+    });
+  }
+
   function handleThreadEvent(workflowName: string, msg: ThreadEventMessage): void {
     const state = states.get(workflowName);
     if (state === undefined) return;
@@ -337,6 +362,7 @@ export function createWorkflowManager(
       dequeueNext(workflowName);
       const exitCode = extractExitCode(msg.payload);
       logWorkflowEvent(workflowName, msg.runId, msg.eventType, msg.payload, exitCode);
+      maybeDeferredHotReloadDrain(workflowName);
     }
   }
 
@@ -428,6 +454,7 @@ export function createWorkflowManager(
 
     state.active.clear();
     workers.delete(workflowName);
+    pendingDrains.delete(workflowName);
 
     if (stopped || workflowConfig(workflowName) === null) return;
 
@@ -484,6 +511,7 @@ export function createWorkflowManager(
         dequeueNext(workflowName);
       }
       logWorkflowEvent(workflowName, msg.runId, "failed", { error: msg.error }, msg.exitCode);
+      maybeDeferredHotReloadDrain(workflowName);
       return;
     }
 
@@ -681,9 +709,38 @@ export function createWorkflowManager(
     await waitForExit(entry.process, drainTimeoutMs);
     // The exit handler (draining branch) will respawn the worker automatically
   }
+  function drainWhenIdle(workflowName: string): void {
+    const state = states.get(workflowName);
+    const hasActiveRuns = state !== undefined && state.active.size > 0;
+
+    if (!hasActiveRuns) {
+      pendingDrains.delete(workflowName);
+      drainAndRespawn(workflowName).catch((e) => {
+        const msg = e instanceof Error ? e.message : String(e);
+        process.stderr.write(
+          `[workflow-manager] drainAndRespawn error for "${workflowName}": ${msg}\n`,
+        );
+      });
+      return;
+    }
+
+    // Defer until all active runs finish
+    if (pendingDrains.has(workflowName)) {
+      process.stderr.write(
+        `[workflow-manager] hot-reload already pending for "${workflowName}", skipping duplicate\n`,
+      );
+      return;
+    }
+
+    pendingDrains.add(workflowName);
+    process.stderr.write(
+      `[workflow-manager] deferring hot-reload for "${workflowName}" until ${state.active.size} active run(s) complete\n`,
+    );
+  }
 
   async function stop(): Promise<void> {
     stopped = true;
+    pendingDrains.clear();
     const exitPromises: Promise<void>[] = [];
     for (const entry of workers.values()) {
       sendShutdown(entry.process, entry);
@@ -701,6 +758,7 @@ export function createWorkflowManager(
     totalActiveCount,
     updateConfig,
     drainAndRespawn,
+    drainWhenIdle,
     stop,
   };
 }