fix(background-agent): coalesce parent wake races

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/parent-wake-notifier.ts

@@ -194,6 +194,12 @@ export class ParentWakeNotifier {
       if (promptResult.status === "failed") {
         throw promptResult.error
       }
+      if (promptResult.status === "reserved" && promptResult.reservedBy === "background-agent-parent-wake") {
+        log("[background-agent] Ignored duplicate parent wake flush already reserved by promptAsync gate:", {
+          sessionID,
+        })
+        return
+      }
       if (promptResult.status !== "dispatched") {
         this.requeueWake(sessionID, latestWake)
         this.schedulePendingParentWakeFlush(sessionID)
@@ -451,7 +457,7 @@ export class ParentWakeNotifier {
         if (createdAt === undefined) {
           return false
         }
-        return Date.now() - createdAt < this.options.userMessageInProgressWindowMs
+        return Date.now() - createdAt <= this.options.userMessageInProgressWindowMs
       }
       if (role === "assistant" || role === "tool") {
         // An assistant/tool message is more recent than the last user message,

src/features/background-agent/parent-wake-user-message-race.test.ts

@@ -67,6 +67,155 @@ function createNotifier(args: {
 }
 
 describe("ParentWakeNotifier — user message race guard (issue #4120)", () => {
+  test("#given user message was created exactly at the race-window boundary #when flushing pending wake #then dispatch is still deferred", async () => {
+    // given
+    const originalDateNow = Date.now
+    Date.now = () => 10_000
+    const { notifier, promptAsyncCalls } = createNotifier({
+      sessionMessages: [
+        {
+          info: {
+            role: "user",
+            time: { created: 8_000 },
+          },
+        },
+      ],
+      userMessageInProgressWindowMs: 2_000,
+    })
+    notifier.queuePendingParentWake(
+      "parent-boundary",
+      "task complete",
+      { agent: "sisyphus" },
+      true,
+    )
+
+    try {
+      // when
+      await notifier.flushPendingParentWake("parent-boundary")
+
+      // then
+      expect(promptAsyncCalls).toHaveLength(0)
+      expect(notifier.getPendingParentWakes().has("parent-boundary")).toBe(true)
+    } finally {
+      Date.now = originalDateNow
+      notifier.shutdown()
+      releaseAllPromptAsyncReservationsForTesting()
+    }
+  })
+
+  test("#given user message is just outside the race-window boundary #when flushing pending wake #then dispatch proceeds", async () => {
+    // given
+    const originalDateNow = Date.now
+    Date.now = () => 10_001
+    const { notifier, promptAsyncCalls } = createNotifier({
+      sessionMessages: [
+        {
+          info: {
+            role: "user",
+            time: { created: 8_000 },
+          },
+        },
+      ],
+      userMessageInProgressWindowMs: 2_000,
+    })
+    notifier.queuePendingParentWake(
+      "parent-boundary-open",
+      "task complete",
+      { agent: "sisyphus" },
+      true,
+    )
+
+    try {
+      // when
+      await notifier.flushPendingParentWake("parent-boundary-open")
+
+      // then
+      expect(promptAsyncCalls).toHaveLength(1)
+      expect(notifier.getPendingParentWakes().has("parent-boundary-open")).toBe(false)
+    } finally {
+      Date.now = originalDateNow
+      notifier.shutdown()
+      releaseAllPromptAsyncReservationsForTesting()
+    }
+  })
+
+  test("#given two flushes race for one pending wake #when both reach the prompt gate #then the skipped duplicate is not requeued", async () => {
+    // given
+    const { notifier, promptAsyncCalls } = createNotifier({
+      sessionMessages: [
+        {
+          info: {
+            role: "assistant",
+            finish: "stop",
+            time: { created: Date.now() - 10_000 },
+          },
+        },
+      ],
+    })
+    notifier.queuePendingParentWake(
+      "parent-concurrent",
+      "task complete",
+      { agent: "sisyphus" },
+      true,
+    )
+
+    // when
+    await Promise.all([
+      notifier.flushPendingParentWake("parent-concurrent"),
+      notifier.flushPendingParentWake("parent-concurrent"),
+    ])
+
+    // then
+    expect(promptAsyncCalls).toHaveLength(1)
+    expect(notifier.getPendingParentWakes().has("parent-concurrent")).toBe(false)
+    expect(notifier.getPendingParentWakeTimers().has("parent-concurrent")).toBe(false)
+
+    notifier.shutdown()
+    releaseAllPromptAsyncReservationsForTesting()
+  })
+
+  test("#given burst notifications share a parent session #when the pending wake flushes #then one dispatch drains the coalesced wake", async () => {
+    // given
+    const { notifier, promptAsyncCalls } = createNotifier({
+      sessionMessages: [
+        {
+          info: {
+            role: "assistant",
+            finish: "stop",
+            time: { created: Date.now() - 10_000 },
+          },
+        },
+      ],
+    })
+    notifier.queuePendingParentWake(
+      "parent-burst",
+      "task one complete",
+      { agent: "sisyphus" },
+      false,
+    )
+    notifier.queuePendingParentWake(
+      "parent-burst",
+      "task two complete",
+      { agent: "sisyphus" },
+      true,
+    )
+
+    // when
+    await Promise.all([
+      notifier.flushPendingParentWake("parent-burst"),
+      notifier.flushPendingParentWake("parent-burst"),
+    ])
+
+    // then
+    expect(promptAsyncCalls).toHaveLength(1)
+    expect(promptAsyncCalls[0]?.body.noReply).toBe(false)
+    expect(notifier.getPendingParentWakes().has("parent-burst")).toBe(false)
+    expect(notifier.getPendingParentWakeTimers().has("parent-burst")).toBe(false)
+
+    notifier.shutdown()
+    releaseAllPromptAsyncReservationsForTesting()
+  })
+
   test("#given latest message is a user message just added #when flushing pending wake #then dispatch is deferred (no promptAsync)", async () => {
     // given
     const { notifier, promptAsyncCalls } = createNotifier({