fix(background-agent): start promptAsync before blocking tmux callback

- Fixes BackgroundManager to launch promptAsync before invoking the blocking tmux callback.
- Adds regression test to ensure promptAsync is called before tmux callback.

src/features/background-agent/manager.test.ts

@@ -322,6 +322,82 @@ function createToastRemoveTaskTracker(): { removeTaskCalls: string[]; resetToast
   }
 }
 
+describe("BackgroundManager tmux callback ordering", () => {
+  test("starts promptAsync before a blocking tmux callback resolves", async () => {
+    //#given
+    const events: string[] = []
+    let resolveTmuxCallback: () => void = () => {}
+    const tmuxCallbackPromise = new Promise<void>((resolve) => {
+      resolveTmuxCallback = resolve
+    })
+
+    const client = {
+      session: {
+        get: async () => {
+          events.push("session.get")
+          return { data: { directory: "/tmp/test" } }
+        },
+        create: async () => {
+          events.push("session.create")
+          return { data: { id: "ses_manager_blocking_tmux" } }
+        },
+        promptAsync: async () => {
+          events.push("promptAsync")
+          return { data: {} }
+        },
+        abort: async () => ({ data: {} }),
+      },
+    }
+
+    const onSubagentSessionCreated = mock(async () => {
+      events.push("tmux.callback.start")
+      await tmuxCallbackPromise
+      events.push("tmux.callback.end")
+    })
+    const manager = new BackgroundManager({
+      pluginContext: createPluginInput(client, "/tmp/test"),
+      tmuxConfig: {
+        enabled: true,
+        layout: "main-vertical",
+        main_pane_size: 60,
+        main_pane_min_width: 120,
+        agent_pane_min_width: 40,
+        isolation: "inline",
+      },
+      onSubagentSessionCreated,
+      enableParentSessionNotifications: false,
+    })
+    const originalTmux = process.env.TMUX
+    process.env.TMUX = "/tmp/fake-tmux-socket"
+
+    try {
+      //#when
+      await manager.launch({
+        description: "Blocking tmux test",
+        prompt: "Do work",
+        agent: "general",
+        parentSessionId: "ses_parent",
+        parentMessageId: "msg_parent",
+      })
+      await new Promise((resolve) => setTimeout(resolve, 20))
+
+      //#then
+      expect(events).toContain("session.create")
+      expect(events).toContain("promptAsync")
+      expect(events).toContain("tmux.callback.start")
+      const promptIdx = events.indexOf("promptAsync")
+      const tmuxStartIdx = events.indexOf("tmux.callback.start")
+      expect(promptIdx < tmuxStartIdx).toBe(true)
+      expect(events).not.toContain("tmux.callback.end")
+    } finally {
+      resolveTmuxCallback()
+      if (originalTmux === undefined) delete process.env.TMUX
+      else process.env.TMUX = originalTmux
+      manager.shutdown()
+    }
+  })
+})
+
 describe("BackgroundManager session.error fallback hydration", () => {
   test("hydrates fallbackChain from session fallback state before retrying sync child-session errors", async () => {
     //#given
@@ -3463,7 +3539,7 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
       expect(getConcurrencyManager(manager).getCount("test-agent")).toBe(0)
     })
 
-    test("should keep task cancelled when cancelled during tmux callback before running state is assigned", async () => {
+      test("should start prompt before tmux callback cancellation", async () => {
       // given
       resetClaudeCodeSessionState()
       const originalTmuxEnvironment = process.env.TMUX
@@ -3474,9 +3550,9 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
         const abortCalls: string[] = []
         const promptAsyncSessionIDs: string[] = []
         let taskID: string | undefined
-        let resolveAbortCalled: (() => void) | undefined
-        const abortCalled = new Promise<void>((resolve) => {
-          resolveAbortCalled = resolve
+        let resolveCancelCalled: (() => void) | undefined
+        const cancelCalled = new Promise<void>((resolve) => {
+          resolveCancelCalled = resolve
         })
 
         manager.shutdown()
@@ -3496,7 +3572,6 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
                 status: async () => ({ data: {} }),
                 abort: async ({ path }: { path: { id: string } }) => {
                   abortCalls.push(path.id)
-                  resolveAbortCalled?.()
                   return {}
                 },
               },
@@ -3523,6 +3598,7 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
                 source: "test",
                 abortSession: false,
               })
+              resolveCancelCalled?.()
             }, }
         )
 
@@ -3539,7 +3615,7 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
 
         // when
         await Promise.race([
-          abortCalled,
+          cancelCalled,
           new Promise<never>((_, reject) => setTimeout(() => reject(new Error("timeout")), 500)),
         ])
         await flushBackgroundNotifications()
@@ -3547,12 +3623,12 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
         // then
         const updatedTask = manager.getTask(task.id)
         expect(updatedTask?.status).toBe("cancelled")
-        expect(updatedTask?.sessionId).toBeUndefined()
-        expect(promptAsyncSessionIDs).not.toContain(createdSessionID)
-        expect(abortCalls).toEqual([createdSessionID])
+        expect(updatedTask?.sessionId).toBe(createdSessionID)
+        expect(promptAsyncSessionIDs).toContain(createdSessionID)
+        expect(abortCalls).toEqual([])
         expect(getConcurrencyManager(manager).getCount("test-agent")).toBe(0)
         expect(getRootDescendantCounts(manager).has("parent-session")).toBe(false)
-        expect(subagentSessions.has(createdSessionID)).toBe(false)
+        expect(subagentSessions.has(createdSessionID)).toBe(true)
       } finally {
         resetClaudeCodeSessionState()
         if (originalTmuxEnvironment === undefined) {

src/features/background-agent/manager.ts

@@ -761,33 +761,8 @@ export class BackgroundManager {
     this.settlePreStartDescendantReservation(task)
     subagentSessions.add(sessionID)
 
-    log("[background-agent] tmux callback check", {
-      hasCallback: !!this.onSubagentSessionCreated,
-      tmuxEnabled: this.tmuxEnabled,
-      isInsideTmux: isInsideTmux(),
-      sessionID,
-      parentID: input.parentSessionId,
-    })
-
-    if (!input.suppressTmuxSpawn && this.onSubagentSessionCreated && this.tmuxEnabled && isInsideTmux()) {
-      log("[background-agent] Invoking tmux callback NOW", { sessionID })
-      await this.onSubagentSessionCreated({
-        sessionID,
-        parentID: input.parentSessionId,
-        title: input.description,
-      }).catch((err) => {
-        log("[background-agent] Failed to spawn tmux pane:", err)
-      })
-      log("[background-agent] tmux callback completed, waiting 200ms")
-      await new Promise(r => setTimeout(r, 200))
-    } else {
-      log("[background-agent] SKIP tmux callback - conditions not met", {
-        suppressTmuxSpawn: !!input.suppressTmuxSpawn,
-      })
-    }
-
     if (this.tasks.get(task.id)?.status === "cancelled") {
-      await this.abortSessionWithLogging(sessionID, "cancelled during tmux setup")
+      await this.abortSessionWithLogging(sessionID, "cancelled during launch setup")
       subagentSessions.delete(sessionID)
       if (task.rootSessionId) {
         this.unregisterRootDescendant(task.rootSessionId)
@@ -982,6 +957,29 @@ The fallback retry session is now created and can be inspected directly.
         })
       }
     })
+
+    log("[background-agent] tmux callback check", {
+      hasCallback: !!this.onSubagentSessionCreated,
+      tmuxEnabled: this.tmuxEnabled,
+      isInsideTmux: isInsideTmux(),
+      sessionID,
+      parentID: input.parentSessionId,
+    })
+
+    if (!input.suppressTmuxSpawn && this.onSubagentSessionCreated && this.tmuxEnabled && isInsideTmux()) {
+      log("[background-agent] Invoking tmux callback (fire-and-forget)", { sessionID })
+      void this.onSubagentSessionCreated({
+        sessionID,
+        parentID: input.parentSessionId,
+        title: input.description,
+      }).catch((err) => {
+        log("[background-agent] Failed to spawn tmux pane:", err)
+      })
+    } else {
+      log("[background-agent] SKIP tmux callback - conditions not met", {
+        suppressTmuxSpawn: !!input.suppressTmuxSpawn,
+      })
+    }
   }
 
   getTask(id: string): BackgroundTask | undefined {