fix(background-agent): treat non-active session statuses as terminal to prevent parent session hang

Previously, pollRunningTasks() and checkAndInterruptStaleTasks() treated
any non-"idle" session status as "still running", which caused tasks with
terminal statuses like "interrupted" to be skipped indefinitely — both
for completion detection AND stale timeout. This made the parent session
hang forever waiting for an ALL COMPLETE notification that never came.

Extract isActiveSessionStatus() and isTerminalSessionStatus() that
classify session statuses explicitly. Only known active statuses
("busy", "retry", "running") protect tasks from completion/stale checks.
Known terminal statuses ("interrupted") trigger immediate completion.
Unknown statuses fall through to the standard idle/gone path with output
validation as a conservative default.

Introduced by: a0c93816 (2026-02-14), dc370f7f (2026-03-08)

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

@@ -153,4 +153,42 @@ describe("BackgroundManager pollRunningTasks", () => {
       expect(task.status).toBe("running")
     })
   })
+
+  describe("#given a running task whose session has terminal non-idle status", () => {
+    test('#when session status is "interrupted" #then completes the task', async () => {
+      //#given
+      const manager = createManagerWithClient({
+        status: async () => ({ data: { "ses-interrupted": { type: "interrupted" } } }),
+      })
+      const task = createRunningTask("ses-interrupted")
+      injectTask(manager, task)
+
+      //#when
+      const poll = (manager as unknown as { pollRunningTasks: () => Promise<void> }).pollRunningTasks
+      await poll.call(manager)
+      manager.shutdown()
+
+      //#then
+      expect(task.status).toBe("completed")
+      expect(task.completedAt).toBeDefined()
+    })
+
+    test('#when session status is an unknown type #then completes the task', async () => {
+      //#given
+      const manager = createManagerWithClient({
+        status: async () => ({ data: { "ses-unknown": { type: "some-weird-status" } } }),
+      })
+      const task = createRunningTask("ses-unknown")
+      injectTask(manager, task)
+
+      //#when
+      const poll = (manager as unknown as { pollRunningTasks: () => Promise<void> }).pollRunningTasks
+      await poll.call(manager)
+      manager.shutdown()
+
+      //#then
+      expect(task.status).toBe("completed")
+      expect(task.completedAt).toBeDefined()
+    })
+  })
 })

src/features/background-agent/manager.ts

@@ -52,6 +52,7 @@ import { join } from "node:path"
 import { pruneStaleTasksAndNotifications } from "./task-poller"
 import { checkAndInterruptStaleTasks } from "./task-poller"
 import { removeTaskToastTracking } from "./remove-task-toast-tracking"
+import { isActiveSessionStatus, isTerminalSessionStatus } from "./session-status-classifier"
 import {
   detectRepetitiveToolUse,
   recordToolCall,
@@ -900,18 +901,19 @@ export class BackgroundManager {
       task.progress.lastUpdate = new Date()
 
       if (partInfo?.type === "tool" || partInfo?.tool) {
-        const countedToolPartIDs = task.progress.countedToolPartIDs ?? []
+        const countedToolPartIDs = task.progress.countedToolPartIDs ?? new Set<string>()
         const shouldCountToolCall =
           !partInfo.id ||
           partInfo.state?.status !== "running" ||
-          !countedToolPartIDs.includes(partInfo.id)
+          !countedToolPartIDs.has(partInfo.id)
 
         if (!shouldCountToolCall) {
           return
         }
 
         if (partInfo.id && partInfo.state?.status === "running") {
-          task.progress.countedToolPartIDs = [...countedToolPartIDs, partInfo.id]
+          countedToolPartIDs.add(partInfo.id)
+          task.progress.countedToolPartIDs = countedToolPartIDs
         }
 
         task.progress.toolCalls += 1
@@ -1782,11 +1784,9 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
           }
         }
 
-        // Match sync-session-poller pattern: only skip completion check when
-        // status EXISTS and is not idle (i.e., session is actively running).
-        // When sessionStatus is undefined, the session has completed and dropped
-        // from the status response — fall through to completion detection.
-        if (sessionStatus && sessionStatus.type !== "idle") {
+        // Only skip completion when session status is actively running.
+        // Unknown or terminal statuses (like "interrupted") fall through to completion.
+        if (sessionStatus && isActiveSessionStatus(sessionStatus.type)) {
           log("[background-agent] Session still running, relying on event-based progress:", {
             taskId: task.id,
             sessionID,
@@ -1796,6 +1796,24 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
           continue
         }
 
+        // Explicit terminal non-idle status (e.g., "interrupted") — complete immediately,
+        // skipping output validation (session will never produce more output).
+        // Unknown statuses fall through to the idle/gone path with output validation.
+        if (sessionStatus && isTerminalSessionStatus(sessionStatus.type)) {
+          await this.tryCompleteTask(task, `polling (terminal session status: ${sessionStatus.type})`)
+          continue
+        }
+
+        // Unknown non-idle status — not active, not terminal, not idle.
+        // Fall through to idle/gone completion path with output validation.
+        if (sessionStatus && sessionStatus.type !== "idle") {
+          log("[background-agent] Unknown session status, treating as potentially idle:", {
+            taskId: task.id,
+            sessionID,
+            sessionStatus: sessionStatus.type,
+          })
+        }
+
         // Session is idle or no longer in status response (completed/disappeared)
         const completionSource = sessionStatus?.type === "idle"
           ? "polling (idle status)"

src/features/background-agent/session-status-classifier.test.ts

@@ -0,0 +1,66 @@
+import { describe, test, expect, mock } from "bun:test"
+import { isActiveSessionStatus, isTerminalSessionStatus } from "./session-status-classifier"
+
+const mockLog = mock()
+mock.module("../../shared", () => ({ log: mockLog }))
+
+describe("isActiveSessionStatus", () => {
+  describe("#given a known active session status", () => {
+    test('#when type is "busy" #then returns true', () => {
+      expect(isActiveSessionStatus("busy")).toBe(true)
+    })
+
+    test('#when type is "retry" #then returns true', () => {
+      expect(isActiveSessionStatus("retry")).toBe(true)
+    })
+
+    test('#when type is "running" #then returns true', () => {
+      expect(isActiveSessionStatus("running")).toBe(true)
+    })
+  })
+
+  describe("#given a known terminal session status", () => {
+    test('#when type is "idle" #then returns false', () => {
+      expect(isActiveSessionStatus("idle")).toBe(false)
+    })
+
+    test('#when type is "interrupted" #then returns false and does not log', () => {
+      mockLog.mockClear()
+      expect(isActiveSessionStatus("interrupted")).toBe(false)
+      expect(mockLog).not.toHaveBeenCalled()
+    })
+  })
+
+  describe("#given an unknown session status", () => {
+    test('#when type is an arbitrary unknown string #then returns false and logs warning', () => {
+      mockLog.mockClear()
+      expect(isActiveSessionStatus("some-unknown-status")).toBe(false)
+      expect(mockLog).toHaveBeenCalledWith(
+        "[background-agent] Unknown session status type encountered:",
+        "some-unknown-status",
+      )
+    })
+
+    test('#when type is empty string #then returns false', () => {
+      expect(isActiveSessionStatus("")).toBe(false)
+    })
+  })
+})
+
+describe("isTerminalSessionStatus", () => {
+  test('#when type is "interrupted" #then returns true', () => {
+    expect(isTerminalSessionStatus("interrupted")).toBe(true)
+  })
+
+  test('#when type is "idle" #then returns false (idle is handled separately)', () => {
+    expect(isTerminalSessionStatus("idle")).toBe(false)
+  })
+
+  test('#when type is "busy" #then returns false', () => {
+    expect(isTerminalSessionStatus("busy")).toBe(false)
+  })
+
+  test('#when type is an unknown string #then returns false', () => {
+    expect(isTerminalSessionStatus("some-unknown")).toBe(false)
+  })
+})

src/features/background-agent/session-status-classifier.ts

@@ -0,0 +1,20 @@
+import { log } from "../../shared"
+
+const ACTIVE_SESSION_STATUSES = new Set(["busy", "retry", "running"])
+const KNOWN_TERMINAL_STATUSES = new Set(["idle", "interrupted"])
+
+export function isActiveSessionStatus(type: string): boolean {
+  if (ACTIVE_SESSION_STATUSES.has(type)) {
+    return true
+  }
+
+  if (!KNOWN_TERMINAL_STATUSES.has(type)) {
+    log("[background-agent] Unknown session status type encountered:", type)
+  }
+
+  return false
+}
+
+export function isTerminalSessionStatus(type: string): boolean {
+  return KNOWN_TERMINAL_STATUSES.has(type) && type !== "idle"
+}

src/features/background-agent/task-poller.test.ts

@@ -417,6 +417,56 @@ describe("checkAndInterruptStaleTasks", () => {
     expect(task.status).toBe("cancelled")
     expect(onTaskInterrupted).toHaveBeenCalledWith(task)
   })
+
+  it('should NOT protect task when session has terminal non-idle status like "interrupted"', async () => {
+    //#given — lastUpdate is 5min old, session is "interrupted" (terminal, not active)
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 2,
+        lastUpdate: new Date(Date.now() - 300_000),
+      },
+    })
+
+    //#when — session status is "interrupted" (terminal)
+    await checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { staleTimeoutMs: 180_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: { "ses-1": { type: "interrupted" } },
+    })
+
+    //#then — terminal statuses should not protect from stale timeout
+    expect(task.status).toBe("cancelled")
+    expect(task.error).toContain("Stale timeout")
+  })
+
+  it('should NOT protect task when session has unknown status type', async () => {
+    //#given — lastUpdate is 5min old, session has an unknown status
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 2,
+        lastUpdate: new Date(Date.now() - 300_000),
+      },
+    })
+
+    //#when — session has unknown status type
+    await checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { staleTimeoutMs: 180_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: { "ses-1": { type: "some-weird-status" } },
+    })
+
+    //#then — unknown statuses should not protect from stale timeout
+    expect(task.status).toBe("cancelled")
+    expect(task.error).toContain("Stale timeout")
+  })
 })
 
 describe("pruneStaleTasksAndNotifications", () => {

src/features/background-agent/task-poller.ts

@@ -14,6 +14,7 @@ import {
 } from "./constants"
 import { removeTaskToastTracking } from "./remove-task-toast-tracking"
 
+import { isActiveSessionStatus } from "./session-status-classifier"
 const TERMINAL_TASK_STATUSES = new Set<BackgroundTask["status"]>([
   "completed",
   "error",
@@ -120,7 +121,7 @@ export async function checkAndInterruptStaleTasks(args: {
     if (!startedAt || !sessionID) continue
 
     const sessionStatus = sessionStatuses?.[sessionID]?.type
-    const sessionIsRunning = sessionStatus !== undefined && sessionStatus !== "idle"
+    const sessionIsRunning = sessionStatus !== undefined && isActiveSessionStatus(sessionStatus)
     const runtime = now - startedAt.getTime()
 
     if (!task.progress?.lastUpdate) {