diff --git a/src/features/background-agent/manager.test.ts b/src/features/background-agent/manager.test.ts
index d8eba0980..789fac212 100644
--- a/src/features/background-agent/manager.test.ts
+++ b/src/features/background-agent/manager.test.ts
@@ -2289,10 +2289,221 @@ describe("BackgroundManager.checkAndInterruptStaleTasks", () => {
 
     getTaskMap(manager).set(task.id, task)
 
-    await manager["checkAndInterruptStaleTasks"]()
+     await manager["checkAndInterruptStaleTasks"]()
 
     expect(task.status).toBe("cancelled")
   })
+
+  test("should NOT interrupt task when session is running, even with stale lastUpdate", async () => {
+    //#given
+    const client = {
+      session: {
+        prompt: async () => ({}),
+        promptAsync: async () => ({}),
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput, { staleTimeoutMs: 180_000 })
+
+    const task: BackgroundTask = {
+      id: "task-running-session",
+      sessionID: "session-running",
+      parentSessionID: "parent-rs",
+      parentMessageID: "msg-rs",
+      description: "Task with running session",
+      prompt: "Test",
+      agent: "test-agent",
+      status: "running",
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 2,
+        lastUpdate: new Date(Date.now() - 300_000),
+      },
+    }
+
+    getTaskMap(manager).set(task.id, task)
+
+    //#when — session is actively running
+    await manager["checkAndInterruptStaleTasks"]({ "session-running": { type: "running" } })
+
+    //#then — task survives because session is running
+    expect(task.status).toBe("running")
+  })
+
+  test("should interrupt task when session is idle and lastUpdate exceeds stale timeout", async () => {
+    //#given
+    const client = {
+      session: {
+        prompt: async () => ({}),
+        promptAsync: async () => ({}),
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput, { staleTimeoutMs: 180_000 })
+    stubNotifyParentSession(manager)
+
+    const task: BackgroundTask = {
+      id: "task-idle-session",
+      sessionID: "session-idle",
+      parentSessionID: "parent-is",
+      parentMessageID: "msg-is",
+      description: "Task with idle session",
+      prompt: "Test",
+      agent: "test-agent",
+      status: "running",
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 2,
+        lastUpdate: new Date(Date.now() - 300_000),
+      },
+    }
+
+    getTaskMap(manager).set(task.id, task)
+
+    //#when — session is idle
+    await manager["checkAndInterruptStaleTasks"]({ "session-idle": { type: "idle" } })
+
+    //#then — killed because session is idle with stale lastUpdate
+    expect(task.status).toBe("cancelled")
+    expect(task.error).toContain("Stale timeout")
+  })
+
+  test("should NOT interrupt running session even with very old lastUpdate (no safety net)", async () => {
+    //#given
+    const client = {
+      session: {
+        prompt: async () => ({}),
+        promptAsync: async () => ({}),
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput, { staleTimeoutMs: 180_000 })
+
+    const task: BackgroundTask = {
+      id: "task-long-running",
+      sessionID: "session-long",
+      parentSessionID: "parent-lr",
+      parentMessageID: "msg-lr",
+      description: "Long running task",
+      prompt: "Test",
+      agent: "test-agent",
+      status: "running",
+      startedAt: new Date(Date.now() - 900_000),
+      progress: {
+        toolCalls: 5,
+        lastUpdate: new Date(Date.now() - 900_000),
+      },
+    }
+
+    getTaskMap(manager).set(task.id, task)
+
+    //#when — session is running, lastUpdate 15min old
+    await manager["checkAndInterruptStaleTasks"]({ "session-long": { type: "running" } })
+
+    //#then — running sessions are NEVER stale-killed
+    expect(task.status).toBe("running")
+  })
+
+  test("should NOT interrupt running session with no progress (undefined lastUpdate)", async () => {
+    //#given — no progress at all, but session is running
+    const client = {
+      session: {
+        prompt: async () => ({}),
+        promptAsync: async () => ({}),
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput, { messageStalenessTimeoutMs: 600_000 })
+
+    const task: BackgroundTask = {
+      id: "task-running-no-progress",
+      sessionID: "session-rnp",
+      parentSessionID: "parent-rnp",
+      parentMessageID: "msg-rnp",
+      description: "Running no progress",
+      prompt: "Test",
+      agent: "test-agent",
+      status: "running",
+      startedAt: new Date(Date.now() - 15 * 60 * 1000),
+      progress: undefined,
+    }
+
+    getTaskMap(manager).set(task.id, task)
+
+    //#when — session is running despite no progress
+    await manager["checkAndInterruptStaleTasks"]({ "session-rnp": { type: "running" } })
+
+    //#then — running sessions are NEVER killed
+    expect(task.status).toBe("running")
+  })
+
+  test("should interrupt task with no lastUpdate after messageStalenessTimeout", async () => {
+    //#given
+    const client = {
+      session: {
+        prompt: async () => ({}),
+        promptAsync: async () => ({}),
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput, { messageStalenessTimeoutMs: 600_000 })
+    stubNotifyParentSession(manager)
+
+    const task: BackgroundTask = {
+      id: "task-no-update",
+      sessionID: "session-no-update",
+      parentSessionID: "parent-nu",
+      parentMessageID: "msg-nu",
+      description: "No update task",
+      prompt: "Test",
+      agent: "test-agent",
+      status: "running",
+      startedAt: new Date(Date.now() - 15 * 60 * 1000),
+      progress: undefined,
+    }
+
+    getTaskMap(manager).set(task.id, task)
+
+    //#when — no progress update for 15 minutes
+    await manager["checkAndInterruptStaleTasks"]({})
+
+    //#then — killed after messageStalenessTimeout
+    expect(task.status).toBe("cancelled")
+    expect(task.error).toContain("no activity")
+  })
+
+  test("should NOT interrupt task with no lastUpdate within messageStalenessTimeout", async () => {
+    //#given
+    const client = {
+      session: {
+        prompt: async () => ({}),
+        promptAsync: async () => ({}),
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput, { messageStalenessTimeoutMs: 600_000 })
+
+    const task: BackgroundTask = {
+      id: "task-fresh-no-update",
+      sessionID: "session-fresh",
+      parentSessionID: "parent-fn",
+      parentMessageID: "msg-fn",
+      description: "Fresh no-update task",
+      prompt: "Test",
+      agent: "test-agent",
+      status: "running",
+      startedAt: new Date(Date.now() - 5 * 60 * 1000),
+      progress: undefined,
+    }
+
+    getTaskMap(manager).set(task.id, task)
+
+    //#when — only 5 min since start, within 10min timeout
+    await manager["checkAndInterruptStaleTasks"]({})
+
+    //#then — task survives
+    expect(task.status).toBe("running")
+  })
 })
 
 describe("BackgroundManager.shutdown session abort", () => {
diff --git a/src/features/background-agent/manager.ts b/src/features/background-agent/manager.ts
index 3a680dd6a..7d75af85b 100644
--- a/src/features/background-agent/manager.ts
+++ b/src/features/background-agent/manager.ts
@@ -12,6 +12,7 @@ import { ConcurrencyManager } from "./concurrency"
 import type { BackgroundTaskConfig, TmuxConfig } from "../../config/schema"
 import { isInsideTmux } from "../../shared/tmux"
 import {
+  DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS,
   DEFAULT_STALE_TIMEOUT_MS,
   MIN_IDLE_TIME_MS,
   MIN_RUNTIME_BEFORE_STALE_MS,
@@ -1437,24 +1438,54 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     }
   }
 
-  private async checkAndInterruptStaleTasks(): Promise<void> {
+  private async checkAndInterruptStaleTasks(
+    allStatuses: Record<string, { type: string }> = {},
+  ): Promise<void> {
     const staleTimeoutMs = this.config?.staleTimeoutMs ?? DEFAULT_STALE_TIMEOUT_MS
+    const messageStalenessMs = this.config?.messageStalenessTimeoutMs ?? DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS
     const now = Date.now()
 
     for (const task of this.tasks.values()) {
       if (task.status !== "running") continue
-      if (!task.progress?.lastUpdate) continue
-      
+
       const startedAt = task.startedAt
       const sessionID = task.sessionID
       if (!startedAt || !sessionID) continue
 
+      const sessionIsRunning = allStatuses[sessionID]?.type === "running"
       const runtime = now - startedAt.getTime()
+
+      if (!task.progress?.lastUpdate) {
+        if (sessionIsRunning) continue
+        if (runtime <= messageStalenessMs) continue
+
+        const staleMinutes = Math.round(runtime / 60000)
+        task.status = "cancelled"
+        task.error = `Stale timeout (no activity for ${staleMinutes}min since start)`
+        task.completedAt = new Date()
+
+        if (task.concurrencyKey) {
+          this.concurrencyManager.release(task.concurrencyKey)
+          task.concurrencyKey = undefined
+        }
+
+        this.client.session.abort({ path: { id: sessionID } }).catch(() => {})
+        log(`[background-agent] Task ${task.id} interrupted: no progress since start`)
+
+        try {
+          await this.enqueueNotificationForParent(task.parentSessionID, () => this.notifyParentSession(task))
+        } catch (err) {
+          log("[background-agent] Error in notifyParentSession for stale task:", { taskId: task.id, error: err })
+        }
+        continue
+      }
+
+      if (sessionIsRunning) continue
+
       if (runtime < MIN_RUNTIME_BEFORE_STALE_MS) continue
 
       const timeSinceLastUpdate = now - task.progress.lastUpdate.getTime()
       if (timeSinceLastUpdate <= staleTimeoutMs) continue
-
       if (task.status !== "running") continue
 
       const staleMinutes = Math.round(timeSinceLastUpdate / 60000)
@@ -1467,10 +1498,7 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
         task.concurrencyKey = undefined
       }
 
-      this.client.session.abort({
-        path: { id: sessionID },
-      }).catch(() => {})
-
+      this.client.session.abort({ path: { id: sessionID } }).catch(() => {})
       log(`[background-agent] Task ${task.id} interrupted: stale timeout`)
 
       try {
@@ -1483,11 +1511,12 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
 
   private async pollRunningTasks(): Promise<void> {
     this.pruneStaleTasksAndNotifications()
-    await this.checkAndInterruptStaleTasks()
 
     const statusResult = await this.client.session.status()
     const allStatuses = (statusResult.data ?? {}) as Record<string, { type: string }>
 
+    await this.checkAndInterruptStaleTasks(allStatuses)
+
     for (const task of this.tasks.values()) {
       if (task.status !== "running") continue
       
@@ -1497,7 +1526,6 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
       try {
         const sessionStatus = allStatuses[sessionID]
         
-        // Don't skip if session not in status - fall through to message-based detection
         if (sessionStatus?.type === "idle") {
           // Edge guard: Validate session has actual output before completing
           const hasValidOutput = await this.validateSessionHasOutput(sessionID)
diff --git a/src/features/background-agent/poll-running-tasks.ts b/src/features/background-agent/poll-running-tasks.ts
index 6c5a4461d..023fbf556 100644
--- a/src/features/background-agent/poll-running-tasks.ts
+++ b/src/features/background-agent/poll-running-tasks.ts
@@ -34,7 +34,7 @@ export async function pollRunningTasks(args: {
   tasks: Iterable<BackgroundTask>
   client: OpencodeClient
   pruneStaleTasksAndNotifications: () => void
-  checkAndInterruptStaleTasks: () => Promise<void>
+  checkAndInterruptStaleTasks: (statuses: Record<string, { type: string }>) => Promise<void>
   validateSessionHasOutput: (sessionID: string) => Promise<boolean>
   checkSessionTodos: (sessionID: string) => Promise<boolean>
   tryCompleteTask: (task: BackgroundTask, source: string) => Promise<boolean>
@@ -54,11 +54,12 @@ export async function pollRunningTasks(args: {
   } = args
 
   pruneStaleTasksAndNotifications()
-  await checkAndInterruptStaleTasks()
 
   const statusResult = await client.session.status()
   const allStatuses = ((statusResult as { data?: unknown }).data ?? {}) as SessionStatusMap
 
+  await checkAndInterruptStaleTasks(allStatuses)
+
   for (const task of tasks) {
     if (task.status !== "running") continue
 
diff --git a/src/features/background-agent/task-poller.test.ts b/src/features/background-agent/task-poller.test.ts
index 29ff47447..128ef978d 100644
--- a/src/features/background-agent/task-poller.test.ts
+++ b/src/features/background-agent/task-poller.test.ts
@@ -136,6 +136,125 @@ describe("checkAndInterruptStaleTasks", () => {
     expect(task.error).toContain("no activity")
   })
 
+  it("should NOT interrupt task when session is running, even if lastUpdate exceeds stale timeout", async () => {
+    //#given — lastUpdate is 5min old but session is actively running
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 2,
+        lastUpdate: new Date(Date.now() - 300_000),
+      },
+    })
+
+    //#when — session status is "running"
+    await checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { staleTimeoutMs: 180_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: { "ses-1": { type: "running" } },
+    })
+
+    //#then — task should survive because session is actively running
+    expect(task.status).toBe("running")
+  })
+
+  it("should interrupt task when session is idle and lastUpdate exceeds stale timeout", async () => {
+    //#given — lastUpdate is 5min old and session is idle
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 2,
+        lastUpdate: new Date(Date.now() - 300_000),
+      },
+    })
+
+    //#when — session status is "idle"
+    await checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { staleTimeoutMs: 180_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: { "ses-1": { type: "idle" } },
+    })
+
+    //#then — task should be killed because session is idle with stale lastUpdate
+    expect(task.status).toBe("cancelled")
+    expect(task.error).toContain("Stale timeout")
+  })
+
+  it("should NOT interrupt running session task even with very old lastUpdate", async () => {
+    //#given — lastUpdate is 15min old, but session is still running
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 900_000),
+      progress: {
+        toolCalls: 2,
+        lastUpdate: new Date(Date.now() - 900_000),
+      },
+    })
+
+    //#when — session running, lastUpdate far exceeds any timeout
+    await checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { staleTimeoutMs: 180_000, messageStalenessTimeoutMs: 600_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: { "ses-1": { type: "running" } },
+    })
+
+    //#then — running sessions are NEVER stale-killed (babysitter + TTL prune handle these)
+    expect(task.status).toBe("running")
+  })
+
+  it("should NOT interrupt running session even with no progress (undefined lastUpdate)", async () => {
+    //#given — task has no progress at all, but session is running
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 15 * 60 * 1000),
+      progress: undefined,
+    })
+
+    //#when — session is running
+    await checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { messageStalenessTimeoutMs: 600_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: { "ses-1": { type: "running" } },
+    })
+
+    //#then — running sessions are NEVER killed, even without progress
+    expect(task.status).toBe("running")
+  })
+
+  it("should use default stale timeout when session status is unknown/missing", async () => {
+    //#given — lastUpdate exceeds stale timeout, session not in status map
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 1,
+        lastUpdate: new Date(Date.now() - 200_000),
+      },
+    })
+
+    //#when — empty sessionStatuses (session not found)
+    await checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { staleTimeoutMs: 180_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: {},
+    })
+
+    //#then — unknown session treated as potentially stale, apply default timeout
+    expect(task.status).toBe("cancelled")
+    expect(task.error).toContain("Stale timeout")
+  })
+
   it("should release concurrency key when interrupting a never-updated task", async () => {
     //#given
     const releaseMock = mock(() => {})
diff --git a/src/features/background-agent/task-poller.ts b/src/features/background-agent/task-poller.ts
index a9f63a9a8..c8cafd724 100644
--- a/src/features/background-agent/task-poller.ts
+++ b/src/features/background-agent/task-poller.ts
@@ -57,14 +57,17 @@ export function pruneStaleTasksAndNotifications(args: {
   }
 }
 
+export type SessionStatusMap = Record<string, { type: string }>
+
 export async function checkAndInterruptStaleTasks(args: {
   tasks: Iterable<BackgroundTask>
   client: OpencodeClient
   config: BackgroundTaskConfig | undefined
   concurrencyManager: ConcurrencyManager
   notifyParentSession: (task: BackgroundTask) => Promise<void>
+  sessionStatuses?: SessionStatusMap
 }): Promise<void> {
-  const { tasks, client, config, concurrencyManager, notifyParentSession } = args
+  const { tasks, client, config, concurrencyManager, notifyParentSession, sessionStatuses } = args
   const staleTimeoutMs = config?.staleTimeoutMs ?? DEFAULT_STALE_TIMEOUT_MS
   const now = Date.now()
 
@@ -77,9 +80,11 @@ export async function checkAndInterruptStaleTasks(args: {
     const sessionID = task.sessionID
     if (!startedAt || !sessionID) continue
 
+    const sessionIsRunning = sessionStatuses?.[sessionID]?.type === "running"
     const runtime = now - startedAt.getTime()
 
     if (!task.progress?.lastUpdate) {
+      if (sessionIsRunning) continue
       if (runtime <= messageStalenessMs) continue
 
       const staleMinutes = Math.round(runtime / 60000)
@@ -103,6 +108,8 @@ export async function checkAndInterruptStaleTasks(args: {
       continue
     }
 
+    if (sessionIsRunning) continue
+
     if (runtime < MIN_RUNTIME_BEFORE_STALE_MS) continue
 
     const timeSinceLastUpdate = now - task.progress.lastUpdate.getTime()