fix(background-agent): detect stale tasks that never received progress updates

Tasks with no progress.lastUpdate were silently skipped in
checkAndInterruptStaleTasks, causing them to hang forever when the model
hangs before its first tool call. Now falls back to checking startedAt
against a configurable messageStalenessTimeoutMs (default: 10 minutes).

Closes #1769

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

@@ -6,6 +6,7 @@ import type { ConcurrencyManager } from "./concurrency"
 import type { OpencodeClient } from "./opencode-client"
 
 import {
+  DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS,
   DEFAULT_STALE_TIMEOUT_MS,
   MIN_RUNTIME_BEFORE_STALE_MS,
   TASK_TTL_MS,
@@ -67,15 +68,41 @@ export async function checkAndInterruptStaleTasks(args: {
   const staleTimeoutMs = config?.staleTimeoutMs ?? DEFAULT_STALE_TIMEOUT_MS
   const now = Date.now()
 
+  const messageStalenessMs = config?.messageStalenessTimeoutMs ?? DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS
+
   for (const task of tasks) {
     if (task.status !== "running") continue
-    if (!task.progress?.lastUpdate) continue
 
     const startedAt = task.startedAt
     const sessionID = task.sessionID
     if (!startedAt || !sessionID) continue
 
     const runtime = now - startedAt.getTime()
+
+    if (!task.progress?.lastUpdate) {
+      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) {
+        concurrencyManager.release(task.concurrencyKey)
+        task.concurrencyKey = undefined
+      }
+
+      client.session.abort({ path: { id: sessionID } }).catch(() => {})
+      log(`[background-agent] Task ${task.id} interrupted: no progress since start`)
+
+      try {
+        await notifyParentSession(task)
+      } catch (err) {
+        log("[background-agent] Error in notifyParentSession for stale task:", { taskId: task.id, error: err })
+      }
+      continue
+    }
+
     if (runtime < MIN_RUNTIME_BEFORE_STALE_MS) continue
 
     const timeSinceLastUpdate = now - task.progress.lastUpdate.getTime()
@@ -92,10 +119,7 @@ export async function checkAndInterruptStaleTasks(args: {
       task.concurrencyKey = undefined
     }
 
-    client.session.abort({
-      path: { id: sessionID },
-    }).catch(() => {})
-
+    client.session.abort({ path: { id: sessionID } }).catch(() => {})
     log(`[background-agent] Task ${task.id} interrupted: stale timeout`)
 
     try {