fix(background-agent): retain completed tasks via archive fallback after cleanup

MessageAbortedError/worker shutdown could race with scheduled removeTask, leaving background_output's manager.getTask returning 'Task not found' even though the task had completed cleanly.

Fix: add completedTaskArchive (max 500, FIFO eviction). On removeTask, archive non-running/pending tasks with sessionId. getTask falls back to archive on active-map miss. addTask clears stale archive entries on re-registration.

Fixes #3895

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

@@ -5991,6 +5991,33 @@ describe("BackgroundManager regression fixes - resume and aborted notification",
 
     manager.shutdown()
   })
+
+  test("should keep completed task retrievable after scheduled removal", () => {
+    //#given
+    const manager = createBackgroundManager()
+    const task: BackgroundTask = {
+      id: "task-archive-regression",
+      sessionId: "session-archive-regression",
+      parentSessionId: "parent-session",
+      parentMessageId: "msg-1",
+      description: "archive regression",
+      prompt: "test",
+      agent: "explore",
+      status: "completed",
+      startedAt: new Date(),
+      completedAt: new Date(),
+    }
+    getTaskMap(manager).set(task.id, task)
+
+    //#when
+    ;(cast<{ removeTask: (task: BackgroundTask) => void }>(manager)).removeTask(task)
+
+    //#then
+    expect(getTaskMap(manager).has(task.id)).toBe(false)
+    expect(manager.getTask(task.id)?.sessionId).toBe(task.sessionId)
+
+    manager.shutdown()
+  })
 })
 
 describe("BackgroundManager - tool permission spread order", () => {

src/features/background-agent/manager.ts

@@ -188,6 +188,7 @@ export interface SubagentSessionCreatedEvent {
 export type OnSubagentSessionCreated = (event: SubagentSessionCreatedEvent) => Promise<void>
 
 const MAX_TASK_REMOVAL_RESCHEDULES = 6
+const MAX_COMPLETED_TASK_ARCHIVE_SIZE = 500
 
 export interface BackgroundManagerConfig {
   pluginContext: PluginInput
@@ -222,6 +223,7 @@ export class BackgroundManager {
   private queuesByKey: Map<string, QueueItem[]> = new Map()
   private processingKeys: Set<string> = new Set()
   private completionTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
+  private completedTaskArchive: Map<string, BackgroundTask> = new Map()
   private completedTaskSummaries: Map<string, BackgroundTaskNotificationTask[]> = new Map()
   private idleDeferralTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
   private notificationQueueByParent: Map<string, Promise<void>> = new Map()
@@ -347,6 +349,7 @@ export class BackgroundManager {
   }
 
   private addTask(task: BackgroundTask): void {
+    this.completedTaskArchive.delete(task.id)
     this.tasks.set(task.id, task)
     if (!task.parentSessionId) {
       return
@@ -358,10 +361,30 @@ export class BackgroundManager {
   }
 
   private removeTask(task: BackgroundTask): void {
+    this.archiveCompletedTask(task)
     this.tasks.delete(task.id)
     this.removeTaskFromParentIndex(task.id, task.parentSessionId)
   }
 
+  private archiveCompletedTask(task: BackgroundTask): void {
+    if (!task.sessionId) {
+      return
+    }
+    if (task.status === "running" || task.status === "pending") {
+      return
+    }
+
+    this.completedTaskArchive.set(task.id, task)
+    if (this.completedTaskArchive.size <= MAX_COMPLETED_TASK_ARCHIVE_SIZE) {
+      return
+    }
+
+    const oldestTaskID = this.completedTaskArchive.keys().next().value
+    if (typeof oldestTaskID === "string") {
+      this.completedTaskArchive.delete(oldestTaskID)
+    }
+  }
+
   private updateTaskParent(task: BackgroundTask, parentSessionID: string): void {
     if (task.parentSessionId === parentSessionID) {
       return
@@ -830,7 +853,7 @@ The fallback retry session is now created and can be inspected directly.
   }
 
   getTask(id: string): BackgroundTask | undefined {
-    return this.tasks.get(id)
+    return this.tasks.get(id) ?? this.completedTaskArchive.get(id)
   }
 
   getTasksByParentSession(sessionID: string): BackgroundTask[] {