fix: add user notifications for background task completion, errors, and timeouts

- Add BackgroundTaskRunnerCallbacks interface with onTaskTimeout and onTaskError
- Wire VS Code notifications in ClineProvider: info on completion, warning on timeout/error
- Document auto-approval design decision for read-only background tasks
- Add 2 new tests for callback invocation (19 total, all passing)

src/core/task/BackgroundTaskRunner.ts

@@ -35,17 +35,31 @@ export interface BackgroundTaskInfo {
 	timeoutHandle: ReturnType<typeof setTimeout>
 }
 
+/**
+ * Optional callbacks that allow the owner (e.g. ClineProvider) to react to
+ * background task lifecycle events such as completion, timeout, or errors.
+ */
+export interface BackgroundTaskRunnerCallbacks {
+	/** Called when a background task times out. */
+	onTaskTimeout?: (taskId: string, parentTaskId: string) => void
+	/** Called when aborting a background task throws an error. */
+	onTaskError?: (taskId: string, parentTaskId: string, error: Error) => void
+}
+
 export class BackgroundTaskRunner {
 	private backgroundTasks: Map<string, BackgroundTaskInfo> = new Map()
 	private maxConcurrentTasks: number
 	private taskTimeoutMs: number
+	private callbacks: BackgroundTaskRunnerCallbacks
 
 	constructor(
 		maxConcurrentTasks: number = DEFAULT_MAX_BACKGROUND_TASKS,
 		taskTimeoutMs: number = DEFAULT_BACKGROUND_TASK_TIMEOUT_MS,
+		callbacks: BackgroundTaskRunnerCallbacks = {},
 	) {
 		this.maxConcurrentTasks = maxConcurrentTasks
 		this.taskTimeoutMs = taskTimeoutMs
+		this.callbacks = callbacks
 	}
 
 	/**
@@ -163,11 +177,13 @@ export class BackgroundTaskRunner {
 		try {
 			await info.task.abortTask(true)
 		} catch (error) {
-			console.error(
-				`[BackgroundTaskRunner] Error aborting background task ${taskId}: ${
-					error instanceof Error ? error.message : String(error)
-				}`,
-			)
+			const err = error instanceof Error ? error : new Error(String(error))
+			console.error(`[BackgroundTaskRunner] Error aborting background task ${taskId}: ${err.message}`)
+			try {
+				this.callbacks.onTaskError?.(taskId, info.parentTaskId, err)
+			} catch {
+				// Callback errors must not break cleanup.
+			}
 		}
 
 		this.backgroundTasks.delete(taskId)
@@ -193,7 +209,17 @@ export class BackgroundTaskRunner {
 	 * Handle timeout of a background task.
 	 */
 	private async timeoutTask(taskId: string): Promise<void> {
+		const info = this.backgroundTasks.get(taskId)
+		const parentTaskId = info?.parentTaskId ?? "unknown"
+
 		console.warn(`[BackgroundTaskRunner] Background task ${taskId} timed out after ${this.taskTimeoutMs}ms`)
+
+		try {
+			this.callbacks.onTaskTimeout?.(taskId, parentTaskId)
+		} catch {
+			// Callback errors must not break cleanup.
+		}
+
 		await this.cancelTask(taskId)
 	}
 }

src/core/task/Task.ts

@@ -1397,6 +1397,12 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 		let timeouts: NodeJS.Timeout[] = []
 
 		// Background tasks auto-approve all asks immediately (no user interaction).
+		// Design decision: Full auto-approval is safe here because background tasks
+		// are restricted to read-only tools only (read_file, list_files, search_files,
+		// codebase_search). They cannot modify files, execute commands, or perform any
+		// destructive operations. If a future phase introduces write-capable background
+		// tasks, this auto-approval should be revisited to allow selective user input
+		// for dangerous operations.
 		if (this.isBackgroundTask) {
 			this.approveAsk()
 			await pWaitFor(() => this.askResponse !== undefined || this.lastMessageTs !== askTs, { interval: 100 })

src/core/task/__tests__/BackgroundTaskRunner.spec.ts

@@ -129,6 +129,19 @@ describe("BackgroundTaskRunner", () => {
 		it("should handle canceling unknown task gracefully", async () => {
 			await runner.cancelTask("unknown") // should not throw
 		})
+
+		it("should invoke onTaskError callback when abort throws", async () => {
+			const onTaskError = vi.fn()
+			const customRunner = new BackgroundTaskRunner(3, undefined, { onTaskError })
+			const task = createMockTask("task-1")
+			task.abortTask.mockRejectedValue(new Error("abort failed"))
+			customRunner.registerTask(task, "parent-1")
+
+			await customRunner.cancelTask("task-1")
+
+			expect(onTaskError).toHaveBeenCalledWith("task-1", "parent-1", expect.any(Error))
+			expect(customRunner.activeCount).toBe(0)
+		})
 	})
 
 	describe("cancelTasksByParent", () => {
@@ -164,6 +177,18 @@ describe("BackgroundTaskRunner", () => {
 			expect(task.abortTask).toHaveBeenCalledWith(true)
 			expect(customRunner.activeCount).toBe(0)
 		})
+
+		it("should invoke onTaskTimeout callback when task times out", async () => {
+			const onTaskTimeout = vi.fn()
+			const customRunner = new BackgroundTaskRunner(3, 5000, { onTaskTimeout })
+			const task = createMockTask("task-1")
+			customRunner.registerTask(task, "parent-1")
+
+			vi.advanceTimersByTime(5000)
+			await vi.runAllTimersAsync()
+
+			expect(onTaskTimeout).toHaveBeenCalledWith("task-1", "parent-1")
+		})
 	})
 
 	describe("dispose", () => {

src/core/webview/ClineProvider.ts

@@ -85,6 +85,11 @@ import { CustomModesManager } from "../config/CustomModesManager"
 import { Task } from "../task/Task"
 import { buildTaskContext } from "../task/TaskContextBuilder"
 import { BackgroundTaskRunner, BACKGROUND_TASK_ALLOWED_TOOLS } from "../task/BackgroundTaskRunner"
+import {
+	BackgroundTaskRunner,
+	BACKGROUND_TASK_ALLOWED_TOOLS,
+	BackgroundTaskRunnerCallbacks,
+} from "../task/BackgroundTaskRunner"
 
 import { webviewMessageHandler } from "./webviewMessageHandler"
 import type { ClineMessage, TodoItem, SubtaskQueueItem, TaskPermissions, ContextHandoffSummary } from "@roo-code/types"
@@ -144,7 +149,14 @@ export class ClineProvider
 	private recentTasksCache?: string[]
 	public readonly taskHistoryStore: TaskHistoryStore
 	private taskHistoryStoreInitialized = false
-	public readonly backgroundTaskRunner: BackgroundTaskRunner = new BackgroundTaskRunner()
+	public readonly backgroundTaskRunner: BackgroundTaskRunner = new BackgroundTaskRunner(undefined, undefined, {
+		onTaskTimeout: (taskId, _parentTaskId) => {
+			vscode.window.showWarningMessage(`Background task ${taskId} timed out and was cancelled.`)
+		},
+		onTaskError: (taskId, _parentTaskId, error) => {
+			vscode.window.showWarningMessage(`Background task ${taskId} encountered an error: ${error.message}`)
+		},
+	})
 	private globalStateWriteThroughTimer: ReturnType<typeof setTimeout> | null = null
 	private static readonly GLOBAL_STATE_WRITE_THROUGH_DEBOUNCE_MS = 5000 // 5 seconds
 	private pendingOperations: Map<string, PendingEditOperation> = new Map()
@@ -3265,6 +3277,9 @@ export class ClineProvider
 			return
 		}
 
+		// Notify the user that the background task finished.
+		vscode.window.showInformationMessage(`Background task ${taskId} completed.`)
+
 		const parentTaskId = info.parentTaskId
 		const currentTask = this.getCurrentTask()