feat(background-task): add all parameter to cancel all running tasks at once

Allows OmO agent to cleanup all running background tasks before providing final answers.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/agents/omo.ts

@@ -844,6 +844,7 @@ You are allowed to be proactive, but balance this with user expectations:
 - **Stop when you have enough** - don't over-explore
 - **Evidence for everything** - no evidence = not complete
 - **Background pattern** - fire agents, continue working, collect with background_output
+- **Cleanup before answering** - When ready to deliver your final answer, cancel ALL running background tasks with \`background_cancel(all=true)\` first, then respond. This conserves resources and ensures clean workflow completion.
 - Complete accepted tasks fully - don't stop halfway through implementation
 - But if you discover the task is larger or more complex than initially apparent, communicate this and confirm direction before investing significant effort
 </Final_Reminders>

src/tools/background-task/constants.ts

@@ -25,9 +25,12 @@ Arguments:
 
 The system automatically notifies when background tasks complete. You typically don't need block=true.`
 
-export const BACKGROUND_CANCEL_DESCRIPTION = `Cancel a running background task.
+export const BACKGROUND_CANCEL_DESCRIPTION = `Cancel running background task(s).
 
 Only works for tasks with status "running". Aborts the background session and marks the task as cancelled.
 
 Arguments:
-- taskId: Required task ID to cancel.`
+- taskId: Task ID to cancel (optional if all=true)
+- all: Set to true to cancel ALL running background tasks at once (default: false)
+
+**Cleanup Before Answer**: When you have gathered sufficient information and are ready to provide your final answer to the user, use \`all=true\` to cancel ALL running background tasks first, then deliver your response. This conserves resources and ensures clean workflow completion.`

src/tools/background-task/tools.ts

@@ -263,11 +263,42 @@ export function createBackgroundCancel(manager: BackgroundManager, client: Openc
   return tool({
     description: BACKGROUND_CANCEL_DESCRIPTION,
     args: {
-      taskId: tool.schema.string().describe("Task ID to cancel"),
+      taskId: tool.schema.string().optional().describe("Task ID to cancel (required if all=false)"),
+      all: tool.schema.boolean().optional().describe("Cancel all running background tasks (default: false)"),
     },
-    async execute(args: BackgroundCancelArgs) {
+    async execute(args: BackgroundCancelArgs, toolContext) {
       try {
-        const task = manager.getTask(args.taskId)
+        const cancelAll = args.all === true
+
+        if (!cancelAll && !args.taskId) {
+          return `❌ Invalid arguments: Either provide a taskId or set all=true to cancel all running tasks.`
+        }
+
+        if (cancelAll) {
+          const tasks = manager.getTasksByParentSession(toolContext.sessionID)
+          const runningTasks = tasks.filter(t => t.status === "running")
+
+          if (runningTasks.length === 0) {
+            return `✅ No running background tasks to cancel.`
+          }
+
+          const results: string[] = []
+          for (const task of runningTasks) {
+            client.session.abort({
+              path: { id: task.sessionID },
+            }).catch(() => {})
+
+            task.status = "cancelled"
+            task.completedAt = new Date()
+            results.push(`- ${task.id}: ${task.description}`)
+          }
+
+          return `✅ Cancelled ${runningTasks.length} background task(s):
+
+${results.join("\n")}`
+        }
+
+        const task = manager.getTask(args.taskId!)
         if (!task) {
           return `❌ Task not found: ${args.taskId}`
         }

src/tools/background-task/types.ts

@@ -11,5 +11,6 @@ export interface BackgroundOutputArgs {
 }
 
 export interface BackgroundCancelArgs {
-  taskId: string
+  taskId?: string
+  all?: boolean
 }