[Bug]: Cursor Agent subagents disappear into the ether after being spawned

[joshrad-dev]

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Start a thread with the Cursor agent
2. Write a prompt that instructs it to delegate to subagents
3. Notice it calls a "Task" tool to start the subagents
4. Subagents aren't visible anywhere else, Cursor agent claims that it will be notified when they're finished (which doesn't happen) and there's no way to see the result of the subagents

Expected behavior

Some kind of visual showing the progress of the subagents, their output (if possible with the Cursor agent), and pass notifications when finished to the main agent

Actual behavior

The subagents run in the background and aren't seen again

Impact

Blocks work completely

Version or commit

0.0.43-nightly.20260927.2344 (de251fc)

Environment

Cursor Agent CLI 2026.09.23-86fc751

Logs or stack traces

2026-09-27 18:17:15 UTC  Task: Audit iOS SwiftUI architecture
  Cursor ACP tool_call_update: status=completed
  rawOutput: {"durationMs":49,"isBackground":true}

2026-09-27 18:17:21 UTC  Task: Audit Electron desktop app
  Cursor ACP tool_call_update: status=completed
  rawOutput: {"durationMs":43,"isBackground":true}

2026-09-27 18:20:08 UTC  Parent turn completed.

Analysis from Codex:

The completed status describes the launch calls, while isBackground: true says the delegated work continued. T3 persisted these as dynamic_tool_call / tool.completed activities and persisted no task.started, task.progress, or task.completed activities for either child. The current Cursor path forwards ACP tool updates as generic item events (apps/server/src/provider/Layers/CursorAdapter.ts, around line 853); the Agents UI needs agent task activities (packages/client-runtime/src/state/subagentRuntime.ts, around line 477).
Cursor’s local child transcripts contain report text but end with turn_ended: aborted after the later interruption. The available logs do not establish whether Cursor sent a completion notification that T3 missed.

Screenshots, recordings, or supporting files

Screenshot 2026-09-27 at 1.57.59 PM.png
Screenshot 2026-09-27 at 1.58.15 PM.png
Screenshot 2026-09-27 at 1.58.08 PM.png
Screenshot 2026-09-27 at 1.57.55 PM.png

Workaround

Didn't find one

[juliusmarminge]

Tracked by open PR #13906, which maps Cursor ACP Task tool calls onto task.started / task.progress / task.completed so they show in the Agents panel.

Same gap as discussion #6998 (converted from #6644). Near-miss left alone: #13284 (shell backgrounding, not Task subagents).