Background subagent task_status returns 'cancelled' but session data still accessible via task(task_id=) resume #27827

Bug Description

When using task(background=true) to launch a background subagent, the task completes successfully and results are visible in the UI. However, when the parent agent polls for results using task_status(task_id="...", wait=true), it returns state: cancelled with empty error.

The session data is not lost — using task(task_id="...", prompt="...") to resume the session retrieves the full completed results.

Steps to Reproduce

1. Set OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true
2. Launch background subagent: task(background=true, ...)
3. The subagent completes its work (results visible in parent session context)
4. Parent agent calls task_status(task_id="...", wait=true) to get results
5. state: cancelled is returned, no result data

Expected Behavior

task_status(wait=true) should return state: completed with the full task result data, matching what is visible in the session UI.

Workaround

Instead of using task_status, use task(task_id="<same_id>", prompt="return your previous result") to resume the session and retrieve results. But this is clunky — the parent agent shouldn't need to know the prompt again.

Environment

• OpenCode version: latest
• Feature flag: OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true
• Plugin: @tarquinen/opencode-dcp@latest (DCP)