Background task_status reports cancelled after compaction while child session keeps running #28995

Summary

When a background subagent session triggers compaction/context overflow handling, the background job can be marked cancelled even though the child session continues running and eventually finishes. After this happens, task_status reports state: cancelled while session status events still show the child session as busy.

This looks like a mismatch between BackgroundJob fiber state and the actual child session lifecycle.

Repro

1. Start OpenCode with native background subagents enabled.
2. Launch a background task that reads enough large files to trigger compaction/context overflow.
3. Reuse/resume the same background task/session and continue reading large files.
4. Wait for compaction to occur.
5. Call task_status for the task id.

Observed task id in my repro: ses_1aa9421abffeuASVxya3Kva4Th.

Expected

If the child session is still busy/running after compaction, task_status should report running or otherwise reflect the live child session state. Compaction should not make the background task look cancelled unless the child session was actually stopped.

Actual

task_status returned:

task_id: ses_1aa9421abffeuASVxya3Kva4Th
state: cancelled

<task_error>

</task_error>

But the child session continued running after that and eventually reached idle/finished.

Sanitized log timeline

OpenCode log:

15:21:30 context_length_exceeded for child session ses_1aa9421abffeuASVxya3Kva4Th
15:21:30 compaction agent started for same session
15:21:32 bus type=session.compacted publishing
15:22:01 task_status result observed as state=cancelled
15:22:07+ repeated session busy events for the same child session after cancelled state
15:22:59 child session eventually emitted idle/finished behavior

Plugin-side lifecycle logs from a local OpenCode plugin also showed the mismatch:

15:22:01 parsed task status output { taskID: ses_1aa9421abffeuASVxya3Kva4Th, state: cancelled }
15:22:01 background job status updated { state: cancelled }
15:22:07 session busy event received { sessionId: ses_1aa9421abffeuASVxya3Kva4Th, backgroundJobState: cancelled }
15:22:08 session busy event received { sessionId: ses_1aa9421abffeuASVxya3Kva4Th, backgroundJobState: cancelled }
...
15:22:59 session status idle received for same child session

Source pointers

From current source, this appears related to:

• packages/opencode/src/background/job.ts
  • BackgroundJob.start marks interrupted job fibers as cancelled via Cause.hasInterruptsOnly(cause) ? "cancelled" : "error".
• packages/opencode/src/tool/task.ts
  • background task wraps child session execution in BackgroundJob.start({ id: nextSession.id, run: runTask() ... }).
• packages/opencode/src/tool/task_status.ts
  • task_status trusts BackgroundJob.status before checking live session status / latest assistant state.
• packages/opencode/src/session/compaction.ts and packages/opencode/src/session/prompt.ts
  • compaction can create synthetic continuation messages and continue the same session after context overflow.

Possible fix direction

Either:

1. task_status should not trust BackgroundJob.status === cancelled if the child session status is currently busy/retry; or
2. BackgroundJob should not mark a background task cancelled until the child session is actually stopped/deleted, not merely because the wrapper fiber was interrupted during compaction/continuation.

This matters for background orchestration because callers will treat cancelled as terminal and may stop tracking or reuse the session incorrectly while it is still doing work.