compaction: "Compaction produced no summary" kills every prompt in a long session #56857

Summary

In a long-running session that had already compacted successfully many times, automatic compaction began failing with Session.StepFailedError: Compaction produced no summary. Every new prompt now aborts before the user's message is processed — the session is permanently unusable and the failure is not surfaced as an actionable error.

Environment

• opencode version: 2.0.32
• OS: Windows 11 x64 (Microsoft Windows [Version 10.0.26300.9711])
• Terminal: VS Code integrated terminal (TERM_PROGRAM=vscode, TERM=xterm-256color, COLORTERM=truecolor)
• Shell: cmd.exe
• Install/channel: latest release channel; native Windows installer (%LOCALAPPDATA%\opencode\bin\opencode.exe); update check reports up-to-date
• Active plugins: none (global config is only {"$schema": "https://opencode.ai/config.json"})

Reproduction

Not yet reduced to a minimal case, but 100% reproducible on the affected session:

1. Use one session heavily over several days with many turns that produce very large tool outputs, so it compacts repeatedly. This session compacted ~19 times (~1,650 stored messages).
2. Send any new message.
3. Automatic compaction runs first, completes without producing a summary, and the whole turn aborts.

The user's message is never sent to the model; the TUI appears to just do nothing / fail. Retrying reproduces it every time (three attempts across ~12 hours).

Expected Behavior

Either compaction produces a summary and the prompt is processed, or a clear, actionable error is surfaced. An empty compaction summary should not silently make a session permanently unusable.

Actual Behavior

Each attempt logs:

timestamp=2026-10-18T17:32:16.964Z level=ERROR run=<run> message="Failed to drain Session" cause="Session.StepFailedError: Compaction produced no summary
    at <anonymous> (B:/~BUN/root/chunk-z20sjgqk.js:382:3088)
    at SessionRunner.runStep (B:/~BUN/root/chunk-z20sjgqk.js:382:2672)
    at SessionRunner.runStep (definition) (B:/~BUN/root/chunk-z20sjgqk.js:382:2826)
    at SessionRunner.drain (B:/~BUN/root/chunk-4e2rqd16.js:18:2552)
    at SessionRunner.drain (definition) (B:/~BUN/root/chunk-z20sjgqk.js:382:516)
    at ServerProcess.start (B:/~BUN/root/chunk-94gxrctf.js:2:7603)
    at ServerProcess.start (definition) (B:/~BUN/root/chunk-z20sjgqk.js:453:25665)" role=server

Same error at 2026-10-18T05:08:04.418Z and 2026-10-18T05:14:07.065Z.

After each failure the session is left idle with a failed outcome. Observed in the session database:

• session_v2.time_idle set and idle_outcome = 'failed', resume_attempts = 0
• message sequence per attempt: user → compaction → idle, i.e. a compaction message is recorded even though no summary was produced

Additional Context

• Not a service or connectivity problem: opencode service status is healthy, the version is current, and other sessions on the same model keep working.
• No agent=compaction stream record appears on the failing attempts. Successful compactions in the same session earlier (e.g. 10-13 through 10-16) logged:
  level=INFO message=stream providerID=opencode modelID=<model> small=false agent=compaction mode=primary
  On the failing attempts no such line is logged — compaction appears to fail without reaching a provider stream, and no underlying provider/stream error is recorded (no stream error entries for these attempts either).
• Compaction uses the session's own model (per the V2 compaction docs), and that model had already summarised this same session successfully ~17 times, so a permanently unsupported or misconfigured model seems unlikely.
• Hypothesis: the session's history is enormous (many very large tool outputs), so compaction is on the "shortened text version" path; if the model returns an empty summary there, the result is treated as fatal (StepFailedError) instead of retried or reported. Logging the raw compaction response and treating an empty summary as retryable would help.
• Workarounds tried: retrying the prompt (fails every time); waiting ~12 hours and retrying (fails). The only recovery was exporting the session and starting a new one. Manual POST /api/session/{id}/compact was not attempted.
• Sensitive data redacted: session id, local paths, workspace contents, run ids.