Windows Codex: Guardian checkpoint compatibility error blocks messaging in existing task; fresh task succeeds

Summary

An existing long-running Codex desktop task cannot send even a harmless diagnostic acknowledgment. Automatic approval review fails with a parent-compaction checkpoint compatibility error.

A fresh diagnostic-only task successfully sent a comparable harmless acknowledgment. This suggests the failure may be specific to the existing task’s conversation state, but the root cause is unconfirmed.

Environment

• Windows desktop
• Codex app reports it is up to date
• Exact app version: [INSERT VERSION IF AVAILABLE]
• Normal approval safeguards remain enabled

Actual error

“Automatic approval review failed: parent compaction checkpoint is incompatible with the Guardian review model or its compatibility is unknown”

Observed behavior

1. An existing task attempted task-to-task messaging and received the error above.
2. I completely quit and restarted Codex.
3. After restarting, a single diagnostic was performed from the affected task. The outgoing message contained only:
   DIAGNOSTIC_ACK_POST_RESTART
4. That diagnostic failed with the same error at approximately 2026-09-14 19:06:07 UTC.
5. A fresh, projectless diagnostic-only task sent:
   FRESH_TASK_DIAGNOSTIC_ACK_ONLY
6. That message was successfully received.

These are observations from existing sessions, not a guaranteed reproduction procedure for a clean installation.

Expected behavior

The platform should be able to evaluate an ordinary messaging action under its normal approval safeguards, or provide an actionable recovery procedure when its reviewer cannot process the saved conversation state.

Troubleshooting completed

• Confirmed the app reports it is up to date.
• Fully restarted Codex.
• Confirmed basic task reading and read-only terminal commands work.
• Compared affected-task messaging with fresh-task messaging.
• Submitted in-app feedback from the affected conversation.

Impact

The affected task cannot coordinate further work. No rejected project action was moved into the diagnostic task. No approval protections were disabled.

Requested assistance

Please investigate the checkpoint/reviewer compatibility failure and provide a supported way to repair or migrate the affected conversation while preserving approval safeguards.

I can provide the in-app feedback/session reference through an appropriate support channel. Private project data and full transcripts are intentionally omitted from this public report.

Start by comparing the affected existing task with the successful fresh task, focusing on the saved conversation state, parent-compaction checkpoint, and Guardian review path described in the report. Reproduce the compatibility failure if possible and verify that any recovery or migration preserves the normal approval safeguards and restores messaging.