Best possible solution:

Close this PR as superseded by current main, keep the merged #75606 session-repair implementation, and track any remaining pre-compaction flush behavior separately under #75305 if it still reproduces.

Do we have a high-confidence way to reproduce the issue?

Yes. #75313 provides a concrete persisted-session JSONL reproduction, and current main has focused unit fixtures that verify blank user rows are rewritten and retained during session-file repair.

Is this the best way to solve the issue?

Yes for the code direction, but merging this exact branch is no longer the best next step because #75606 already landed the behavior on main with additional trailing-assistant repair coverage.

Security review:

Security review cleared: The PR diff only changes TypeScript session repair logic, colocated tests, and changelog text, with no dependency, workflow, package, secret, download, or release-script changes.

What I checked:

• Current main rewrites blank users: BLANK_USER_FALLBACK_TEXT is defined as (continue), and repairUserEntryWithBlankTextContent rewrites blank string content and all-blank text-block arrays instead of returning a drop result. (src/agents/session-file-repair.ts:5, 51affb81b9a5)
• Repair loop preserves rewritten entries: The repair loop pushes rewritten user entries and increments rewrittenUserMessages, so repaired JSONL keeps the user-role row instead of skipping it. (src/agents/session-file-repair.ts:242, 51affb81b9a5)
• Regression tests cover the behavior: Current tests assert that blank text-block user messages and blank string-content user messages are rewritten and retained with the fallback placeholder. (src/agents/session-file-repair.test.ts:154, 51affb81b9a5)
• Runtime surface matches the report: Transcript hygiene docs identify repairSessionFileIfNeeded as the on-disk session-file repair pass called before embedded runner load and compaction. Public docs: docs/reference/transcript-hygiene.md. (docs/reference/transcript-hygiene.md:57, 51affb81b9a5)
• Changelog records superseding fix: Current main has a 2026.4.30 changelog entry for the merged Agents/session-repair fix covering Session repair leaves JSONL ending on assistant turn → triggers Anthropic 400 prefill loop #75271 and Session-file repair drops blank user-role messages, breaking strict OpenAI-compat providers (Qwen3.6 / mlx-vlm) #75313, credited to @amknight. (CHANGELOG.md:93, 51affb81b9a5)
• Release check: The checkout only has release tag v2026.4.29 for the provided latest release, and git show v2026.4.29:CHANGELOG.md | rg '75313|75271|Agents/session-repair|Qwen|mlx|blank user' returned no match, so this is implemented on current main but not proven shipped in a release. (CHANGELOG.md:93, 51affb81b9a5)

Likely related people:

• amknight: Authored the merged fix(agents): trim trailing assistant turns and rewrite blank user messages in session repair #75606 follow-up that current main and the changelog identify as covering Session-file repair drops blank user-role messages, breaking strict OpenAI-compat providers (Qwen3.6 / mlx-vlm) #75313 and the same session-file repair behavior. (role: merged fix contributor; confidence: high; commits: deaa6f656758; files: src/agents/session-file-repair.ts, src/agents/session-file-repair.test.ts, CHANGELOG.md)
• steipete: Local shallow blame for the current session repair file points at the current main proof tree under Peter Steinberger, and the prior ClawSweeper review context traces related session/replay repair history to this maintainer. (role: recent maintainer / proof-tree owner; confidence: medium; commits: 51affb81b9a5, f0a2b09df69a, 9d33da6ddf2c; files: src/agents/session-file-repair.ts, src/agents/session-file-repair.test.ts, src/agents/pi-embedded-runner/replay-history.ts)
• openperf: Prior review context identifies this contributor on nearby Bedrock empty-assistant session-file repair and transcript hygiene documentation in the same persisted replay repair area. (role: adjacent feature contributor; confidence: medium; commits: 930d81aa41d2; files: src/agents/session-file-repair.ts, src/agents/session-file-repair.test.ts, docs/reference/transcript-hygiene.md)

Codex review notes: model gpt-5.5, reasoning high; reviewed against 51affb81b9a5; fix evidence: commit 51affb81b9a5, main fix timestamp 2026-05-01T16:46:53+01:00.