Best possible solution:

Keep normal assistant final replies in stored session history while preserving provider-specific outbound prefill stripping; land after maintainer review and a rebase/merge that keeps the current debug logging behavior.

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

Yes. Current main can be reproduced statically with a JSONL session ending in a plain assistant message: repairSessionFileIfNeeded pops that entry, and the PR adds focused tests for HEARTBEAT_OK and consecutive trailing assistant finals staying unchanged.

Is this the best way to solve the issue?

Yes, directionally. Removing persistent disk trimming is the narrowest fix for transcript history loss because current main already has outbound Anthropic/Gemini prefill stripping at the stream boundary; the final branch should preserve the newer debug logging change.

What I checked:

• Protected label: The live PR metadata includes the maintainer label, which is protected for ClawSweeper cleanup decisions. (fb3c8e5bc501)
• Current main trims assistant finals on disk: Current main has isTrimmableTrailingAssistantEntry and a post-parse loop that pops trailing non-tool assistant messages from the session JSONL. (src/agents/session-file-repair.ts:271, 7f83ba8ddf84)
• PR preserves assistant finals: The PR head removes the trailing-assistant trim pass from repairSessionFileIfNeeded and adds tests that keep HEARTBEAT_OK and consecutive assistant finals unchanged on disk. (src/agents/session-file-repair.test.ts:316, fb3c8e5bc501)
• Outbound replay guard remains: Current main still strips trailing assistant prefill turns at the provider stream boundary when Anthropic or Gemini turn validation is active. (src/agents/pi-embedded-runner/run/attempt.tool-call-normalization.ts:904, 7f83ba8ddf84)
• Guard coverage exists: Focused tests assert trailing assistant prefill stripping for Anthropic and Gemini outbound replay. (src/agents/pi-embedded-runner/run/attempt.test.ts:1764, 7f83ba8ddf84)
• Related provenance: Merged PR fix(agents): trim trailing assistant turns and rewrite blank user messages in session repair #75606 introduced the disk trim in commit 5245289 to close 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; this PR is a follow-up after live sessions showed the trim erasing healthy finals. (src/agents/session-file-repair.ts:267, 524528944f05)

Likely related people:

• amknight: Merged fix(agents): trim trailing assistant turns and rewrite blank user messages in session repair #75606 introduced the disk-level trailing assistant trim and blank-user rewrite in the central session-file repair path. (role: introduced behavior; confidence: high; commits: 524528944f05; files: src/agents/session-file-repair.ts, src/agents/session-file-repair.test.ts, CHANGELOG.md)
• steipete: Recently changed the same session repair logging surface and has adjacent replay-history maintenance commits in the agents area. (role: recent maintainer; confidence: high; commits: 929df0f5567f, d80a8eb3adb6, c5c08c074a44; files: src/agents/session-file-repair.ts, src/agents/session-file-repair.test.ts, src/agents/pi-embedded-runner/replay-history.ts)
• openperf: Added the earlier empty-assistant Bedrock session repair behavior that this file still preserves. (role: adjacent owner; confidence: medium; commits: 930d81aa41d2; files: src/agents/session-file-repair.ts, src/agents/session-file-repair.test.ts, docs/reference/transcript-hygiene.md)
• shakkernerd: Earlier session-file repair hardening and malformed session repair work appear in the history for this module. (role: feature-history maintainer; confidence: medium; commits: e6fdac7bfb6d, 0da6de6624a9; files: src/agents/session-file-repair.ts)

Remaining risk / open question:

• This read-only review did not run the PR's tests; validation here is static plus the PR author's reported targeted test and live gateway smoke.
• The branch predates current main's 929df0f session-repair debug logging change, so a maintainer should ensure the final merge preserves debug-level success logging while removing only disk-level assistant-final trimming.

Codex review notes: model gpt-5.5, reasoning high; reviewed against 7f83ba8ddf84.