Summary

agent/embedded session-repair logic leaves session JSONL files ending on a role=assistant message after "repair", which then resubmits to Anthropic with assistant prefill — Anthropic rejects with HTTP 400 (This model does not support assistant message prefill). When the agent's failover chain has only one candidate, this surfaces as a user-visible error and can wedge the embedded agent in a silent loop.

Environment

• OpenClaw: latest (running via LaunchAgent on macOS)
• Model: anthropic/claude-opus-4-7
• Platform: Darwin 25.3.0 arm64, Node v25.8.1

Repro / Observed Behaviour

1. Session JSONL ends with role=assistant (e.g. previous run interrupted before the next user turn was appended).
2. agent/embedded attempts repair on session resume.
3. Repair rewrites the assistant message but file still ends on role=assistant.
4. Request submitted to Anthropic ends with assistant content → HTTP 400 format error.
5. Failover decision = surface_error because primary is the only candidate.
6. Loop continues until process is SIGTERM'd.

Real-world Impact

Out of 877 session files on this gateway, 250 (~28%) had this corruption. Yesterday alone: 300 prefill rejections + 49 repair attempts. Today before manual restart (~7h window): 84 + 14. Continuous, not one-off.

Suggested Fix

Either:

• Drop trailing assistant entries during repair until the file ends on a role=user turn, OR
• Append a synthetic user "(continue)" turn before resubmission.

Also worth considering: a session-janitor cron that quarantines any JSONL whose last message entry isn't role=user, run weekly or on startup.

Workaround (current)

• Quarantined 250 corrupt sessions to ~/.openclaw/agents/<id>/sessions/_quarantine_YYYY-MM-DD/ with a MANIFEST.
• Added fallbacks array to agents.defaults.model so single-candidate 4xx no longer surfaces directly.

Logs (sanitized)

06:51:36  agent/embedded repair → session e9256c82-...
06:51:38  Anthropic 400: "This model does not support assistant message prefill"
06:51:38  failover decision: surface_error (1 candidate)
06:51:53→06:53:21  90s silence, embedded lane stuck
06:53:21  SIGTERM (manual)

Happy to provide the quarantine MANIFEST or full logs offline if useful.