The empty-response recovery path in run_agent.py appends synthetic
messages tagged with _empty_recovery_synthetic (and the agent loop uses
_thinking_prefill / _empty_terminal_sentinel similarly). These are
internal bookkeeping markers — they must never reach the wire.
chat_completions' convert_messages only stripped Codex Responses leak
fields (codex_reasoning_items, call_id, etc.), not these _-prefixed
markers. Permissive providers (real OpenAI, Anthropic) silently ignore
unknown message keys so the bug stayed hidden, but strict
OpenAI-compatible gateways reject them outright. Observed against
codex.nekos.me:
502: [ObjectParam] [input[617]._empty_recovery_synthetic]
[unknown_parameter] Unknown parameter:
'_empty_recovery_synthetic'
Because the synthetic messages persist in the session, every
subsequent request in that session carries the poisoned key and
fails identically — a deterministic 502 the retry loop mistakes for
a transient server error.
Fix: convert_messages now drops any top-level message key starting
with '_'. OpenAI's message schema has no '_'-prefixed fields, so this
is safe and future-proofs against new internal markers.
Origin: local-author
Upstream-PR: none
Patch-State: local-only
775a17284f