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fix(agent): strip trailing empty assistant messages before API calls to prevent prefill rejection #2129

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fix(agent): strip trailing empty assistant messages before API calls to prevent prefill rejection #2129

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13 changes: 12 additions & 1 deletion run_agent.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2217,7 +2217,18 @@ def _sanitize_api_messages(messages: List[Dict[str, Any]]) -> List[Dict[str, Any
"Pre-call sanitizer: added %d stub tool result(s)",
len(missing_results),
)

# 3. Strip trailing empty assistant messages to prevent prefill rejection.
# These can leak from Responses API reasoning-only turns (Codex/MiniMax)
# where an empty assistant message is required by the Responses API but
# must NOT be sent to Chat Completions or Anthropic Messages API providers.
while (
messages
and messages[-1].get("role") == "assistant"
and not (messages[-1].get("content") or "").strip()
and not messages[-1].get("tool_calls")
):
logger.debug("Pre-call sanitizer: removed trailing empty assistant message")
messages = messages[:-1]
return messages

@staticmethod
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