fix: merge stray top-level system kwarg into messages array in Chat Completions transport (#60821)

[kyssta-exe]

When request_overrides or a provider profile accidentally forwards system as a top-level OpenAI Chat Completions kwarg, merge it into the messages array instead of crashing.

[falkoro]

Community review — verified locally. Direction is correct: a top-level system kwarg is invalid for Chat Completions and merging it into the messages array is the right recovery. Also checked the mutation concern: sanitized comes from convert_messages(), which deep-copies per call, so the in-place insert(0, …) is safe.

Three things worth tightening:

1. The guard elif k == "system" and sanitized: means an empty messages list still falls through to api_kwargs["system"] = v — the original 400 in that edge. sanitized.insert(0, …) works fine on an empty list, so the and sanitized can drop (with the exact-match branch guarded by if sanitized and sanitized[0].get("role") == "system").
2. System message content may be a content-parts list rather than a string on some providers; (list or "") + "\n" raises TypeError. An isinstance(content, str) check (falling back to inserting a separate system message) covers it.
3. No tests — this is exactly the kind of fix a small unit test pins cheaply (overrides with system + existing system message / no system message / empty messages).

[tonydwb]

Code Review Summary

Verdict: Comment (token read-only)

PR 60891 merges a stray top-level system kwarg into the messages array in Chat Completions transport. Fixes a protocol handling edge case. Well-scoped (1 file, 14 additions). No security issues or debug artifacts detected.

LGTM - awaiting maintainer approval.

[teknium1]

Thanks for the narrowly scoped defensive fix. It needs rework before it can reliably address the reported failure.

Problems

• The confirmed #60821 injector was llm_request middleware. Current main builds transport kwargs at agent/conversation_loop.py:1161, then middleware replaces them at agent/conversation_loop.py:1175-1192; this PR only handles request_overrides, so it does not intercept that path.
• The added and sanitized guard leaves messages=[] on the existing fallback that forwards system into api_kwargs (agent/transports/chat_completions.py:601-608).
• The merge expression assumes string content. Existing system messages may use content parts (tests/providers/test_provider_profiles.py:471), for which list-plus-string raises TypeError.

Suggested changes

• Re-scope the reported fix to the post-middleware boundary or present this as separate request_overrides hardening.
• Handle empty messages and content-part system messages without forwarding a top-level system kwarg.
• Add regression tests in tests/agent/transports/test_chat_completions.py for text, absent, empty, and content-parts system-message cases.

Automated hermes-sweeper review.

[teknium1]

and sanitized leaves an empty messages list on the final else, which still forwards the invalid top-level system kwarg. Handle the empty-list case by inserting a system message too.

[teknium1]

System content can be a non-empty content-parts list (tests/providers/test_provider_profiles.py:471); adding "\n" to it raises TypeError. Merge only strings, or insert a separate system message for non-string content.

[kyssta-exe]

Stale — no merge activity for 4-6 days. Can resubmit if still needed.