ai-agents-openai-api-compatibi-when-using-langchain-openai-any-additional-fields--6ba47d04

THEN Modify the internal conversion functions _convert_dict_to_message and _convert_message_to_dict to preserve unknown fields by passing through any keys not explicitly handled. This matches the openai library's own forward-compatible design and allows custom or future fields to survive the round trip. Implement the fix as shown in the linked PR (LangChain #26617).

from langchain_core.messages import HumanMessage

# Before fix: extra field 'custom' is lost
msg = HumanMessage(content='hello', custom='value')
d = _convert_message_to_dict(msg)  # yields {'content': 'hello', 'role': 'user'} — 'custom' dropped

# After fix: include extra fields
class HumanMessage(BaseMessage):
    pass  # allow_extra_fields via model_config
# Then dict conversion iterates over message fields and includes all keys