langchain - 💡(How to fix) Fix ChatOpenAI fails to provide thought signatures on multi-turn conversations when used with Gemini models, resulting in 400 BadRequestError

import os

from dotenv import load_dotenv
from langchain_core.messages import HumanMessage, ToolMessage
from langchain_core.tools import tool
from langchain_openai import ChatOpenAI

load_dotenv()

@tool
def greet() -> str:
    """Say hello."""
    return "Hello!"


llm = ChatOpenAI(
    model="gemini-3-flash-preview",
    base_url="https://generativelanguage.googleapis.com/v1beta/openai/",
    api_key=os.environ["GEMINI_API_KEY"],
    temperature=0,
).bind_tools([greet])


def main() -> None:
    msgs: list = [HumanMessage("Use the greet tool")]
    ai = llm.invoke(msgs)
    print(f"Turn 1 OK; tool_calls={ai.tool_calls}")
    print(f"additional_kwargs keys: {list(ai.additional_kwargs)}")

    tc_id = ai.tool_calls[0]["id"]
    msgs += [ai, ToolMessage(content="Hello!", tool_call_id=tc_id)]
    final = llm.invoke(msgs)
    print(f"Turn 2 OK: {final.content!r}")


if __name__ == "__main__":
    main()

---

Turn 1 OK; tool_calls=[{'name': 'greet', 'args': {}, 'id': 'zv7ul4f5', 'type': 'tool_call'}]
additional_kwargs keys: ['refusal']
Traceback (most recent call last):
  File "gemini3_openai_issue/main.py", line 38, in <module>
    main()
    ~~~~^^
  File "gemini3_openai_issue/main.py", line 33, in main
    final = llm.invoke(msgs)
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/langchain_core/runnables/base.py", line 5753, in invoke
    return self.bound.invoke(
           ~~~~~~~~~~~~~~~~~^
        input,
        ^^^^^^
        self._merge_configs(config),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        **{**self.kwargs, **kwargs},
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/langchain_core/language_models/chat_models.py", line 472, in invoke
    self.generate_prompt(
    ~~~~~~~~~~~~~~~~~~~~^
        [self._convert_input(input)],
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<6 lines>...
        **kwargs,
        ^^^^^^^^^
    ).generations[0][0],
    ^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/langchain_core/language_models/chat_models.py", line 1752, in generate_prompt
    return self.generate(prompt_messages, stop=stop, callbacks=callbacks, **kwargs)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/langchain_core/language_models/chat_models.py", line 1559, in generate
    self._generate_with_cache(
    ~~~~~~~~~~~~~~~~~~~~~~~~~^
        m,
        ^^
    ...<2 lines>...
        **kwargs,
        ^^^^^^^^^
    )
    ^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/langchain_core/language_models/chat_models.py", line 1899, in _generate_with_cache
    result = self._generate(
        messages, stop=stop, run_manager=run_manager, **kwargs
    )
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/langchain_openai/chat_models/base.py", line 1653, in _generate
    _handle_openai_bad_request(e)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/langchain_openai/chat_models/base.py", line 1650, in _generate
    raw_response = self.client.with_raw_response.create(**payload)
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/openai/_legacy_response.py", line 367, in wrapped
    return cast(LegacyAPIResponse[R], func(*args, **kwargs))
                                      ~~~~^^^^^^^^^^^^^^^^^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/openai/_utils/_utils.py", line 298, in wrapper
    return func(*args, **kwargs)
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/openai/resources/chat/completions/completions.py", line 1215, in create
    return self._post(
           ~~~~~~~~~~^
        "/chat/completions",
        ^^^^^^^^^^^^^^^^^^^^
    ...<51 lines>...
        stream_cls=Stream[ChatCompletionChunk],
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/openai/_base_client.py", line 1332, in post
    return cast(ResponseT, self.request(cast_to, opts, stream=stream, stream_cls=stream_cls))
                           ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "gemini3_openai_issue/.venv-buggy/lib/python3.13/site-packages/openai/_base_client.py", line 1105, in request
    raise self._make_status_error_from_response(err.response) from None
openai.BadRequestError: Error code: 400 - [{'error': {'code': 400, 'message': 'Function call is missing a thought_signature in functionCall parts. This is required for tools to work correctly, and missing thought_signature may lead to degraded model performance. Additional data, function call `default_api:greet` , position 2. Please refer to https://ai.google.dev/gemini-api/docs/thought-signatures for more details.', 'status': 'INVALID_ARGUMENT'}}]