Please read this first

• Have you read the custom model provider docs, including the 'Common issues' section? Yes
• Have you searched for related issues? Yes

Describe the question

When using the Agents SDK with Google's Gemini API (OpenAI-compatible endpoint https://generativelanguage.googleapis.com/v1beta/openai/) as a custom model provider, using the gemini-2.5-flash-preview-05-20 model with tools specified in the Agent leads to an AttributeError: 'NoneType' object has no attribute 'model_dump' within the SDK's openai_chatcompletions.py file.

This error occurs because the Gemini API returns a ChatCompletion object where choices[0].message is None. Debugging reveals the finish_reason for this None message is content_filter: OTHER.

Key observations:

• The AttributeError (and the underlying None message with content_filter: OTHER) does not occur with the older gemini-2.5-flash-preview-04-17 model when tools are specified.
• The AttributeError does not occur with the gemini-2.5-flash-preview-05-20 model if tools are omitted from the Agent initialization.

This suggests an issue with how the gemini-2.5-flash-preview-05-20 model (via its OpenAI-compatible API) handles requests that include the tools parameter, leading to a response structure that causes an error in the Agents SDK.

Debug information

• Agents SDK version: v0.0.16 (approximated, based on User-Agent during testing)
• Python version: Python 3.12
• Relevant Traceback (from initial user report):

  Traceback (most recent call last):
    File "c:\Users\S\Documents\Projects-2\DumbCompUse\agenttest copy.py", line 298, in main
      result = await Runner.run(agent, input=conversation, max_turns=100)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Users\S\AppData\Roaming\Python\Python312\site-packages\agents\run.py", line 218, in run
      input_guardrail_results, turn_result = await asyncio.gather(
                                             ^^^^^^^^^^^^^^^^^^^^^
    File "C:\Users\S\AppData\Roaming\Python\Python312\site-packages\agents\run.py", line 762, in _run_single_turn
      new_response = await cls._get_new_response(
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Users\S\AppData\Roaming\Python\Python312\site-packages\agents\run.py", line 921, in _get_new_response
      new_response = await model.get_response(
                     ^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Users\S\AppData\Roaming\Python\Python312\site-packages\agents\models\openai_chatcompletions.py", line 78, in get_response
      f"LLM resp:\n{json.dumps(response.choices[0].message.model_dump(), indent=2)}\n"
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  AttributeError: 'NoneType' object has no attribute 'model_dump'
  

Repro steps

1. Setup: Use openai-agents, openai libraries. Point OpenAIChatCompletionsModel to Gemini's OpenAI-compatible endpoint with a valid API key.
2. Script (ensure openai_chatcompletions.py is in its original state, without the manual error handling for None messages):

   import asyncio
   from openai import AsyncOpenAI
   from agents import Agent, Runner, function_tool, OpenAIChatCompletionsModel, set_tracing_disabled
   import logging
   # Set to logging.DEBUG to see underlying API responses if SDK is modified to not crash
   logging.basicConfig(level=logging.INFO) 
   
   BASE_URL = "https://generativelanguage.googleapis.com/v1beta/openai/"
   API_KEY = "YOUR_GEMINI_API_KEY" # Replace with your actual key
   MODEL_WITH_ISSUE = "gemini-2.5-flash-preview-05-20"
   MODEL_WORKING_FINE = "gemini-2.5-flash-preview-04-17"
   
   # Test this model to see the AttributeError
   CURRENT_MODEL_TO_TEST = MODEL_WITH_ISSUE 
   
   client = AsyncOpenAI(base_url=BASE_URL, api_key=API_KEY)
   set_tracing_disabled(True) # Keep it simple
   
   @function_tool
   def get_weather(city: str):
       """Gets the current weather."""
       return f"The weather in {city} is sunny."
   
   async def main():
       print(f"--- Testing: {CURRENT_MODEL_TO_TEST} with tools (expecting AttributeError) ---")
       agent = Agent(
           name="TestBot", 
           model=OpenAIChatCompletionsModel(model=CURRENT_MODEL_TO_TEST, openai_client=client), 
           tools=[get_weather], 
           instructions="Respond in haikus."
       )
       try:
           result = await Runner.run(agent, "What is the weather in Tokyo?")
           print(f"Output (with tools): {result.final_output if result else 'None - this should not be reached if AttributeError occurs'}")
       except AttributeError as e:
           print(f"Caught expected AttributeError: {e}")
       except Exception as e:
           print(f"Caught other exception: {e}")
   
       # For comparison, this works:
       print(f"\n--- Testing: {MODEL_WORKING_FINE} with tools (expected to work) ---")
       agent_ok = Agent(
           name="TestBotOK", 
           model=OpenAIChatCompletionsModel(model=MODEL_WORKING_FINE, openai_client=client), 
           tools=[get_weather], 
           instructions="Respond in haikus."
       )
       result_ok = await Runner.run(agent_ok, "What is the weather in Tokyo?")
       print(f"Output (MODEL_WORKING_FINE with tools): {result_ok.final_output if result_ok else 'None'}")
   
   asyncio.run(main())

3. Run: Execute the script with CURRENT_MODEL_TO_TEST set to MODEL_WITH_ISSUE.
4. Observed: The script raises AttributeError: 'NoneType' object has no attribute 'model_dump' at agents\models\openai_chatcompletions.py", line 78. If the SDK is modified to inspect the response before this line, it's seen that response.choices[0].message is None and response.choices[0].finish_reason is content_filter: OTHER.

Expected behavior

The gemini-2.5-flash-preview-05-20 model, when used with the Agents SDK via its OpenAI-compatible endpoint and with tools specified, should return a ChatCompletion object where choices[0].message is properly populated (with text content or tool_calls). This would prevent the AttributeError in the SDK and allow for normal tool use, consistent with gemini-2.5-flash-preview-04-17 or when tools are omitted for the 05-20 model. The API should not return a None message with content_filter: OTHER solely due to the presence of the tools parameter for innocuous prompts.