Function tool choice causing pydantic validation error

[qqaatw]

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• Have you searched for related issues? Yes

Describe the bug

A clear and concise description of what the bug is.

This only happens when run with Runner.run_streamed

When a function name is provided to ModelSettings, the converter here will convert it to a dict structure:

openai-agents-python/src/agents/models/openai_chatcompletions.py

Line 247 in 18cb55e

tool_choice = Converter.convert_tool_choice(model_settings.tool_choice)

@classmethod
    def convert_tool_choice(
        cls, tool_choice: Literal["auto", "required", "none"] | str | None
    ) -> ChatCompletionToolChoiceOptionParam | NotGiven:
        if tool_choice is None:
            return NOT_GIVEN
        elif tool_choice == "auto":
            return "auto"
        elif tool_choice == "required":
            return "required"
        elif tool_choice == "none":
            return "none"
        else:
            return {
                "type": "function",
                "function": {
                    "name": tool_choice,
                },
            }

However, at the below line, we restrict the input to be one of the three Literals.

openai-agents-python/src/agents/models/openai_chatcompletions.py

Line 305 in 18cb55e

tool_choice=cast(Literal["auto", "required", "none"], tool_choice)

Resulting in the below pydantic validation error:

    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 5 validation errors for Response
tool_choice.literal['none','auto','required']
  Input should be 'none', 'auto' or 'required' [type=literal_error, input_value={'type': 'function', 'fun...ognize_named_entities'}}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/literal_error
tool_choice.ToolChoiceTypes.type
  Input should be 'file_search', 'web_search_preview', 'computer_use_preview', 'web_search_preview_2025_03_11', 'image_generation' or 'code_interpreter' [type=literal_error, input_value='function', input_type=str]
    For further information visit https://errors.pydantic.dev/2.11/v/literal_error
tool_choice.ToolChoiceFunction.name
  Field required [type=missing, input_value={'type': 'function', 'fun...ognize_named_entities'}}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing
tool_choice.ToolChoiceMcp.server_label
  Field required [type=missing, input_value={'type': 'function', 'fun...ognize_named_entities'}}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing
tool_choice.ToolChoiceMcp.type
  Input should be 'mcp' [type=literal_error, input_value='function', input_type=str]
    For further information visit https://errors.pydantic.dev/2.11/v/literal_error

Debug information

• Agents SDK version: v0.1.0
• Python version: Python 3.12

Repro steps

Run with Runner.run_streamed and set the model settings to a tool function name instead of the default three Literals.

Expected behavior

Without any validation issues.