Fix error when OpenRouter API returns None tool call arguments

[barp]

This only happens if you have a tool with all optional parameters.

for example the following response can return from open router:

{'id': 'gen-1764223032-90is29WrhzbOG2XawxiT', 'choices': [{'finish_reason': 'tool_calls', 'index': 0, 'logprobs': None, 'messag
e': {'content': '', 'refusal': None, 'role': 'assistant', 'annotations': None, 'audio': None, 'function_call': None, 'tool_call
s': [{'id': 'toolu_vrtx_0142STg84ZeUdDEVtucWMjys', 'function': {'arguments': None, 'name': 'find_education_content'}, 'type': '
function', 'index': 0}], 'reasoning': None}, 'native_finish_reason': 'tool_calls'}], 'created': 1764223033, 'model': 'anthropic
/claude-sonnet-4.5', 'object': 'chat.completion', 'service_tier': None, 'system_fingerprint': None, 'usage': {'completion_token
s': 23, 'prompt_tokens': 8146, 'total_tokens': 8169, 'completion_tokens_details': None, 'prompt_tokens_details': None}, 'provid
er': 'Google'}

as you can see the function arguments can be None.

The following code reproduces the issue:

from pydantic import BaseModel
from pydantic_ai import RunContext, Agent
from pydantic_ai.models.openrouter import OpenRouterModel

model = OpenRouterModel(
    'anthropic/claude-sonnet-4.5',
)

agent = Agent[None, str](
    model=model,
    output_type=str,
    instrument=True,
    retries=5,
)

class FindEducationContentFilters(BaseModel):
    title: str | None = None

@agent.tool
async def find_education_content(
    ctx: RunContext[None], filters: FindEducationContentFilters
) -> str:
    return "This is a content about education"

print(agent.run_sync("Can you find me education content?"))

[DouweM]

Thanks for the report. For context, the error looks like this:

  File "/home/DouweM/pydantic-ai/pydantic_ai_slim/pydantic_ai/models/openai.py", line 598, in _process_response
    raise UnexpectedModelBehavior(f'Invalid response from {self.system} chat completions endpoint: {e}') from e
pydantic_ai.exceptions.UnexpectedModelBehavior: Invalid response from openrouter chat completions endpoint: 3 validation errors for _OpenRouterChatCompletion
choices.0.message.tool_calls.0.ChatCompletionMessageFunctionToolCall.function.arguments
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.11/v/string_type

[DouweM]

@barp Thanks for working on this! In addition to my comments, can you please make sure we have a test for this?

[barp]

@barp Thanks for working on this! In addition to my comments, can you please make sure we have a test for this?

Can you point me to where the test should be added?

Edit: NVM I think I got it.

[barp]

The failure seems unrelated to my change and the CI should be re-run