Google Vertex OpenAI Models - Error 400 INVALID ARGUMENT

[Mgczacki]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

As of the latest versions of PydanticAI, using the "google-vertex:gpt-oss-120b-maas" model from Google Vertex's model garden is not possible due to this error:

google.genai.errors.ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': 'Request contains an invalid argument.', 'status': 'INVALID_ARGUMENT'}}

Example Code

from pydantic_ai import Agent

a = Agent("google-vertex:gpt-oss-120b-maas")

await a.run("Hello, world!")

Python, Pydantic AI & LLM client version

Python 3.13
pydantic-ai==1.10.0
google-genai==1.48.0

[DouweM]

@Mgczacki It's not indicated on https://docs.cloud.google.com/vertex-ai/generative-ai/docs/maas/openai/gpt-oss-120b, but https://console.cloud.google.com/vertex-ai/publishers/openai/model-garden/gpt-oss-120b-maas clarifies that the complete model ID is publishers/openai/models/gpt-oss-120b-maas. I tried your example with that, and it worked. Let me know if you run into any more issues.