OpenAI compatibility endpoint, 400 invalid argument

Gemini API
,
1

I have a test case for a problem I am finding with the OAI compatibility layer. Minimal-ish case gist here. It is based on the sample code here. The code is set up to easily swap in another endpoint for sxs comparison with working endpoints.

I also capture the request/responses here.

The outgoing request sending the tool response is well formed. The final response is:

INCOMING RESPONSE: 400
Response body: [
  {
    "error": {
      "code": 400,
      "message": "Request contains an invalid argument.",
      "status": "INVALID_ARGUMENT"
    }
  }
]

There are no parallel tool calls at play, this is about as simple an exchange as it gets.