Error when using OpenAI GPT-5 based models

[raphaelpor]

When running Github Models (both on the CLI and on the web interface) with GPT-5 based models, we get the error:

Error: unexpected response from the server: 500 Internal Server Error
{
  "error": {
    "message": "The server had an error while processing your request. Sorry about that!",
    "type": "server_error",
    "param": null,
    "code": null
  }
}

It happens with any kind of request or evals task with these models.