OpenAI API 400: unsupported parameter with reasoning models

Symptom

Your AI coding tool or backend integration fails immediately after switching models or upgrading the SDK. The request reaches the API, but the response is a 400 Bad Request.

BadRequestError: 400 Unsupported parameter: 'temperature' is not supported with this model.

Likely Cause

Reasoning-focused models expose a smaller or different parameter surface than older chat models. Agent frameworks often reuse a default configuration that includes sampling parameters, response format options, or tool settings the selected model does not accept.

Fast Fix

• Remove unsupported sampling options from the request for the affected model.
• Keep model-specific request builders instead of sharing one global config across all models.
• Upgrade the SDK and check the model capability table before enabling structured output or tool options.
• Log the final payload sent by the agent, not only the high-level prompt.

Verification

Run a minimal request with only the model and input. Add parameters back one by one until the failing option is identified. Keep this minimal request as a regression test for future model upgrades.

Prevention

Create a small compatibility map in your app: model name, supported controls, structured output mode, tool mode, and retry behavior. AI agents fail less mysteriously when the request layer validates options before sending them.