Gemini 3 Preview (OpenAI-compatible) rejects reasoning_effort: "medium" — works with low and high

Gemini API
, ,
1

Starting today, requests to Gemini 3 Preview using the OpenAI compatibility interface fail when reasoning_effort (thinking level) is set to "medium". The same request succeeds with "low" and "high".

This appears to be a model capability/config regression or a validation bug specific to the "medium" level.

Product / API: Gemini 3 Preview via OpenAI compatibility layer (Chat Completions API)

Issue type: Regression / breaking change

Note: There is no issue for gemini-3-flash-preview

Date first observed: 2025-12-19 (worked until 2025-12-16)

Steps to reproduce

  1. Send a request to Gemini 3 Preview (OpenAI compatibility endpoint) with:

    • model: "gemini-3-pro-preview" (or your exact Gemini 3 preview model id)

    • reasoning_effort: "medium"

    • Any basic prompt/messages

  2. Observe the error response.

  3. Repeat the same request with reasoning_effort: "low" → succeeds.

  4. Repeat with reasoning_effort: "high" → succeeds.

Minimal repro (example payload)

POST https://generativelanguage.googleapis.com/v1beta/openai/v1/chat/completions

{
  "model": "gemini-3-pro-preview",
  "reasoning_effort": "medium",
  "messages": [
    { "role": "user", "content": "Say hello." }
  ]
}

Actual result

Request fails with:

Thinking level MEDIUM is not supported for this model. Please retry with other thinking level