Bug: 'Invalid request body' with github-copilot/gemini-3-pro-preview model #5161

Description

When using the opencode CLI with the github-copilot/gemini-3-pro-preview model, the tool fails with an "invalid request body" error.

Environment

• OS: Linux
• opencode version: Standalone binary installation located at /home/dani/.opencode/bin/opencode.

Steps to Reproduce

1. Configure the opencode CLI to use github-copilot/gemini-3-pro-preview as the model.
2. Run a command that invokes the model (e.g., opencode "your prompt here").
3. The request fails with an "invalid request body" error from the upstream API.

Investigation Details

During a debugging session, the following was determined:

• The issue appears to stem from the opencode-gemini-auth plugin, which is bundled within the standalone binary.
• This plugin transforms requests for Gemini models to a format expected by the cloudcode-pa.googleapis.com endpoint.
• This transformation seems to be incorrectly applied to the github-copilot/gemini-3-pro-preview model, causing the API to reject the request body.
• Since the opencode installation is a single binary, it was not possible to patch the bundled plugin code to bypass this transformation for the specific model.
• Attempts to override the behavior via opencode.json configuration were also unsuccessful.

Suggested Fix

The opencode-gemini-auth plugin should likely not apply its specific request body transformation when the model is github-copilot/gemini-3-pro-preview. The request should be passed through to the standard Gemini API endpoint without modification.