Gemini OAuth Plugin for Opencode

Authenticate the Opencode CLI with your Google account so you can use your existing Gemini plan and its included quota instead of API billing.

Setup

1. Add the plugin to your Opencode config:

   {
     "$schema": "https://opencode.ai/config.json",
     "plugin": ["opencode-gemini-auth"]
   }

2. Run opencode auth login.

3. Choose the Google provider and select OAuth with Google (Gemini CLI).

The plugin spins up a local callback listener, so after approving in the browser you'll land on an "Authentication complete" page with no URL copy/paste required. If that port is already taken, the CLI automatically falls back to the classic copy/paste flow and explains what to do.

Updating

Warning

OpenCode does NOT auto-update plugins

To get the latest version:

(cd ~ && sed -i.bak '/"opencode-gemini-auth"/d' .cache/opencode/package.json && \
rm -rf .cache/opencode/node_modules/opencode-gemini-auth && \
echo "Plugin update script finished successfully.")

opencode  # Reinstalls latest

Local Development

First, clone the repository and install dependencies:

git clone https://github.com/jenslys/opencode-gemini-auth.git
cd opencode-gemini-auth
bun install

When you want Opencode to use a local checkout of this plugin, point the plugin entry in your config to the folder via a file:// URL:

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["file:///absolute/path/to/opencode-gemini-auth"]
}

Replace /absolute/path/to/opencode-gemini-auth with the absolute path to your local clone.

Manual Google Cloud Setup

If automatic provisioning fails, use the console:

1. Go to the Google Cloud Console and create (or select) a project, e.g. gemini.
2. Select that project.
3. Enable the Gemini for Google Cloud API (cloudaicompanion.googleapis.com).
4. Re-run opencode auth login and enter the project ID (not the display name).

Debugging Gemini Requests

Set OPENCODE_GEMINI_DEBUG=1 in the environment when you run an Opencode command to capture every Gemini request/response that this plugin issues. When enabled, the plugin writes to a timestamped gemini-debug-<ISO>.log file in your current working directory so the CLI output stays clean.

OPENCODE_GEMINI_DEBUG=1 opencode

The logger shows the transformed URL, HTTP method, sanitized headers (the Authorization header is redacted), whether the call used streaming, and a truncated preview (2 KB) of both the request and response bodies. This is handy when diagnosing "Bad Request" responses from Gemini. Remember that payloads may still include parts of your prompt or response, so only enable this flag when you're comfortable keeping that information in the generated log file.

404s on gemini-2.5-flash-image. Opencode fires internal summarization/title requests at gemini-2.5-flash-image. The plugin automatically remaps those payloads to gemini-2.5-flash, eliminating the extra 404s for accounts without image access. If you still see a 404, confirm your project actually has access to the fallback model.