Gemini OAuth Plugin for Opencode

!License
!Version

Authenticate the Opencode CLI with your Google account. This plugin enables
you to use your existing Gemini plan and quotas (including the free tier)
directly within Opencode, bypassing separate API billing.

Prerequisites

- Opencode CLI installed.
- A Google account with access to Gemini.

Installation

Add the plugin to your Opencode configuration file
(~/.config/opencode/opencode.json or similar):

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

> [!IMPORTANT]
> If you're using a paid Gemini Code Assist subscription (Standard/Enterprise),
> explicitly configure a Google Cloud projectId. Free tier accounts should
> auto-provision a managed project, but you can still set projectId to force
> a specific project.

Usage

1. Login: Run the authentication command in your terminal:

`bash
opencode auth login
`

2. Select Provider: Choose Google from the list.
3. Authenticate: Select OAuth with Google (Gemini CLI).
- A browser window will open for you to approve the access.
- The plugin spins up a temporary local server to capture the callback.
- If the local server fails (e.g., port in use or headless environment),
you can manually paste the callback URL or just the authorization code.

Once authenticated, Opencode will use your Google account for Gemini requests.

Configuration

$3

By default, the plugin attempts to provision or find a suitable Google Cloud
project. To force a specific project, set the projectId in your configuration
or via environment variables:

File: ~/.config/opencode/opencode.json

`json
{
"provider": {
"google": {
"options": {
"projectId": "your-specific-project-id"
}
}
}
}
`

You can also set OPENCODE_GEMINI_PROJECT_ID, GOOGLE_CLOUD_PROJECT, or
GOOGLE_CLOUD_PROJECT_ID to supply the project ID via environment variables.

$3

Below are example model entries you can add under provider.google.models in your
Opencode config. Each model can include an options.thinkingConfig block to
enable "thinking" features.

`json
{
"provider": {
"google": {
"models": {
"gemini-2.5-flash": {
"options": {
"thinkingConfig": {
"thinkingBudget": 8192,
"includeThoughts": true
}
}
},
"gemini-2.5-pro": {
"options": {
"thinkingConfig": {
"thinkingBudget": 8192,
"includeThoughts": true
}
}
},
"gemini-3-flash-preview": {
"options": {
"thinkingConfig": {
"thinkingLevel": "high",
"includeThoughts": true
}
}
},
"gemini-3-pro-preview": {
"options": {
"thinkingConfig": {
"thinkingLevel": "high",
"includeThoughts": true
}
}
}
}
}
}
}
`

Note: Available model names and previews may change—check Google's documentation or
the Gemini product page for the current model identifiers.

$3

The plugin supports configuring Gemini "thinking" features per-model via
thinkingConfig. The available fields depend on the model family:

- For Gemini 3 models: use thinkingLevel with values "low" or "high".
- For Gemini 2.5 models: use thinkingBudget (token count).
- includeThoughts (boolean) controls whether the model emits internal thoughts.

A combined example showing both model types:

`json
{
"provider": {
"google": {
"models": {
"gemini-3-pro-preview": {
"options": {
"thinkingConfig": {
"thinkingLevel": "high",
"includeThoughts": true
}
}
},
"gemini-2.5-flash": {
"options": {
"thinkingConfig": {
"thinkingBudget": 8192,
"includeThoughts": true
}
}
}
}
}
}
}
`

If you don't set a thinkingConfig for a model, the plugin will use default
behavior for that model.

Troubleshooting

$3

If automatic provisioning fails, you may need to set up the project manually:

1. Go to the Google Cloud Console.
2. Create or select a project.
3. Enable the Gemini for Google Cloud API
(cloudaicompanion.googleapis.com).
4. Configure the projectId in your Opencode config as shown above.

$3

To view detailed logs of Gemini requests and responses, set the
OPENCODE_GEMINI_DEBUG environment variable:

`bash
OPENCODE_GEMINI_DEBUG=1 opencode
`

This will generate gemini-debug-.log files in your working
directory containing sanitized request/response details.

Parity Notes

This plugin mirrors the official Gemini CLI OAuth flow and Code Assist
endpoints. In particular, project onboarding and quota retry handling follow
the same behavior patterns as the Gemini CLI.

$3

- Gemini CLI repository: https://github.com/google-gemini/gemini-cli
- Gemini CLI quota documentation: https://developers.google.com/gemini-code-assist/resources/quotas

$3

Opencode does not automatically update plugins. To update to the latest version,
you must clear the cached plugin:

`bash


Clear the specific plugin cache

rm -rf ~/.cache/opencode/node_modules/opencode-gemini-auth

Run Opencode to trigger a fresh install

opencode
`

Development

To develop on this plugin locally:

1. Clone:

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

2. Link:
Update your Opencode config to point to your local directory using a
file:// URL:

`json
{
"plugin": ["file:///absolute/path/to/opencode-gemini-auth"]
}
``

License

MIT