How to Use Claude 4.5 and Gemini 3 for Free with OpenCode AI Agent

The landscape of AI-assisted development is shifting from simple chat interfaces to sophisticated autonomous agents. While proprietary tools like Cursor or Windsurf dominate the market, open-source alternatives like OpenCode are gaining traction by offering unparalleled flexibility. One of the most sought-after features in the developer community is the ability to access high-tier models such as Claude 4.5 and Gemini 3 without the burden of multiple expensive subscriptions. This tutorial explores how to leverage OpenCode and the Antigravity Auth plugin to achieve this workflow.

Understanding the OpenCode Ecosystem

OpenCode is an open-source AI coding agent designed to live within your terminal and IDE. Unlike traditional extensions that lock you into a specific provider, OpenCode acts as a protocol-agnostic bridge. It allows developers to connect to various backends, ranging from local Ollama instances to high-performance aggregators like n1n.ai.

The core strength of OpenCode lies in its plugin architecture. By using specialized plugins, you can extend the authentication layer to support OAuth-based providers that often offer free tiers or experimental access to bleeding-edge models like Claude 4.5. However, for production-grade reliability and higher rate limits, developers often transition to professional API hubs like n1n.ai to ensure their CI/CD pipelines never stall.

Step 1: Installing the OpenCode CLI

OpenCode supports a wide array of package managers. While npm and brew are common, the community has seen a significant performance boost when using bun due to its faster execution of lifecycle scripts.

To install OpenCode globally, run the following command:

bun install -g opencode-ai

If you prefer other environments, you can use:

• npm: npm install -g opencode-ai
• Homebrew: brew install opencode-ai
• Scoop: scoop install opencode

Verify the installation by checking the version:

opencode --version

Step 2: Configuring Antigravity Auth

Antigravity Auth is a plugin that facilitates Google OAuth connections, allowing OpenCode to tap into enterprise-level model pools. To enable this, you need to modify the OpenCode configuration file. On most Unix-based systems, this is located at ~/.config/opencode/opencode.json.

Open the file with your preferred editor:

vim ~/.config/opencode/opencode.json

Inject the following plugin definition:

{
  "plugin": ["opencode-antigravity-auth@beta"]
}

This tells OpenCode to load the Antigravity module upon startup. This module is specifically designed to handle the complex handshake required for Google-based authentication, which is the gateway to Gemini 3 and other experimental models.

Step 3: The Authentication Handshake

Once the plugin is registered, you need to link your account. OpenCode provides a seamless CLI-to-Browser flow for this. Execute the login command:

opencode auth login

You will be presented with a list of providers. Use the arrow keys to navigate and select Google, then choose the specific method:

> OAuth with Google (Antigravity)

Your default web browser will open a secure Google sign-in page. After authorizing the application, the CLI will receive a callback. You should see a success message indicating Multi-account setup complete. This token-based authentication is secure and allows OpenCode to sign requests on your behalf.

Step 4: Model Selection and Advanced Configuration

With authentication out of the way, you need to define which models you want to use. The power of this setup is the ability to switch between Claude 4.5 for complex architectural reasoning and Gemini 3 for large-context window tasks (like reading an entire repository).

Update your opencode.json with the full model block provided in the official Antigravity documentation. This block maps the internal OpenCode IDs to the specific endpoints provided by the auth plugin.

To launch the interactive environment, simply type:

opencode

Inside the OpenCode interface, use the shortcut Ctrl + P. This opens the command palette. Navigate to Switch model and select Claude Opus 4.5 (Antigravity). You are now running one of the world's most powerful LLMs directly in your terminal.

Why Developers Move to n1n.ai for Scaling

While the "free" method via Antigravity is excellent for hobbyist projects and testing, it often suffers from instability, frequent re-authentication requirements, and strict rate limiting. For developers building commercial software or teams requiring 99.9% uptime, relying on community plugins can be risky.

This is where n1n.ai becomes essential. As a premier LLM API aggregator, n1n.ai provides:

1. Unified API: Access Claude, GPT, and Gemini through a single endpoint.
2. High Throughput: No more "Rate limit exceeded" messages during critical deployments.
3. Low Latency: Optimized routing ensures that your AI agent responds in milliseconds, not seconds.

Comparison: Free vs. Professional Access

Pro Tips for OpenCode Power Users

• Context Optimization: When using Gemini 3, take advantage of the 2M+ token context window. You can feed OpenCode your entire documentation folder to get hyper-relevant code suggestions.
• Security: Never share your opencode.json file. It contains sensitive session tokens that could allow unauthorized access to your cloud accounts.
• Fallback Logic: Configure OpenCode to fallback to a local model (via Ollama) if the cloud provider is unreachable. This ensures you can keep coding even when offline.

Conclusion

Setting up OpenCode with Antigravity Auth is a fantastic way to explore the capabilities of next-generation models like Claude 4.5 and Gemini 3. By following the steps outlined above—installing the CLI, configuring the plugin, and completing the OAuth handshake—you can significantly upgrade your development workflow without immediate cost.

However, as your needs grow, remember that stability and speed are the true currencies of productivity. For a more robust and professional experience, consider integrating your tools with a reliable provider.

Get a free API key at n1n.ai.