Antigravity Auth Quick Install: Configure Plugin in 5 Minutes

Antigravity Auth Quick Install lets you complete OpenCode plugin configuration in 5 minutes and start using Claude and Gemini 3 advanced models. This tutorial provides two installation methods (AI-assisted/manual), covering plugin installation, OAuth authentication, model definitions, and verification steps, ensuring you can get started quickly.

What You'll Learn

• ✅ Complete Antigravity Auth plugin installation in 5 minutes
• ✅ Configure access to Claude and Gemini 3 models
• ✅ Perform Google OAuth authentication and verify successful installation

Your Current Challenge

You want to try the powerful features of Antigravity Auth (Claude Opus 4.5, Sonnet 4.5, Gemini 3 Pro/Flash), but don't know how to install the plugin or configure models, worried that one wrong step will leave you stuck.

When to Use This

• When using Antigravity Auth plugin for the first time
• When installing OpenCode on a new machine
• When you need to reconfigure the plugin

🎒 Prerequisites

Prerequisites Check

Before starting, please confirm:

• [ ] OpenCode CLI is installed (opencode command is available)
• [ ] You have an available Google account (for OAuth authentication)
• [ ] You understand Antigravity Auth basic concepts (read What is Antigravity Auth?)

Core Approach

The Antigravity Auth installation process consists of 4 steps:

1. Install Plugin → Enable the plugin in OpenCode configuration
2. OAuth Authentication → Log in with your Google account
3. Configure Models → Add Claude/Gemini model definitions
4. Verify Installation → Send a first request to test

Important Note: The configuration file path is ~/.config/opencode/opencode.json on all systems (on Windows, ~ automatically resolves to the user directory, such as C:\Users\YourName).

Follow Along

Step 1: Choose Installation Method

Antigravity Auth provides two installation methods. Choose either one.

Recommended Method

If you use an LLM Agent (such as Claude Code, Cursor, or OpenCode), AI-assisted installation is recommended for a faster and smoother experience.

Method 1: AI-Assisted Installation (Recommended)

Simply copy the following prompt and paste it to any LLM Agent:

Install opencode-antigravity-auth plugin and add Antigravity model definitions to ~/.config/opencode/opencode.json by following: https://raw.githubusercontent.com/NoeFabris/opencode-antigravity-auth/dev/README.md

The AI will automatically:

• Edit ~/.config/opencode/opencode.json
• Add plugin configuration
• Add complete model definitions
• Execute opencode auth login for authentication

You should see: The AI outputs "plugin installed successfully" or a similar message.

Method 2: Manual Installation

If you prefer manual control, follow these steps:

Step 1.1: Add Plugin to Configuration File

Edit ~/.config/opencode/opencode.json (create the file if it doesn't exist):

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

Beta Version: If you want to experience the latest features, use opencode-antigravity-auth@beta instead of @latest.

You should see: The configuration file contains a plugin field with an array value.

---

Step 2: Perform Google OAuth Authentication

Run in your terminal:

opencode auth login

The system will automatically:

1. Start a local OAuth server (listening on localhost:51121)
2. Open a browser and navigate to the Google authorization page
3. Receive the OAuth callback and exchange tokens
4. Automatically retrieve the Google Cloud project ID

You need to:

1. Click "Allow" in the browser to authorize access
2. If you're in WSL or Docker environment, you may need to manually copy the callback URL

You should see:

✅ Authentication successful
✅ Account added: [email protected]
✅ Project ID resolved: cloud-project-id-xxx

Multi-Account Support

Need to add more accounts to increase quota? Run opencode auth login again. The plugin supports up to 10 accounts and automatically rotates load balancing.

---

Step 3: Configure Model Definitions

Copy the following complete configuration and append it to ~/.config/opencode/opencode.json (be careful not to overwrite the existing plugin field):

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["opencode-antigravity-auth@latest"],
  "provider": {
    "google": {
      "models": {
        "antigravity-gemini-3-pro": {
          "name": "Gemini 3 Pro (Antigravity)",
          "limit": { "context": 1048576, "output": 65535 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] },
          "variants": {
            "low": { "thinkingLevel": "low" },
            "high": { "thinkingLevel": "high" }
          }
        },
        "antigravity-gemini-3-flash": {
          "name": "Gemini 3 Flash (Antigravity)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] },
          "variants": {
            "minimal": { "thinkingLevel": "minimal" },
            "low": { "thinkingLevel": "low" },
            "medium": { "thinkingLevel": "medium" },
            "high": { "thinkingLevel": "high" }
          }
        },
        "antigravity-claude-sonnet-4-5": {
          "name": "Claude Sonnet 4.5 (Antigravity)",
          "limit": { "context": 200000, "output": 64000 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] }
        },
        "antigravity-claude-sonnet-4-5-thinking": {
          "name": "Claude Sonnet 4.5 Thinking (Antigravity)",
          "limit": { "context": 200000, "output": 64000 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] },
          "variants": {
            "low": { "thinkingConfig": { "thinkingBudget": 8192 } },
            "max": { "thinkingConfig": { "thinkingBudget": 32768 } }
          }
        },
        "antigravity-claude-opus-4-5-thinking": {
          "name": "Claude Opus 4.5 Thinking (Antigravity)",
          "limit": { "context": 200000, "output": 64000 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] },
          "variants": {
            "low": { "thinkingConfig": { "thinkingBudget": 8192 } },
            "max": { "thinkingConfig": { "thinkingBudget": 32768 } }
          }
        },
        "gemini-2.5-flash": {
          "name": "Gemini 2.5 Flash (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] }
        },
        "gemini-2.5-pro": {
          "name": "Gemini 2.5 Pro (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] }
        },
        "gemini-3-flash-preview": {
          "name": "Gemini 3 Flash Preview (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] }
        },
        "gemini-3-pro-preview": {
          "name": "Gemini 3 Pro Preview (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65535 },
          "modalities": { "input": ["text", "image", "pdf"], "output": ["text"] }
        }
      }
    }
  }
}

Model Classification

• Antigravity Quota (Claude + Gemini 3): antigravity-gemini-*, antigravity-claude-*
• Gemini CLI Quota (Separate): gemini-2.5-*, gemini-3-*-preview

For more model configuration details, refer to Complete List of Available Models.

You should see: The configuration file contains the complete provider.google.models definition, and the JSON format is valid (no syntax errors).

---

Step 4: Verify Installation

Run the following command to test if the plugin is working correctly:

opencode run "Hello" --model=google/antigravity-claude-sonnet-4-5-thinking --variant=max

You should see:

Currently using: google/antigravity-claude-sonnet-4-5-thinking (max)
...

Claude: Hello! I'm Claude Sonnet 4.5 Thinking.

Checkpoint ✅

If you see the AI responding normally, congratulations! The Antigravity Auth plugin has been successfully installed and configured.

---

Common Pitfalls

Issue 1: OAuth Authentication Failure

Symptoms: After running opencode auth login, an error message appears, such as invalid_grant or the authorization page cannot be opened.

Causes: Google account password changed, security events, or incomplete callback URL.

Solutions:

1. Check if the browser correctly opens the Google authorization page
2. If in WSL/Docker environment, manually copy the callback URL displayed in the terminal to the browser
3. Delete ~/.config/opencode/antigravity-accounts.json and re-authenticate

Issue 2: Model Not Found (400 Error)

Symptoms: When running a request, 400 Unknown name 'xxx' is returned.

Causes: Model name spelling error or configuration file format issue.

Solutions:

1. Check if the --model parameter matches the key in the configuration file exactly (case-sensitive)
2. Verify that opencode.json is valid JSON (use cat ~/.config/opencode/opencode.json | jq to check)
3. Confirm that the corresponding model definition exists under the provider.google.models field

Issue 3: Incorrect Configuration File Path

Symptoms: Prompt "configuration file does not exist" or modifications have no effect.

Causes: Using the wrong path on different systems.

Solutions: Use ~/.config/opencode/opencode.json on all systems, including Windows (where ~ automatically resolves to the user directory).

System	Correct Path	Incorrect Path
macOS/Linux	~/.config/opencode/opencode.json	/usr/local/etc/...
Windows	C:\Users\YourName\.config\opencode\opencode.json	%APPDATA%\opencode\...

Summary

In this lesson, we completed:

1. ✅ Two installation methods (AI-assisted / manual configuration)
2. ✅ Google OAuth authentication process
3. ✅ Complete model configuration (Claude + Gemini 3)
4. ✅ Installation verification and common issue troubleshooting

Key Takeaways:

• Unified configuration file path: ~/.config/opencode/opencode.json
• OAuth authentication automatically retrieves Project ID, no manual configuration needed
• Supports multiple accounts to increase quota limits
• Use the variant parameter to control thinking depth for Thinking models

Next Lesson Preview

In the next lesson, we'll learn First Authentication: Deep Dive into OAuth 2.0 PKCE Flow.

You'll learn:

• How OAuth 2.0 PKCE works
• Token refresh mechanism
• Project ID automatic resolution process
• Account storage format

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Appendix: Source Code Reference

Click to expand source code locations

Last updated: 2026-01-23

Feature	File Path	Lines
OAuth Authorization URL Generation	src/antigravity/oauth.ts	91-113
PKCE Key Pair Generation	src/antigravity/oauth.ts	1-2
Token Exchange	src/antigravity/oauth.ts	201-270
Project ID Automatic Retrieval	src/antigravity/oauth.ts	131-196
User Info Retrieval	src/antigravity/oauth.ts	231-242

Key Constants:

• ANTIGRAVITY_CLIENT_ID: OAuth client ID (used for Google authentication)
• ANTIGRAVITY_REDIRECT_URI: OAuth callback address (fixed to http://localhost:51121/oauth-callback)
• ANTIGRAVITY_SCOPES: List of OAuth permission scopes

Key Functions:

• authorizeAntigravity(): Build OAuth authorization URL, including PKCE challenge
• exchangeAntigravity(): Exchange authorization code for access token and refresh token
• fetchProjectID(): Automatically resolve Google Cloud project ID
• encodeState() / decodeState(): Encode/decode OAuth state parameters (including PKCE verifier)