0xkoji

Posted on Jan 13 • Edited on Jan 26

How to Use Claude Opus 4.5 & Gemini 3 for Free with OpenCode

#ai #opencode #llm #programming

🧵 TL;DR (Too Long; Didn’t Read)
Want to use powerful AI models like Claude Opus 4.5 and Gemini 3 for free? This guide walks you through setting up OpenCode with the Antigravity plugin and Google OAuth. You’ll be up and running with top-tier models in minutes—no paid API keys required.

🚀 Getting Started with OpenCode + Antigravity Auth

OpenCode is an open-source AI coding agent that lets you connect to models like Claude, GPT, Gemini, and more. It comes with free models out of the box, or you can bring your own from any provider.

In this guide, I’ll walk you through setting up OpenCode with Antigravity Auth using Google OAuth.

🛠 Step 1: Install OpenCode

You can install OpenCode using various package managers like Homebrew, npm, pnpm, yarn, scoop, choco, etc.

For this guide, I’ll use bun:

bun install -g opencode-ai

More install options: Installation Docs

⚙️ Step 2: Configure the Plugin

We’ll use opencode-antigravity-auth plugin. Follow the installation instructions in the repo. I’ll go with Option B here:

vim ~/.config/opencode/opencode.json

Add the plugin:

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

🔐 Step 3: Authenticate with Google (Antigravity)

Make sure you have a Google account that supports Antigravity.

Run:

opencode auth login

You’ll see a provider list like this:

> Google
  OpenAI
  Anthropic
  GitHub Copilot
  ...

Select Google, then choose:

> OAuth with Google (Antigravity)

Your browser will open—sign in with your Google account. Once complete, you should see:

•  Multi-account setup complete (1 account(s)).
o  Login successful
—  Done

🧩 Step 4: Update Your Config

Now let’s update the full config file to include model definitions. Open:

vim ~/.config/opencode/opencode.json

And paste the full configuration block from the plugin’s documentation. (You can keep the version you shared above—no changes needed unless you want to customize models.)

{
  "$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"] }
        }
      }
    }
  }
}

🧠 Step 5: Select Claude Opus 4.5

Launch OpenCode:

opencode

Then press Ctrl + P and choose:

Switch model → Claude Opus 4.5 (Antigravity)

That’s it! You’re now ready to use OpenCode with Antigravity-authenticated models. Happy coding! 🌱

Top comments (5)

ominiverdi • Jan 29 • Edited

Building on opencode-antigravity-auth, there are now two companion plugins that extend its capabilities:

opencode-antigravity-web (npmjs.com/package/opencode-antigra...) - Web search and URL reading

Google Search with source citations
URL content extraction
Dynamic model selection (auto-picks best available model)
Smart fallback when models hit capacity

opencode-antigravity-img (npmjs.com/package/opencode-antigra...) - Image generation

Gemini 3 Pro Image model
Multiple aspect ratios and resolutions (up to 4K)
Quota tracking

Setup is simple - just add them alongside the auth plugin:
{
plugins: [
opencode-antigravity-auth,
opencode-antigravity-web,
opencode-antigravity-img
]
}

All three work together using the same Google OAuth authentication.

Ellis • Mar 11

Is this safe? Google banned a lot of accounts

0xkoji • Mar 12

there will be a risk to be banned by Google. but so far i haven't had any issue.
i guess recent bans are that users use for openclaw or something similar.