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+# Antigravity Claude Proxy
+
+A proxy server that exposes an **Anthropic-compatible API** backed by **Antigravity's Cloud Code**, letting you use Claude models like `claude-sonnet-4-5` with any Anthropic client including Claude Code CLI.
+
+## How It Works
+
+```
+┌──────────────────┐     ┌─────────────────────┐     ┌────────────────────────────┐
+│   Claude Code    │────▶│  This Proxy Server  │────▶│  Antigravity Cloud Code    │
+│   (Anthropic     │     │  (Anthropic → Google│     │  (daily-cloudcode-pa.      │
+│    API format)   │     │   Generative AI)    │     │   sandbox.googleapis.com)  │
+└──────────────────┘     └─────────────────────┘     └────────────────────────────┘
+```
+
+1. Receives requests in **Anthropic Messages API format**
+2. Extracts OAuth token from Antigravity's local database
+3. Transforms to **Google Generative AI format** with Cloud Code wrapping
+4. Sends to Antigravity's Cloud Code API (`v1internal:streamGenerateContent`)
+5. Converts responses back to **Anthropic format**
+
+## Prerequisites
+
+- **Antigravity** installed and running (you must be logged in)
+- **Node.js** 18 or later
+
+## Quick Start
+
+### 1. Install Dependencies
+
+```bash
+npm install
+```
+
+### 2. Start the Proxy Server
+
+```bash
+npm start
+```
+
+The server runs on `http://localhost:8080` by default.
+
+### 3. Test It
+
+```bash
+# Health check
+curl http://localhost:8080/health
+
+# Simple message (non-streaming)
+curl http://localhost:8080/v1/messages \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "claude-sonnet-4-5",
+    "messages": [{"role": "user", "content": "Say hello!"}],
+    "max_tokens": 100
+  }'
+
+# Streaming
+curl http://localhost:8080/v1/messages \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "claude-sonnet-4-5",
+    "messages": [{"role": "user", "content": "Say hello!"}],
+    "max_tokens": 100,
+    "stream": true
+  }'
+```
+
+## Using with Claude Code CLI
+
+Configure Claude Code to use this proxy:
+
+```bash
+# Set the API base URL
+export ANTHROPIC_BASE_URL=http://localhost:8080
+
+# Use any API key (it's not actually used - auth comes from Antigravity)
+export ANTHROPIC_API_KEY=dummy-key
+
+# Run Claude Code
+claude
+```
+
+Or in your Claude Code config:
+
+```json
+{
+  "apiBaseUrl": "http://localhost:8080",
+  "apiKey": "dummy-key"
+}
+```
+
+## Available Models
+
+| Model ID | Description |
+|----------|-------------|
+| `claude-sonnet-4-5` | Claude Sonnet 4.5 via Antigravity |
+| `claude-sonnet-4-5-thinking` | Claude Sonnet 4.5 with extended thinking |
+| `claude-opus-4-5-thinking` | Claude Opus 4.5 with extended thinking |
+
+You can also use standard Anthropic model names - they'll be automatically mapped:
+- `claude-3-5-sonnet-20241022` → `claude-sonnet-4-5`
+- `claude-3-opus-20240229` → `claude-opus-4-5-thinking`
+
+## API Endpoints
+
+| Endpoint | Method | Description |
+|----------|--------|-------------|
+| `/health` | GET | Health check and token status |
+| `/v1/messages` | POST | Anthropic Messages API |
+| `/v1/models` | GET | List available models |
+| `/refresh-token` | POST | Force token refresh |
+
+## Files
+
+```
+src/
+├── index.js            # Entry point
+├── server.js           # Express server with Anthropic API endpoints
+├── cloudcode-client.js # Cloud Code API client with proper wrapping
+├── format-converter.js # Anthropic ↔ Google format conversion
+├── constants.js        # Endpoints, headers, model mappings
+└── token-extractor.js  # Extracts OAuth token from Antigravity
+```
+
+## Troubleshooting
+
+### "Could not extract token from Antigravity"
+
+Make sure:
+1. Antigravity app is running
+2. You're logged in to Antigravity
+
+### 401 Authentication Errors
+
+The token might have expired. Try:
+```bash
+curl -X POST http://localhost:8080/refresh-token
+```
+
+### Rate Limiting (429)
+
+Antigravity enforces rate limits on Cloud Code requests. Wait and retry, or switch to a different model.
+
+## Credits
+
+Based on insights from:
+- [opencode-antigravity-auth](https://github.com/NoeFabris/opencode-antigravity-auth) - Antigravity OAuth plugin for OpenCode
+- [claude-code-proxy](https://github.com/1rgs/claude-code-proxy) - Anthropic API proxy using LiteLLM
+
+## License
+
+MIT