docs/opencode.md

Using with OpenCode

OpenCode is a terminal-based AI coding assistant similar to Claude Code. You can configure it to use this proxy, which unlocks both Claude and Gemini models in a single interactive session.

The Problem with Claude Code

The proxy exposes both Claude and Gemini models via /v1/models. However, Claude Code's /model picker is hardcoded by Anthropic to only show Claude variants. Gemini models are effectively invisible in the UI.

The only workaround within Claude Code is hardcoding a single Gemini model in settings.json — no interactive switching, no mid-session changes.

What OpenCode Solves

OpenCode supports custom providers via a config file. Once pointed at this proxy, all models (Claude and Gemini) appear in one interactive picker and can be switched freely at any time.

Feature	Claude Code	OpenCode (with this proxy)
Claude models	yes	yes
Gemini models	hardcoded only	interactive picker
Switch models mid-session	no	yes
Multi-account load balancing	yes (via proxy)	yes (via proxy)

Prerequisites

• OpenCode installed (npm install -g opencode-ai@latest)
• Antigravity Claude Proxy running on port 8080
• At least one Google account linked to the proxy

Global Configuration

mkdir -p ~/.config/opencode

cat > ~/.config/opencode/opencode.json <<'EOF'
{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "antigravity": {
      "npm": "@ai-sdk/anthropic",
      "name": "Antigravity Local",
      "options": {
        "baseURL": "http://localhost:8080/v1",
        "apiKey": "{env:ANTHROPIC_API_KEY}"
      },
      "models": {
        "claude-sonnet-4-6": { "id": "claude-sonnet-4-6", "name": "Claude Sonnet 4.6" },
        "claude-opus-4-6-thinking": { "id": "claude-opus-4-6-thinking", "name": "Claude Opus 4.6 Thinking" },
        "gemini-2.5-pro": { "id": "gemini-2.5-pro", "name": "Gemini 2.5 Pro" },
        "gemini-2.5-flash": { "id": "gemini-2.5-flash", "name": "Gemini 2.5 Flash" },
        "gemini-2.5-flash-lite": { "id": "gemini-2.5-flash-lite", "name": "Gemini 2.5 Flash Lite" },
        "gemini-2.5-flash-thinking": { "id": "gemini-2.5-flash-thinking", "name": "Gemini 2.5 Flash Thinking" },
        "gemini-3-flash": { "id": "gemini-3-flash", "name": "Gemini 3 Flash" },
        "gemini-3-flash-agent": { "id": "gemini-3-flash-agent", "name": "Gemini 3 Flash Agent" },
        "gemini-pro-agent": { "id": "gemini-pro-agent", "name": "Gemini 3.1 Pro Agent" },
        "gemini-3.1-pro-low": { "id": "gemini-3.1-pro-low", "name": "Gemini 3.1 Pro Low" },
        "gemini-3.1-flash-lite": { "id": "gemini-3.1-flash-lite", "name": "Gemini 3.1 Flash Lite" },
        "gemini-3.1-flash-image": { "id": "gemini-3.1-flash-image", "name": "Gemini 3.1 Flash Image" },
        "gemini-3.5-flash-low": { "id": "gemini-3.5-flash-low", "name": "Gemini 3.5 Flash" }
      }
    }
  },
  "model": "antigravity/claude-sonnet-4-6",
  "small_model": "antigravity/gemini-3.5-flash-low"
}
EOF

This config is global and applies automatically every time you run opencode from any folder.

Shell Environment

grep -q 'ANTHROPIC_API_KEY=' ~/.zshrc || echo 'export ANTHROPIC_API_KEY="dummy"' >> ~/.zshrc
grep -q 'ANTHROPIC_BASE_URL=' ~/.zshrc || echo 'export ANTHROPIC_BASE_URL="http://localhost:8080/v1"' >> ~/.zshrc
source ~/.zshrc

The ANTHROPIC_API_KEY value can be anything — the proxy does not validate it. Authentication is handled via your linked Google accounts.

Start Both Services

# Terminal 1: Start the proxy
antigravity-claude-proxy start

# Terminal 2: Open OpenCode from any project folder
opencode

Switching Models

Inside OpenCode, all registered models appear as antigravity/<model-id>. The small_model field handles background operations — setting it to a Gemini Flash model conserves your Claude quota, the same way ANTHROPIC_DEFAULT_HAIKU_MODEL works in Claude Code.

Available Models

Model ID	Type	Best For
claude-sonnet-4-6	Claude	Coding, refactoring
claude-opus-4-6-thinking	Claude	Architecture, deep reasoning
gemini-2.5-pro	Gemini	Long context, analysis
gemini-2.5-flash	Gemini	Fast general tasks
gemini-2.5-flash-lite	Gemini	Background tasks
gemini-2.5-flash-thinking	Gemini	Reasoning tasks
gemini-3-flash	Gemini	Fast, lightweight
gemini-3-flash-agent	Gemini	Agentic workflows
gemini-pro-agent	Gemini	Agentic workflows
gemini-3.1-pro-low	Gemini	Strong reasoning, 1M context
gemini-3.1-flash-lite	Gemini	Lightweight tasks
gemini-3.1-flash-image	Gemini	Vision/image tasks
gemini-3.5-flash-low	Gemini	Fast tasks, small_model recommended

Verify Configuration

curl "http://localhost:8080/v1/messages" 
  -H "content-type: application/json" 
  -H "anthropic-version: 2023-06-01" 
  -H "x-api-key: dummy" 
  -d '{"model": "claude-sonnet-4-6", "max_tokens": 100, "messages": [{"role":"user","content":"say hi"}]}'

Troubleshooting

Endpoint POST /messages not found

Your baseURL is missing /v1. Set it to http://localhost:8080/v1, not http://localhost:8080.

Model returns 400 INVALID_ARGUMENT

The model is rejected by Google's backend. Remove it from your models block.

Models not appearing in picker

1. Check the config is valid JSON: cat ~/.config/opencode/opencode.json
2. Check the env var is set: echo $ANTHROPIC_API_KEY
3. Open a fresh terminal after config changes

Connection refused

acc status
curl http://localhost:8080/health

Further Reading

• OpenCode Documentation
• OpenCode Providers
• Proxy Load Balancing
• Proxy Configuration
• Available Models