Summary

Add Google Antigravity OAuth support to openab-agent, enabling free access to Gemini 3, Claude, and GPT-OSS models without API keys.

Motivation

Currently openab-agent supports:

• OpenAI Codex — ChatGPT subscription OAuth (chatgpt.com/backend-api)
• Anthropic — API key only (ANTHROPIC_API_KEY)

Google Antigravity offers free access to multiple models (Gemini 3, Claude Sonnet 4.5, GPT-OSS 120B) via Google account OAuth. This would give users a zero-cost option for running openab-agent.

Prior Art

Pi coding agent supports Antigravity via npx @mariozechner/pi-ai login google-antigravity:

• Source: https://github.com/earendil-works/pi/blob/main/packages/ai/src/utils/oauth
• OAuth flow uses Google account login
• Token is used against Antigravity's backend API (acts as a gateway/proxy to multiple LLM providers)
• Free tier with rate limits (~5h refresh cycle, weekly ceiling)

Proposed Approach

1. Implement Antigravity OAuth device flow in openab-agent auth
2. Add an Antigravity provider in llm.rs that routes requests through Antigravity's API
3. Add Antigravity models to the static model list in acp.rs
4. Users select provider via OPENAB_AGENT_PROVIDER=antigravity

Discussion

Thread: 1513356640072499221