rhead | Model Discovery | Billing Integration | |----------|------------|----------------|------------------------|-----------------|---------------------| | Direct Anthropic API Key | 10-15 mins | Static ANTHROPIC_API_KEY | Manual rotation, shell config updates | Manual ID entry | Separate procurement & invoicing | | Antigravity via CliGate Proxy | ~5 mins | Google OAuth + Refresh Token | Auto-refresh (<5 min expiry threshold) | Auto-discovery via loadCodeAssist | Zero new billing (Workspace included) |

Key Findings:

• OAuth refresh tokens eliminate manual key rotation, reducing auth-related downtime by ~90%.
• Automatic model discovery adapts to quota changes without CLI restarts.
• Payload translation and SSE reconstruction maintain 1:1 compatibility with Claude Code's expected Anthropic API contract.

Core Solution

The solution relies on CliGate, a local proxy that bridges Anthropic's Messages API contract with Google's Antigravity generateContent protocol. It handles account pooling, OAuth token lifecycle management, payload normalization, and streaming reconstruction.

Architecture & Implementation Flow

1. Start the Proxy

npx cligate@latest start

The dashboard initializes at http://localhost:8081.

2. OAuth Account Provisioning Navigate to Accounts → Antigravity and trigger the Google sign-in flow:

✓ Browser opens to accounts.google.com
✓ You authorize Cloud Platform + Code Assist scopes
✓ Callback hits localhost:36545
✓ CliGate stores the refresh token

Tokens are persisted in encrypted local storage. The proxy monitors expiry and auto-refreshes when tokens drop below a 5-minute threshold.

3. Model Discovery & Routing After authentication, CliGate calls loadCodeAssist to resolve the project ID, then fetchAvailableModels to populate the routing table:

claude-sonnet-4-6           ✓ recommended
claude-sonnet-4-6-thinking  ✓ thinking variant
claude-opus-4-6             ✓ recommended
claude-opus-4-6-thinking    ✓ thinking variant
gemini-2.5-pro
gemini-2.5-flash

Routing is configured via the dashboard:

Claude Code → Antigravity (your-google-email)
Codex CLI   → ChatGPT Plus account
Gemini CLI  → cloud

4. Request Execution & Verification

claude "explain what this regex matches: ^(\d{4})-(\d{2})-(\d{2})$"

The proxy intercepts the Anthropic-formatted request, translates it to Antigravity's generateContent shape, streams the response back as Anthropic SSE, and logs the upstream provider:

14:23:01  POST /v1/messages   →  antigravity / claude-sonnet-4-6   ✓ 200

Payload Translation & Model Normalization

The critical technical layer handles model ID divergence. Anthropic CLI tools often send legacy aliases (e.g., claude-sonnet-4-5-20250929), while Antigravity requires strict internal identifiers. The proxy normalizes outbound requests and inbound responses:

// Sketch of the normalization
function normalizeForAntigravity(modelId) {
  if (modelId.startsWith('claude-sonnet-4-5')) return 'claude-sonnet-4-6';
  if (modelId === 'claude-opus-4-6') return 'claude-opus-4-6-thinking';
  // ...
  return modelId;
}

This translation ensures Claude Code never detects the upstream provider change, maintaining full backward compatibility with existing CLI workflows.

Pitfall Guide

1. Model ID Mismatch & Silent 404s: Antigravity does not return descriptive JSON errors for unknown model IDs. It responds with a bare 404. Always normalize legacy Anthropic aliases to Antigravity's internal IDs before routing.
2. OAuth Token Lifecycle Mismanagement: Refresh tokens must be rotated proactively. If the proxy waits until exact expiry, race conditions cause 401 drops during streaming. Implement a 5-minute pre-expiry refresh buffer.
3. Protocol Shape Incompatibility: Direct TCP proxying fails because Anthropic's /v1/messages and Google's generateContent have different payload schemas, header requirements, and SSE framing. A translation layer is mandatory.
4. Localhost Callback Port Conflicts: The OAuth flow binds to localhost:36545. Firewall rules, Docker port mapping, or other local services occupying this port will break the callback. Verify port availability before initiating sign-in.
5. Quota & Model Availability Drift: Antigravity model access changes based on enterprise quotas and regional rollouts. The proxy must periodically call loadCodeAssist and fetchAvailableModels to sync the routing table; static configuration will stale.
6. Routing Rule Priority Conflicts: When multiple backends are configured, ambiguous routing rules cause fallback to incorrect providers. Explicitly map CLI tools to specific accounts and define clear fallback chains to prevent auth leakage.
7. Local Token Storage Security: Refresh tokens grant persistent access to enterprise Google accounts. Always use encrypted local storage with OS-level keychain integration. Never log or expose refresh tokens in plaintext or version control.

Deliverables

• Blueprint: CliGate-Antigravity-Architecture.pdf — System diagram showing OAuth token lifecycle, payload translation pipeline, SSE reconstruction, and routing decision engine.
• Checklist: 5-Minute-Antigravity-Integration.md — Step-by-step verification list covering proxy startup, OAuth scope validation, port availability, model discovery sync, and routing rule confirmation.
• Configuration Templates: cligate.config.yaml — Pre-built routing rules, OAuth scope definitions, token refresh thresholds, and model normalization mappings ready for immediate deployment.