anthropic-proxy

Reverse proxy for the Anthropic Messages API that authenticates with a Claude subscription (OAuth) instead of an API key. Lets you use tools like OpenCode through your existing Claude Pro/Team plan.

How it works

Clients send standard Anthropic API requests to the proxy. The proxy authenticates upstream using OAuth credentials from your Claude subscription, forwards the request, and streams the response back. Requests are optionally sanitized (tool name remapping, string replacement) before forwarding and de-sanitized on return.

Features

• OAuth credential management — reuses ~/.claude/.credentials.json, auto-refreshes tokens
• Request sanitization — rename tools, replace strings in system prompts and body (configurable, hot-reloadable)
• Rate limit tracking — polls Anthropic usage API and reads response headers to track 5h/7d utilization windows
• OpenTelemetry metrics — request counts, latency, token usage, errors (optional OTLP export)
• Structured logging — zerolog with file rotation via lumberjack

Quick start

cp config.example.yaml config.yaml
# edit config.yaml — set api_keys and optionally claude_binary

go build -o anthropic-proxy .
./anthropic-proxy

On first run, if no credentials exist at ~/.claude/.credentials.json, an OAuth login flow starts in your browser. If running headlessly, the authorization URL is printed to stdout. If you've already logged in with Claude Code CLI, the proxy reuses those credentials.

Nix

nix develop   # dev shell with Go
nix build     # build the binary

Client configuration

Point any Anthropic-compatible client at the proxy:

export ANTHROPIC_API_KEY=your-proxy-api-key
export ANTHROPIC_BASE_URL=http://localhost:8082

Endpoints

Method	Path	Description
POST	/v1/messages	Anthropic Messages API (proxied)
POST	/messages	Same, without /v1 prefix
GET	/healthz	Health check
POST	/reload	Hot-reload config (sanitize rules + API keys)

Configuration

See config.example.yaml for all options. Key sections:

• api_keys — keys clients use to authenticate with the proxy
• sanitize — tool renames, system prompt replacements, body replacements
• telemetry — OTLP endpoint, service name, auth headers
• logging — level, file path, rotation settings
• claude_binary — path to claude CLI for request fingerprinting (optional)