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Documentation Index

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LMArena Bridge is a self-hosted FastAPI server that lets you interact with LMArena models — including experimental stealth models — using the OpenAI API format. Point any OpenAI-compatible client, library, or UI at http://localhost:8000/api/v1 and it works without any code changes.

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What it does

The bridge sits between your client and LMArena’s internal API. It handles the hard parts automatically:
  • Anti-bot challenges: Cloudflare Turnstile, reCAPTCHA v3, and reCAPTCHA v2 are solved in the background using real browser automation
  • Auth token management: Rotates through multiple arena-auth-prod-v1 tokens in round-robin, refreshing expired ones automatically
  • Streaming: Returns Server-Sent Events in OpenAI format — compatible with any SSE-capable client
  • Image uploads: Accepts base64-encoded images, uploads them to LMArena R2 storage, and includes signed URLs in the request

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Architecture

Your client (curl / OpenWebUI / SDK)
        │
        │  POST /api/v1/chat/completions
        ▼
  LMArena Bridge (FastAPI, port 8000)
        │
        ├── Direct httpx  ──────────────────────► LMArena API
        ├── Chrome fetch (Playwright)  ──────────► LMArena API
        ├── Camoufox fetch (Firefox stealth)  ───► LMArena API
        └── Userscript proxy (browser tab)  ─────► LMArena API
The bridge automatically selects the best transport based on the model and current challenge state.

Quickstart

Install and make your first API call in minutes.

Authentication

Get your LMArena token and configure API keys.

API Reference

Full endpoint documentation.

Deployment

Run in production with nginx and systemd.
LMArena actively deploys Cloudflare checkpoints, captchas, and reCAPTCHA Enterprise protections. The bridge handles these automatically, but reliability may vary as LMArena updates its defenses.

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