LMArena

platform

closed(hosted arena.ai, proprietary live leaderboard + private vote data)

source

partial(FastChat is a real arena implementation but stale relative to the operating platform

reference-code

FastChat Apache-2.0 (powers Chatbot Arena but stale, last release Feb 2024, 39.5k stars)

aux-OSS

Arena-Hard-Auto Apache-2.0 (automatic LLM eval, highest correlation to LMArena)

Scored as the hosted platform: the live evaluation product, its vote stream and the current ranking pipeline are all closed, while a substantial open-source heritage (FastChat) and an open automatic-eval companion (Arena-Hard-Auto) sit alongside them. That open code is stale relative to the platform actually running - FastChat's newest published release is still v0.2.36 - so what is public is a real arena implementation, just not the one operating. This is not an open core with functionality withheld for a paid tier; it is an open periphery around a runtime that never ships, which makes the source only partial and the product source-available rather than closed. The platform's canonical domain is now arena.ai, with lmarena.ai redirecting to it, and the ranking pipeline behind it still does not ship.

• https://github.com/lm-sys/FastChat recorded 2026-06-04

  Apache-2.0; 'FastChat powers Chatbot Arena (lmarena.ai), serving over 10M chat requests for 70+ LLMs'; latest release v0.2.36 Feb 2024

• https://github.com/lmarena/arena-hard-auto recorded 2026-06-04

  Apache-2.0 automatic LLM eval tool, highest correlation to LMArena

• https://arena.ai/ recorded 2026-06-04

  hosted 'Official AI Ranking & LLM Leaderboard' with Battle Mode; conversations disclosed to providers (proprietary platform)

• https://raw.githubusercontent.com/lm-sys/FastChat/main/README.md recorded 2026-08-13

  README still opens 'FastChat powers Chatbot Arena (lmarena.ai), serving over 10 million chat requests for 70+ LLMs' and documents launching an arena instance locally. Apache-2.0. The repo's newest release is v0.2.36, published 2024-02-11, so the published arena implementation is more than two years behind the operating platform: a real implementation, not the running one.

• https://arena.ai/ recorded 2026-08-13

  Still the hosted ranking and battle platform, and now the canonical domain - lmarena.ai 301-redirects here. It offers no repository, no self-hostable ranking pipeline and no license to the evaluation code, and states that conversations are disclosed to the relevant AI providers.