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)

Chatbot Arena runs as the hosted arena.ai platform, where the live ranking pipeline and vote data are closed. FastChat, its original Apache-2.0 implementation, has had no release since February 2024, and Arena-Hard-Auto is a separate Apache-2.0 automatic-eval companion, so the open code is real but is not the pipeline that runs the platform today.

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

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

• https://arena.ai/ recorded 2026-09-29

  Hosted ranking and battle platform titled "Arena AI: The Official AI Ranking" that invites visitors to compare and vote on models. Searched the body for open source, repository, self-host, license and FastChat: none, so the page offers no repository or license for the ranking pipeline.

• 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://raw.githubusercontent.com/lm-sys/FastChat/main/LICENSE recorded 2026-09-29

  The LICENSE file is the Apache License, "Version 2.0, January 2004", so the FastChat code is Apache-2.0.

• https://raw.githubusercontent.com/lm-sys/FastChat/main/pyproject.toml recorded 2026-09-29

  pyproject.toml declares version = "0.2.36" and the classifier "License :: OSI Approved :: Apache Software License".

• https://raw.githubusercontent.com/lm-sys/FastChat/main/README.md recorded 2026-09-29

  README says "FastChat powers Chatbot Arena" and "Currently, Chatbot Arena is powered by FastChat. Here is how you can launch an instance of Chatbot Arena locally." with the launch command for the side-by-side battle UI.

• https://raw.githubusercontent.com/lmarena/arena-hard-auto/main/LICENSE recorded 2026-09-29

  The LICENSE file is the Apache License, "Version 2.0, January 2004", so Arena-Hard-Auto is Apache-2.0.

Chatbot Arena's reach is taken from the chat-request volume its FastChat README reports for the hosted platform. That figure predates the current arena.ai platform and has not been updated, so it is a dated, conservative measure rather than current usage.

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

  hosted live leaderboard / battle platform in active operation June 2026

• https://arena.ai/ recorded 2026-09-29

  Hosted leaderboard platform titled "Arena AI: The Official AI Ranking" in active operation. Searched the body for a user count, traffic or request figure: none published on the page.

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

  'serving over 10 million chat requests for 70+ LLMs'

• https://raw.githubusercontent.com/lm-sys/FastChat/main/README.md recorded 2026-09-29

  README states "serving over 10 million chat requests for 70+ LLMs" and "Chatbot Arena has collected over 1.5M human votes from side-by-side LLM battles".

The hosted arena runs blind pairwise human-preference battles across many models, and Arena-Hard-Auto adds an automatic-judge companion built from hard real-world queries and creative-writing prompts. The evaluation code is largely closed and hosted, and it covers agentic and task-execution work less than harnesses built around running tasks.

• https://arena.ai/ recorded 2026-09-29

  Hosted platform titled "Arena AI: The Official AI Ranking" whose description reads "Join the community shaping the public leaderboard" through real-world evaluation.

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

  500 SWE/math queries + 250 creative prompts, GPT-4.1/Gemini-2.5 judges, highest correlation to LMArena

• https://raw.githubusercontent.com/lmarena/arena-hard-auto/main/README.md recorded 2026-09-29

  V2.0 contains "500 fresh, challenging real-world user queries" and "250 creative writing queries sourced from Chatbot Arena"; judges are "GPT-4.1 and Gemini-2.5".