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 closed, while substantial OSS heritage (FastChat) and an OSS automatic-eval companion (Arena-Hard-Auto) exist. The OSS code is stale relative to the operating platform, which is what makes this source_available rather than closed: FastChat is a real arena implementation, just not the one running. Class corrected from open_core on 2026-07-30. In this ladder open_core means an OSI core with functionality withheld for a paid tier, and there is no open core here, only an open periphery. A repo that exists while the runtime does not ship is source:partial, which the ladder scores 2/source_available. The score itself was already 2.

• 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)