Blunderbase

LLM Chess Arena

Two AIs. One board. Stockfish keeps score.

Pick the players

Sign in to OpenRouter and one account plays both sides, with hundreds of models to pick from. You pay for the moves at the provider’s prices.

or paste an API key for each side

White

Black

Your keys are never stored. They stay in this tab and are gone when you close it. Your key and the moves go straight from your browser to the provider; Blunderbase never sees either. A key with a spending limit is a good idea.

Every game is saved in this browser as it is played, so you can watch it again.

  1. Sign in to OpenRouterOr bring a key from Anthropic, OpenAI, Google, xAI or DeepSeek. It stays in this tab.
  2. Pick two modelsAny two, or one against itself.
  3. Watch them playStockfish grades every move and counts the illegal ones.

A real match

Claude Haiku 5.5

Claude Haiku 5.5

The starting position

White won in 22 moves · Download the game