Stockfish 19 is the current release of Stockfish, the strongest chess engine there is and the top engine on the CCRL 40/15 list. It is also the engine you meet most on Chessy: the Stockfish bots, the coach, hints, the evaluation bar and the game review all run on it.
It runs inside your browser tab, compiled to WebAssembly: the official stockfish.js build of version 19, unmodified. There are two builds, multi-threaded and single-threaded. The multi-threaded one needs SharedArrayBuffer, which needs special security headers; where a page cannot have them, Chessy uses the single-threaded build. On the play page it uses up to half of your device's cores, at most four, and in the Arena the two engines share them equally.
The download is about 95 MB, most of it the neural network. Your browser keeps it after the first time, so later visits only check whether it has changed. On devices that report less than 4 GB of memory, Chessy uses the much smaller Stockfish 18 Lite instead, unless you choose Stockfish 19 in your profile.
At full strength it is far beyond any human. When you play it at a chosen rating, it weakens itself with Stockfish's own UCI_Elo setting, which runs from 1320 to 3190, so it will not play below about 1320 whatever you ask. Its ratings here are borrowed from published lists, not measured on Chessy, and a browser runs it more slowly than the native program those lists test.
What makes it distinctive
- ▸The engine behind Chessy's Stockfish bots, coach, hints, evaluation bar and game review
- ▸Multi-threaded where your browser allows it, with a single-threaded build as the fallback
- ▸About a 95 MB download, kept on your device after the first time
- ▸Has a strength floor — it will not play below a rating of 1320, whatever you request