Source & licences
stockfish.io is an independent chess interface powered by the open-source Stockfish engine. It is not the official Stockfish project and is not affiliated with the Stockfish developers.
Stockfish 19 / Stockfish.js
The optional browser engine is the unmodified lite single-threaded build of Stockfish.js v19.0.0, distributed under the GNU General Public License v3. The engine runs in a local Web Worker using WebAssembly.
Exact browser-build source and build instructions · Download corresponding source archive · Upstream Stockfish project
The JavaScript worker and WebAssembly engine are served by stockfish.io. The matching source archive is hosted by the upstream project.
Chess rules
chess.js 1.4.0 by Jeff Hlywa and contributors handles legal moves, FEN and PGN parsing. Its unminified JavaScript source is self-hosted with its BSD-2-Clause licence. No runtime CDN is required.
Interface
The interface is plain HTML, CSS, and JavaScript: board and UI, game state and validation, piece artwork, UCI integration, and styles. Pieces use original, self-hosted SVG artwork with consistent silhouettes on every platform.