Source & licenses
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
When the engine is installed, its matching source archive is also provided at stockfishjs-19-source.tar.gz. This optional download is installed together with the engine; see installation instructions.
Chess rules
chess.js 1.4.0 by Jeff Hlywa and contributors handles legal moves and FEN parsing. Its unminified JavaScript source is self-hosted with its BSD-2-Clause license. No runtime CDN is required.
Interface
The interface is plain HTML, CSS, and JavaScript: board and UI, UCI integration, and styles. Pieces use system text glyphs; their appearance can vary slightly by platform.