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
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, FEN and PGN parsing. Its unminified JavaScript source is self-hosted with its BSD-2-Clause licence. No runtime CDN is required.
Local image recognition
Fenshot 0.1.5, pinned source by SORTINO LABS S.R.L. supplies the 2D tile classifier and detector. Its unmodified ONNX weights and bundled pure functions are self-hosted with the MIT licence. The detector also credits tensorflow_chessbot / Sameer Ansari (MIT).
ONNX Runtime Web 1.26.0 runs the model in a local worker using single-threaded WASM. Its MIT licence and third-party notices are included. Exact provenance, build details and limitations · Asset sizes and checksums.
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.