Open-source credits

chess42.xyz stands on free and open-source software written by people who gave their work to everyone. This page credits the chess tools that run in your browser, names their licences, and says where their source code is.

Stockfish

The chess engine behind analysis and the Stockfish opponent. It runs in your browser, not on the server, and is shipped exactly as released, unmodified.

Version: stockfish.js 10.0.2, a WebAssembly build of multi-variant Stockfish.
Copyright: T. Romstad, M. Costalba, J. Kiiski, G. Linscott and other contributors. Multi-variant support by Daniel Dugovic and contributors. Compiled to JavaScript and WebAssembly by Niklas Fiekas.
Licence: GNU General Public License, version 3. The full text is served beside the engine at chess42.xyz/stockfish/COPYING.txt.
Source code: the exact source this build was compiled from, with its build script, is at github.com/niklasf/stockfish.js/tree/e105072e84cf8ee5dd5219e2c5be29c3b8bf8a5a

chess42.xyz talks to Stockfish only through UCI, the standard text protocol chess programs use to talk to engines. Stockfish is a separate program; the rest of chess42.xyz is not part of it.

Maia-3

The engine that plays like a human of a chosen rating. It was created by the Computational Social Science Lab at the University of Toronto and runs in your browser.

Model: Maia3-5M, from the paper "Chessformer: A Unified Architecture for Chess Modeling" (ICLR 2026).
Licence: GNU Affero General Public License, version 3.
Upstream source: github.com/CSSLab/maia3, with the weights at huggingface.co/UofTCSSLab/Maia3-5M

ONNX Runtime Web

Runs the Maia model in your browser.

Copyright: Microsoft Corporation.
Licence: MIT.
Source code: github.com/microsoft/onnxruntime

chess.js

Checks that every move is legal and reads and writes chess notation.

Licence: BSD 2-Clause.
Source code: github.com/jhlywa/chess.js

Licence texts

MIT (ONNX Runtime Web). Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

BSD 2-Clause (chess.js). Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.