Licences & corresponding source

Everything this site runs on in your browser is open-source software. The chess engine is distributed unmodified; its corresponding source is available from the upstream repositories linked below.

Stockfish

Licence: GNU General Public License, version 3 — full text
Copyright: The Stockfish developers (see AUTHORS)
Distributed files: stockfish-18-lite.js / .wasm and stockfish-18-lite-single.js / .wasm — the lite builds from the npm package stockfish 18.0.5, unmodified. The small NNUE evaluation network is embedded in the WASM binary.
Corresponding source: nmrugg/stockfish.js (the WebAssembly build) · official-stockfish/Stockfish (upstream engine)

Stockfish is used only for hints, analysis and play against the engine. Whether your solution is correct is decided by the composer's published solution, not by the engine.

Popeye

Licence: GNU General Public License, version 2 — full text, served with the program
Copyright: Thomas Maeder, Stephen Emmerson, Norbert Geissler, Torsten Linss, Elmar Bartel, Joshua Green, Mu-Tsun Tsai; begun in 1983 by Philippe Schnoebelen
Distributed files: popeye/py.js and popeye/py.wasm — Popeye 4.103 (tag v4.103) compiled to WebAssembly with Emscripten. The C sources are unmodified; every change made to build it is in popeye-4.103-emscripten.patch, which touches only the makefile and the build scripts. Built with Emscripten 6.0.9, make -f makefile.unx TOOLCHAIN=emcc.
Corresponding source: thomas-maeder/popeye, tag v4.103, plus the patch above

Popeye is the chess problem solving program. It runs in your browser on the page that builds a problem, to work out the solution to a position you have set up yourself. It is not used anywhere else on the site.

Chess piece images

Asset: The standard SVG chess pieces, as shipped with react-chessboard
Author: Colin M.L. Burnett (Cburnett)
Licence: Creative Commons Attribution-ShareAlike 3.0 Unported
Source: Wikimedia Commons · used as-is

chess.js

Version: 1.4.0
Licence: BSD 2-Clause License
Copyright: © 2025, Jeff Hlywa
Source: jhlywa/chess.js (licence text in the repository)

react-chessboard

Version: 4.7.3
Licence: MIT License
Copyright: © 2022 Ryan Gregory
Source: Clariity/react-chessboard

Chess_diagram_to_FEN (the diagram reader's models)

Licence: MIT License
Copyright: © 2024 Jost Triller
What is served: ocr/*.onnx.part* — the chess models (board outline, image rotation, piece recognition), converted to ONNX with the weights quantised to 8 bits, compressed and split into parts; nothing retrained
Source: tsoj/Chess_diagram_to_FEN (licence text in the repository)

Used only to read a position off a photo of a diagram — the Photo button on the diagram generator, and the Diagram to FEN page. It runs in your browser; the photo is not sent anywhere.

ONNX Runtime Web

Version: 1.30.0
Licence: MIT License
Copyright: © Microsoft Corporation
Source: microsoft/onnxruntime

OpenCV.js

Version: 4.12.0 (the @techstark/opencv-js build)
Licence: Apache License 2.0
Copyright: © OpenCV team and contributors
Source: opencv/opencv · TechStark/opencv-js

React

Version: 19.2
Licence: MIT License
Source: facebook/react

Tailwind CSS

Version: 4.2
Licence: MIT License
Source: tailwindlabs/tailwindcss

Fonts

Baloo 2, Inter and IBM Plex Mono are served by Google Fonts under the SIL Open Font License 1.1. They are not redistributed by this site.

Problem data

Positions, stipulations, solutions and bibliographic data come from YACPDB. The problems themselves are the work of their composers, who are named on every problem page once it has been solved.

This site's own code

The site's own code is © 2026 Ushiyutvj, all rights reserved. It is not covered by any of the licences above. The source can be read on GitHub for reference; copying it or using it in another site or product needs the author's permission.