Source Code · Chess Best Move

The License

Chess Best Move is free software. You may run it, study how it works, change it and share it, changed or unchanged, under the terms of the GNU General Public License version 3. It comes without any warranty, to the extent permitted by law.

The app includes the Stockfish chess engine, which is licensed under GPLv3, and is distributed as a whole under the same license. The Terms of Use explain how the App Store license and GPLv3 fit together: nothing in them restricts your rights under GPLv3.

One additional term applies, as GPLv3 section 7(e) permits: Motomatic LLC does not grant rights under trademark law to use the name “Chess Best Move” or the app icon. See section 4 of the Terms of Use.

Where to Get the Source

github.com/ciscoriordan/chess-best-move

Each App Store version of Chess Best Move has its own tag in the repository, so you can get the exact source of the version on your device. A tag is named v followed by the version number shown in the app under Settings > About > Version, without the build number in parentheses: for example, v1.0 for “1.0 (3)”. Tags are never removed, so the source of older versions stays available.

In the app, Settings > About > Licenses links to the source code of that version, and Settings > About > Chess engine links to it again, together with the one change the app makes to Stockfish. Both links are built from the version the app is running, so each copy of the app points at its own source. The App Store description links to this page.

What the Repository Contains

The repository holds the source needed to build the app: the app’s Swift code, its Swift packages for chess rules, the engine (with the Stockfish 19 source) and board recognition, the XcodeGen file (project.yml) that generates the Xcode project, and the build scripts. The piece recognition model is included as its Core ML model package (PieceClassifier.mlpackage) and the compiled model the app loads, and Stockfish’s network as the file nn-1a298aa575a0.nnue. The README lists the tools and the steps to build the app with Xcode on a Mac.

Development material that is not needed to build the app is left out, including the program and the images used to train the recognition model. The README describes exactly what the repository contains.

The recognition model was trained on rendered board images, and some of the board and piece art used to render them is published under licenses that ask for credit. NOTICE.md in the repository names those sets, their authors and their licenses. None of that art is in the repository or in the app: only the trained model ships.

Stockfish

Chess Best Move analyzes positions with Stockfish 19 (release tag sf_19), a free and open-source chess engine by the Stockfish developers.

Stockfish is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Stockfish is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

Modification

Chess Best Move includes a small modification of Stockfish 19. On September 13, 2026, src/shm.h was changed so that an app embedding Stockfish can turn off its cross-process shared-memory store for the neural network. That store creates files in /tmp, a socket server thread and an exit handler. With the store turned off, the network is kept in ordinary app memory, which is Stockfish’s existing fallback. Without the switch, the source behaves exactly like the release. The change is documented in the repository.

The Neural Network

Stockfish’s neural network (nn-1a298aa575a0.nnue) is trained on data provided by the Leela Chess Zero project, which is made available under the Open Database License (ODbL).

Fonts

The app and this website use the following typefaces under the SIL Open Font License 1.1.

Noto Sans Symbols 2: Copyright 2022 The Noto Project Authors. The app’s chess piece characters. License text
Oleo Script Bold: Copyright 2012 Soytutype, with Reserved Font Name “Oleo Script”. The source typeface for the image wordmark shared by the app and this website. License text
GFS Didot: Copyright 2007 Greek Font Society, with Reserved Font Name “GFS Didot”. The headings on this website. License text

The app bundles the font files unmodified. Its wordmark is a rasterized image drawn from Oleo Script Bold; interface text uses the system typefaces, and Noto Sans Symbols 2 supplies the chess pieces. This website serves GFS Didot from this address rather than from a font network, so that visiting a page here sends nothing to anyone else, and the file it serves is the released font compressed to WOFF2 and changed in no other way: “GFS Didot” is a Reserved Font Name, which a changed copy may not carry. The rest of the text on this website is set in the reader’s own system typefaces, which is what the app does too.