Chess Trainer is licensed under the GNU General Public License, version 3 or later. The complete corresponding source code is published at https://github.com/minchopm/chess-trainer. You may use, study, modify and redistribute it under those terms.
Why GPLv3
The app includes Stockfish — the open-source chess engine that does the searching, the evaluating and, through the coaching layer, the explaining. Stockfish is licensed under the GNU General Public License version 3, and the GPL is a copyleft licence: a work that includes GPL-licensed code and is distributed must be distributed under the same terms.
Stockfish is compiled into and linked with the application. So the whole application carries GPLv3. This is not a footnote we discovered late; it is a condition of using the best chess engine in the world, and it is a fair one.
Two things follow, and both are worth knowing before you buy anything.
You get the source. Not a marketing gesture — a licence obligation, and your right. The repository holds the iOS app, the Swift rules engine, the training logic, the puzzle-mining pipeline and this website.
There will never be advertising in it. A proprietary advertising SDK compiled into the same binary would make the combined work undistributable under the GPL. Selling the app is fine — the GPL has never forbidden charging — but bolting a closed SDK onto it is not.
Stockfish
- Copyright © 2004–2026 The Stockfish developers (see the project’s
AUTHORSfile) - Licensed under the GNU General Public License version 3
- Upstream: github.com/official-stockfish/Stockfish
- Modifications: none. The engine source is vendored unchanged. The only added file is a small C interface over its public
Engineclass, written for this project and licensed under GPLv3 along with the rest.
The two NNUE evaluation files are produced by the Stockfish project and distributed under the same licence. They are not committed to the repository; a script fetches the exact versions the bundled engine expects.
Chess Trainer is not affiliated with or endorsed by the Stockfish project. Stockfish is used under the terms of its licence, and it is named here — prominently, on its own page — because that is what those terms and ordinary decency both ask for.
Lichess
Most of the bundled tactics puzzles come from the Lichess puzzle database, and the games behind Guess the Elo come from the Lichess game archives. Both are released into the public domain under CC0 1.0.
No attribution is required by that licence. It is given here because it is deserved: those 14,185 puzzles carry ratings calibrated against millions of real solving attempts, which is something no amount of local computation could produce.
Chess Trainer is not affiliated with or endorsed by Lichess. The bundled games keep the two players’ ratings and moves, and not their usernames.
Every component
| Component | What it does | Licence |
|---|---|---|
| Stockfish The Stockfish developers, 2004–2026 | The chess engine. Search, evaluation and the NNUE networks — the whole coach. | GNU GPL v3 |
| Stockfish NNUE networks The Stockfish project | The two neural network evaluation files the bundled engine expects. | GNU GPL v3 |
| Lichess puzzle database Lichess | 14,185 of the bundled tactics puzzles, with human-calibrated ratings and themes. | CC0 1.0 (public domain) |
| Lichess game archives Lichess | 1,624 rated games, for Guess the Elo. Usernames are not kept. | CC0 1.0 (public domain) |
| chess.js Jeff Hlywa | Used by the project’s build tooling only. It is not in the iOS app. | BSD 2-Clause |
| GameKit Apple Inc. | Apple’s framework, used for online matchmaking and to carry moves. | Apple SDK terms |
| Three.js three.js authors | This website only — the title sequence on the front page. | MIT |
| Angular Google LLC and contributors | This website only. | MIT |
Getting the source
The complete corresponding source for the version you are running is at https://github.com/minchopm/chess-trainer, tagged by release. If you would rather have it another way — a tarball, a physical medium, a copy of a specific tag — write to mincho.milev@gmail.com and you will get it. That offer is part of the licence, not a courtesy.
Names and marks
“Chess Trainer”, the wordmark and the app icon are ours and are not covered by the GPL. You may redistribute modified versions of the software — that is the point of the licence — but not under our name or marks in a way that suggests they came from us.
Apple, App Store, iPhone, iPad and Game Center are trademarks of Apple Inc. This app is not affiliated with, sponsored by or endorsed by Apple.
See also: Privacy Policy · Terms of Service · How the engine is used