FlexiChess

Credits & licenses

FlexiChess is built on the work of others. Game data and core software below are used with permission or under their open-source licenses, with thanks.

Game data

  • Lumbra's Gigabase — Reference game database © Michael Jansen. Used under a commercial license; base collection released under CC BY-NC-SA 4.0.
  • The Week in Chess (TWIC) — Current tournament games compiled by Mark Crowther, with thanks for keeping the chess world's games flowing.
  • Lichess — Opening explorer, endgame tablebase, live broadcast relays, and player games imported at your request, via the Lichess API. Broadcast games are shown with attribution to Lichess and the event organizers, and remain subject to Lichess's terms and the organizers' rights.
  • chess-openings (ECO) — The ECO codes and opening names FlexiChess labels positions with come from the lichess-org/chess-openings dataset, released under the Creative Commons CC0 dedication.
  • Chess.com — Player games imported at your request via the Chess.com Published-Data API. Game data is © Chess.com and remains subject to Chess.com's terms.
  • Lichess puzzle database — Supplied training puzzles for Leakfinder drills until that deck source was retired in August 2026. Released under the Creative Commons CC0 dedication.

Software

  • Stockfish — Chess engine analysis. Licensed under the GNU General Public License v3.
  • chess.js — Move generation and validation. BSD 2-Clause license.
  • ts-fsrs — Spaced-repetition scheduling for repertoire training. MIT license.
  • cozy-chess — Move generation inside the WebAssembly module that reads your PGN files. MIT license — full text, with the other Rust crates compiled into that module, at /wasm-third-party-licenses.txt.

Piece graphics

  • Cburnett pieces — Piece graphics by Colin M.L. Burnett, released on Wikimedia Commons under GFDL, CC BY-SA 3.0, BSD and GPLv2+ with the choice left to whoever uses them. FlexiChess elects the BSD licence, which asks only that this copyright notice travel with the artwork. The shapes are served as SVG files, redrawn in some of the app's own icons, and embedded in its stylesheet; this notice covers all three.
  • Merida pieces — Piece graphics by Armando Hernández Marroquín, used under the GNU GPL v2 or later. A copy of the GPL v2 ships beside the artwork at /piece/merida/GPL-2.0.txt.

The web app is also built with open-source npm packages — React, Next.js and around 150 others. Their copyright notices and license texts are reproduced in full at /third-party-licenses.txt, generated from the shipped dependency list on every build.

The WebAssembly module that reads your PGN files is compiled from Rust, and the crates statically linked into it — cozy-chess, wasm-bindgen, serde and 34 more — carry their own MIT and Apache-2.0 notices. The build strips the symbol table out of that binary, so those notices are reproduced at /wasm-third-party-licenses.txt, derived from the crate graph cargo actually links.

GPL components & source

FlexiChess serves GPL-licensed software to your browser, where it runs locally. The largest component is the Stockfish engine (the multi-threaded stockfish-18-lite WebAssembly build from the Stockfish.js v18.0.0 release by Chess.com, LLC, based on official Stockfish). It runs as a separate program in a Web Worker, communicating with the app only over the UCI text protocol. It is used unmodified.

The complete corresponding source, at the exact shipped version, is available at nmrugg/stockfish.js (release v18.0.0). A copy of the GNU GPL v3 ships with the engine at /stockfish/GPL-3.0.txt. The Merida piece graphics above are GPL-v2+ artwork, served as standalone SVG files that are their own source; a copy of the GNU GPL v2 ships alongside them at /piece/merida/GPL-2.0.txt. The Cburnett set is not a GPL component here: its artist offers it under four licences at the user’s election and FlexiChess elects BSD, which the credit above satisfies. FlexiChess will provide the corresponding source for the GPL components on request — support@flexichess.com. Nothing else FlexiChess serves is GPL-licensed: the board is an in-house implementation (Chessground, the Lichess board library, was removed on August 16, 2026), and the WebAssembly module that reads your PGN files was rebuilt the same day without the GPL chess libraries it previously used (shakmaty v0.30.0 and pgn-reader v0.29.0, both GPL-3.0-or-later). If you used FlexiChess before that date you received those builds; corresponding source for them remains available upstream and on request.

FlexiChess’s own application code is proprietary. GPL components keep their own licenses; the attribution and source links here stay regardless of how the build evolves. Trademarks and game data remain the property of their respective owners. FlexiChess is not affiliated with or endorsed by these projects.

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