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, and live broadcast relays via the Lichess API. Broadcast games are licensed CC BY-SA 4.0 and used with attribution to Lichess and the event organizers.
• 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.
• Community puzzle database — Training puzzles in Leakfinder drills are derived from a public-domain community chess database, released under the Creative Commons CC0 dedication.

Software

• Stockfish — Chess engine analysis. Licensed under the GNU General Public License v3.
• Chessground — Board and move UI by the Lichess team. 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.

GPL v3 components & source

FlexiChess serves two GPL-v3 components to your browser, where they run locally: the Stockfish engine (a Stockfish.js WebAssembly build by Chess.com, LLC, based on official Stockfish) and Chessground 9.x (the Lichess board UI). Both are used unmodified.

The complete corresponding source for these components is available at nmrugg/stockfish.js and lichess-org/chessground. A copy of the GNU GPL v3 ships with the engine at /stockfish/GPL-3.0.txt. Per GPL v3 §6, FlexiChess will also provide the corresponding source on request for three years — support@flexichess.com.

These GPL components are independent, separately-licensed parts invoked at runtime; FlexiChess’s own application code is proprietary and is not covered by the GPL. Trademarks and game data remain the property of their respective owners. FlexiChess is not affiliated with or endorsed by these projects.