The Stockfish project strongly believes in free and open-source software and data. Collaboration is what made this engine the strongest chess engine in the world. We license our software using the GNU General Public License, Version 3 (GPL) with the intent to guarantee all chess enthusiasts the freedom to use, share and change all versions of the program.
Unfortunately, not everybody shares this vision of openness. We have come to realize that ChessBase concealed from their customers Stockfish as the true origin of key parts of their products (see also earlier blog posts by us and the joint Lichess, Leela Chess Zero, and Stockfish teams). Indeed, few customers know they obtained a modified version of Stockfish when they paid for Fat Fritz 2 or Houdini 6 - both Stockfish derivatives - and they thus have good reason to be upset. ChessBase repeatedly violated central obligations of the GPL, which ensures that the user of the software is informed of their rights. These rights are explicit in the license and include access to the corresponding sources, and the right to reproduce, modify and distribute GPLed programs royalty-free.