Stockfish is an open-source project, sharing and freely distributing the code, tools and data needed to deliver this chess engine. We do this because we are convinced that open software and open data are key ingredients to make rapid progress, and thus for the benefit of our users. The recent progress in playing strength of Stockfish confirms the path taken. We proudly provide the tools for free chess analysis to millions of users, with countless titled players using Stockfish for their preparation. Be assured that free engines will serve you well!
Recently, ChessBase has started distribution and sales of the Fat Fritz 2 chess engine. This chess engine is a Stockfish derivative, with a few lines of code modification (engine name, authors list and a few parameters), and a new set of NNUE net weights considered proprietary. ChessBase’s communication on Fat Fritz 2, claiming originality where there is none, has shocked our community. Furthermore, the engine Fat Fritz 2 fails to convince on independent rating lists, casting doubt on the usefulness of those modifications. Indeed, we feel that customers buying Fat Fritz 2 get very little added value for money. Claims to the contrary appear misleading.
Selling Stockfish derivatives is possible with the GPLv3 license we grant, but not without requirements. In particular, the license states that if one redistributes a program derived from our work, the corresponding modifications of our sources and all information needed to build that program must be made available. Only after explicitly informing Albert Silver (the author of the net in Fat Fritz 2) of a license violation have matching C++ sources, but not the net weights, been made available. Obviously, we condemn the approach taken.
—Joost VandeVondele, current maintainer of the Stockfish project