A legal reckoning is brewing in the world of open source chess engines. Stockfish, a GPL-licensed chess engine widely recognized as one of the strongest in the world, has filed a lawsuit against ChessBase. The German-based company makes and sells chess software that relies heavily on the Stockfish engine, maintains a prominent chess news site, and runs a chess server for online games.

Stockfish’s announcement, published this week on International Chess Day, claims that ChessBase has violated the GPL by not releasing the corresponding modifications of its products that are derivative works:

We have come to realize that ChessBase concealed from their customers Stockfish as the true origin of key parts of their products. 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.

In 2020, Stockfish added support for NNUE (Efficiently Updatable Neural Networks). ChessBase’s Fat Fritz 2 product includes a neural network that the company has not released. Stockfish’s previous statement on Fat Fritz 2 identifies these net weights as a derivative:

“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,” current Stockfish maintainer Joost VandeVondele said. “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.”

The GPLv3 permits ChessBase to sell its chess engine but requires the company to make its modifications available, along with all information needed to build the program. Stockfish informed Albert Silver, author of the neural net in Fat Fritz 2, of the license violation, resulting in ChessBase releasing its C++ sources but not the net weights. “Obviously, we condemn the approach taken,” VandeVondele said.

Stockfish contributors have been working with a certified copyright and media law attorney in Germany to enforce their license and were able to force a recall of the Fat Fritz 2 DVD and the termination of the sales of Houdini 6. They are now pursuing the Termination clause of the GPL that would shut down ChessBase’s ability to distribute Stockfish in its products.

“Due to Chessbase’s repeated license violations, leading developers of Stockfish have terminated their GPL license with ChessBase permanently,” the Stockfish team said in the most recent statement. “However, ChessBase is ignoring the fact that they no longer have the right to distribute Stockfish, modified or unmodified, as part of their products.”

In a post titled, “Fat Fritz 2 is a rip-off,” published earlier this year, the Stockfish, Leela Chess Zero, and Lichess teams called out the product as a Stockfish clone, repackaged with a different neural network and “minimal changes that are neither innovative nor appear to make the engine stronger.”

“It is sad to see claims of innovation where there has been none, and claims of improvement in an engine that is weaker than its open-source origins,” the teams wrote. “It is also sad to see people appropriating the open-source work and effort of others and claiming it as their own.” 

Lichess, a free and open-source Internet chess server run by a non-profit organization that also uses Stockfish as a critical part of its infrastructure, has published multiple posts in support of Stockfish revoking ChessBase’s license to sell derivatives of the popular engine. Lichess also publishes the source code of everything they create using Stockfish so its users can see, modify, and redistribute it.

Even if you’re not a connoisseur of chess drama, Lichess’ most recent statement of support for Stockfish identifies why this case is important to the greater open source community:

Free open-source software offers essential freedoms that benefit developers and users alike, and those freedoms should have been extended to users of Fat Fritz 1, 2, and Houdini. Failing that, free-software licenses are only meaningful if they are enforced, making this an important case not only for Stockfish, but also for the open source community as a whole. We are happy that the Stockfish developers have the will and means to take action.

Stockfish’s lawsuit may become an important landmark case for proving that the GPL can be enforced. It will also be interesting to see whether the courts regard the neural network weights that ChessBase trained as a derivative work that must be released as source code in order to be in compliance with the GPL.

Stockfish has gained broad support from the project’s maintainers and developers who have stated they “have the evidence, the financial means, and the determination to bring this lawsuit to a successful end.” The team has promised to update their statement once the case makes progress.