The Stockfish team has filed a lawsuit against Chessbase, claiming that the German chess software company is selling their software without having the license to do so. The lawsuit, filed in a German court, was announced last Tuesday in a blog on the Stockfish website.

The Stockfish team argues that Chessbase has “concealed from their customers Stockfish as the true origin of key parts of their products,” namely Fat Fritz 2 and Houdini 6. According to the Stockfish team, Chessbase has “repeatedly violated central obligations of the GPL,” which refers to informing software users 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,” the Stockfish team writes.

In February, Chess.com reported extensively on the feud between the open-source chess community and Chessbase. Back then, the teams behind Stockfish and another open-source chess program, Leela Chess Zero, claimed that the engine in Fat Fritz 2 is Stockfish with minimal changes, that Fat Fritz 2 violated the GNU General Public License under which Stockfish was released, and that Chessbase’s marketing has made false claims about Fat Fritz 2’s playing strength.

Over the past four months, the Stockfish team, together with a certified copyright and media law attorney in Germany, has continued to point its arrows toward Chessbase. In their blog of this week, the developers claim that they managed to recall the Fat Fritz 2 DVD and ensure the termination of sales of Houdini 6.

While Houdini 6 currently has the status “unavailable” in the Chessbase webshop, the sale of Fat Fritz 2 was discontinued in March 2021. However, at the moment their webshop has a product called Fat Fritz 2.0 SE. One of the Stockfish developers told Chess.com that this is an altered version that contains a different binary, GPL license text, and some sources, and it’s not clear if it’s now GPL-compliant.

Fat Fritz 2.0 SE
The old Fat Fritz 2.0 image (left) and the new Fat Fritz 2.0 SE image. Source: Chessbase.

Despite these changes made by Chessbase, the lawsuit has been filed. The reason is that the Stockfish team has terminated permanently their GPL license with Chessbase, but the group of developers claim that the Hamburg-based company still distributes Stockfish, modified or unmodified, as part of their products. They argue that Chessbase is now selling software without a license, which is a copyright violation.

“We believe we have the evidence, the financial means, and the determination to bring this lawsuit to a successful end,” the Stockfish team writes.

Albert Silver, the main developer behind Fat Fritz 2, wrote in an email to Chess.com:

Even if I wanted to, I could not comment on something I know nothing about. The first I learned anything about this subject was via that same link, and it is still all I know.

Chessbase co-founder Matthias Wullenweber wrote:

We still haven’t got the actual filed papers so we cannot comment. It is probably different from how Stockfish describes it because that would be refutable.

One can righteously criticize the articles about Fat Fritz on Chessbase.com which constituted an unacceptable belittlement of Stockfish. The reverse belittlement of Albert’s innovations by SF does not balance this, even if they now seem to follow his path with SF 14. We behaved insensitively and we apologized for it. However, the legal side felt always OK and normal. E.g. advertisements and the packaging have clearly stated since February that Fat Fritz 2 is based on the open-source engine Stockfish, usually as the first claim/bullet point.

So at the moment, we expect to win, but this will not diminish our respect for the work behind Stockfish.


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