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Copyleft-licensed chess engine wins legal case against proprietary counterpart


Copyleft ensures that Free Software remains Free. Stockfish filed a lawsuit when ChessBase distributed parts of Stockfish work under a proprietary license, violating GNU GPL obligations. Checkmate! Copyleft won. Stockfish secured numerous concessions from ChessBase to respect the GPL.

Chess board with a white horse pawn in between a black horse and a black tower.

The players

Stockfish is is a Free Software chess engine licensed under the GNU General Public License version 3 (GPL-3.0). It was created in 2004 and, through its strong community support, is now the strongest chess engine available to the public. ChessBase is a German software company that develops and sells proprietary chess software.

In 2021, Stockfish filed a lawsuit against ChessBase GmbH, alleging that ChessBase had distributed to customers software products under proprietary licenses despite them being derivative works of Stockfish. The most notable derivatives were the Fat Fritz 2 and Houdini 6 software distributed by ChessBase. By doing so, Stockfish alleged that ChessBase has violated central obligations of the GPL-3.0, which ensures that users of the software are informed of their rights. Despite leading developers with Stockfish terminating their licensing of Stockfish to ChessBase, ChessBase continued to distribute a number of Stockfish derivatives.

This suit is notable as it is among the first of its type to involve a permanent termination of a Free Software license to a specific party. Additionally, Stockfish did not seek damages or other forms of financial compensation in their lawsuit, but rather pressed the court only for judicial actions that would result in the enforcement of the GPL-3.0.

Copyleft licenses protect Free Software

Having a copyleft license means that the derivative and/or combined work has to be licensed under the same license as the original work. As a result, this ensures that a copylefted piece of Free Software can remain free, and it is difficult to transform it into proprietary software.


On 7 November 2022, a settlement was reached between Tord Romstad and Stéphane Nicolet (leading developers of Stockfish), and ChessBase in the District Court of Munich I (Case No. 42 0 9765/21). Under this settlement, ChessBase will no longer be able to distribute Stockfish for a year, whether in its original or modified form. Nevertheless, licensing of the rights to Stockfish under the GPL v3 to ChessBase will resume after this year, provided that distribution of Stockfish or its derivatives in this time must comply fully with the license conditions of the GPL-3.0, which includes informing all its users that their use of Stockfish in their products will be subject to the GPL-3.0. To this end, ChessBase can seek certification from the Software Freedom Conservancy, a non-profit organization based in New York that works for software freedom, that their software is in compliance with the GPL-3.0.

To prevent any future violations, ChessBase will also create the position of a Free Software Compliance Officer and maintain a domain (foss.chessbase.com) that lists any of their products that contain Free Software.

Additionally, in case of future violations ChessBase is required to pay a contractual penalty to the FSFE for each breach of Stockfish’s initial request for ChessBase to cease its violations of the GPL-3.0. Under the terms of the settlement, this amount will be determined at Stockfish’s discretion.

It is heartening to see the enforcement in this settlement of the GPL-3.0 and copyleft principles, and we hope that this contributes to a climate where the freedoms of Free Software are respected by businesses and upheld by the courts. We thank the developers behind Stockfish for their great work and for sticking to the Community Enforcement principles