The agreement between Chessbase and Stockfish was announced on the Stockfish website on Saturday and confirmed as accurate to Chess.com by Matthias Wullenweber, owner and chief programmer of Chessbase.
The two parties involved in the litigation, which appeared before the District Court in Munich, Germany, have agreed that Chessbase, longterm, can continue distributing products as long as the company ensures compliance with the GPL-3.0 license terms and adequately informs the public about the use of the Stockfish software in its products.
The terms of the agreement include that Chessbase refrains from distributing and/or making publicly available the Stockfish software for a period of one year. After one year, Chessbase can exercise the license to the Stockfish software again, under the conditions of the GPL-3.0. In case the terms are breached, Chessbase will have to pay an appropriate amount to the Free Software Foundation Europe eV
For the purpose of informing the public, Chessbase will place a clearly visible notice on all of its websites relating to Fat Fritz 2 and/or Houdini 6, containing a text referring to the GPL-3.0 license terms and the settlement. Also, they will internally introduce the role of a “Free Software Compliance Officer” and list under the domain foss.chessbase.com the products that contain the Stockfish software under the GPL-3.0 or other Free and Open Source Software (FOSS). If the respective licenses require the provision of the source code, Chessbase will enable a download of the source code by anyone from there.
In case Chessbase will continue distributing products based on Stockfish after one year, their advertisements will have to refer to the use of Stockfish, and comparisons of playing strength with Stockfish or other chess engines shall be truthful and provable (eg by reference to appropriate current rankings ) and shall refer to current versions.
Also, neural networks offered by Chessbase for use with Stockfish that are included in the compilation or dynamically loaded at runtime to initialize the data structures and logic of the software must be subject to GPL-3.0 or a compatible license.
In July 2021, the team behind Stockfish sued Chessbase for selling products based on Stockfish, an open-source chess engine, without attaching the license text and/or making available the complete corresponding source code. As the copyright owners, leading developers of Stockfish had terminated the GPL license, and without a license, Chessbase infringed copyright by continuing to distribute the products—hence the lawsuit.
Earlier that year, the teams behind Stockfish and another open-source chess program, Leela Chess Zero, had 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.
In a public hearing on July 4, 2022, in Munich, the judge confirmed that it was indeed possible for the Stockfish team to terminate the GPL license. As German law stipulates, the judge nevertheless requested the two parties to try and work on a settlement. If they had failed to come to an agreement, a new court session was scheduled for March 2023.
Joost VandeVondele, the current maintainer of Stockfish, wrote that the settlement “strengthens the Stockfish project in its aim to deliver the world’s number one chess engine as a Free Software and that allows ChessBase to distribute our software in the future.” He explained the reason for re-instating the license a year from now as follows:
“We feel that this is in the spirit of Free Software and to the benefit of the community. ChessBase has now acknowledged the value and potential of open source software, and Stockfish in particular, and has made a clear commitment to respecting the Free Software principles. ”