Fat Fritz

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Unser Fritz [1]

Fat Fritz,
a commercial chess entity by ChessBase, the first version released in November 2019 featuring a set of custom made neural network weights that work in the open source project Leela Chess Zero within its Lc0 executable, while Fat Fritz 2.0 released in February 2021 is based on Stockfish 12 and custom NNUE technology.

Fat Fritz 1

Initially introduced as a cloud project in summer 2019, inspired by DeepMind's ground breaking AlphaZero approach combining Deep learning with Monte-Carlo Tree Search (MCTS) [2], and Albert Silver's Deus X experience, relaxing Leela Chess' "Zero" paradigm of pure reinforcement learning due to supervised learning, Fat Fritz was released in November 2019 as part the Fritz 17 package, best supported by a GPU card such as Nvidia GeForce 20 series. The project and its underlying techniques were emphasized by a talk of AlphaZero co-author Thore Graepel at ChessBase, attended by German computer chess icon Thomas Nitsche along with his sons and Math42 authors Raphael Nitsche and Maxim Nitsche [3]. Fat Fritz was further supported by Daniel Uranga [4] [5].


Lc0 and DeusX aka Fat Fritz components [6]

Fat Fritz 2

Fat Fritz 2, released on February 09, 2021, is based on NNUE technology and Stockfish 12 with a double sized network, and thus, running on a CPU, not requiring expensive GPU graphic cards for game playing like its predecessor. The network was trained by Albert Silver with the help of Daniel Uranga and Dietrich Kappe, who provided scripts and ideas. Further credits were given to the Stockfish contributors, and in particular Yu Nasu for his groundbreaking work on NNUE, and Hisayori Noda for the initial Stockfish NNUE implementation [7]. Using the original Fat Fritz as initial supervisor to evaluate chess positions, the learning of Fat Fritz 2 was reinforced by Stockfish's alpha-beta search [8]. The initial release had the 40 MiB NNUE file embedded inside the Fat Fritz 2 executable, and was soon separated after the intervention of the Stockfish community due to a possible GPL license violation [9]. While the separated none GPL licensed NNUE file had a replacement on GitHub [10], it was not identical with the commercial purchased network, yielding in weaker play [11]. The Stockfish community, denying Fat Fritz 2's originality as claimed by ChessBase, initially reacted with the release of Stockfish 13 on February 19, 2021 [12]. In summer 2021, the Stockfish team filed a lawsuit against ChessBase to enforce the consequences of the license termination concerning Fat Fritz 2 and the allegedly Stockfish derivative Houdini 6 sold by ChessBase [13].

See also

Forum Posts



Re: Fat Fritz Update by Albert Silver, CCC, December 12, 2020


Re: Fat Fritz 2 by gladius, CCC, February 14, 2021 » CCRL
Re: Fat Fratz 2.0 Copyleft Infringement by Joost VandeVondele, FishCooking, February 10, 2021

Blog Posts


External Links


ChessBase News




Fritz 17

Source Code

ff2 · DanielUranga/Stockfish@e44edde · GitHub
FF2 · DanielUranga/Stockfish@faef72a · GitHub
Stockfish/FatFritz2_v1.bin at ff2 · DanielUranga/Stockfish · GitHub

Rating Lists


  1. Malakov tower (build 1873) of Colliery Unser Fritz, Shaft 1, in Herne, North Rhine-Westphalia, part of The Industrial Heritage Trail of the Ruhr area. Photo by GeorgeIvan, May 15, 2014, Wikimedia Commons, Unser Fritz/Crange is further district of Herne (Wanne), famous for its funfair Cranger Kirmes, and SV Unser Fritz the local chess club - eponym of the coal mine was Frederick III, German Emperor
  2. David Silver, Thomas Hubert, Julian Schrittwieser, Ioannis Antonoglou, Matthew Lai, Arthur Guez, Marc Lanctot, Laurent Sifre, Dharshan Kumaran, Thore Graepel, Timothy Lillicrap, Karen Simonyan, Demis Hassabis (2017). Mastering Chess and Shogi by Self-Play with a General Reinforcement Learning Algorithm. arXiv:1712.01815
  3. Fat Fritz – What on Earth is that? by Albert Silver, ChessBase News, August 13, 2019
  4. Standing on the shoulders of giants by Albert Silver, ChessBase News, September 18, 2019
  5. GitHub - DanielUranga/lc0 at uci-wdl-reporting
  6. My failed attempt to change TCEC NN clone rules by Alexander Lyashuk, CCC, September 14, 2019 » TCEC
  7. Fat Fritz 2: The Best of Both Worlds by Albert Silver, ChessBase News, February 10, 2021
  8. Fat Fritz 2.0 - The new number 1, ChessBase News, February 09, 2021
  9. Statement on Fat Fritz 2 by Joost VandeVondele, Stockfish Blog, February 15, 2021
  10. Stockfish/FatFritz2_v1.bin at ff2 · DanielUranga/Stockfish · GitHub
  11. Re: Fat Fritz 2 by Stefan Pohl, CCC, February 13, 2021
  12. Stockfish 13, The Stockfish Team, February 19, 2021
  13. Our lawsuit against ChessBase, The Stockfish team, Stockfish Blog, July 20, 2021
  14. fatfritz.net
  15. An actual interesting computer chess read about FF2 by Dietrich Kappe, CCC, February 27, 2021
  16. Nice review fo Fat Fritz by Jouni Uski, CCC, November 22, 2019
  17. Fat Fritz 1.1 update and a small gift by Albert Silver. ChessBase News, March 05, 2020

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