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General Feb 22, 2021 | 9:32 AMby Colin McGourty

ChessBase’s Fat Fritz 2 condemned as ‘rip-off’ Stockfish clone

Stockfish and Leela Chess Zero developers have joined together with Lichess to condemn what they see as false advertising by ChessBase of the recently released Fat Fritz 2 chess software. The promotional material suggests the chess engine included with the software is “the new number 1”, and that the key contribution is Albert Silver’s in training its neural network. The Stockfish developers, meanwhile, point out that Fat Fritz 2, which is being sold for around 100 euros, is the free Stockfish with a net that adds no noticeable increase in playing strength.


Stockfish, which powers evaluations here on chess24, is the current king of the computer chess world, playing at a level that’s literally inhuman. It’s also a free, open-source engine. How can a commercial chess engine compete? Well, that’s where Fat Fritz 2 comes in. The software, which includes Stockfish, now complies with the Stockfish licensing requirements (after the developers say they pointed out a violation) so is perfectly legal to sell.

False advertising?

What has enraged the Stockfish developers, however, is the perceived false advertising. Albert Silver’s contribution was to train a different version of the neural network that works within the latest version of Stockfish, but the promotional material around the release puts all the emphasis on that work rather than Stockfish itself. To add insult to injury, the claims of a leap in strength of Fat Fritz 2 relied on comparing it to Stockfish 12, but most if not all of the difference could be explained away by the fact that Fat Fritz 2 was built from a later developer version of Stockfish with a higher rating.

The good news for chess fans is that that advertising trick led to the release of Stockfish 13 ahead of schedule, restoring a situation where pure Stockfish is ahead of or very close to Fat Fritz 2 in rating, depending on which list you check. You can download the latest version 13 here.

A computer chess revolution

An article at ChessBase announcing the new software begins, “Fat Fritz 2.0 is the successor to the revolutionary Fat Fritz, which was based on the famous AlphaZero algorithms.” There have been revolutionary changes in chess computing in the last few years, but Fat Fritz certainly wasn’t the driving force behind them. DeepMind’s AlphaZero shook the chess world in late 2017 by showing that its reinforcement learning algorithm needed just four hours of playing against itself to go from zero knowledge to a level superior to Stockfish.

That four hours relied on having the phenomenal computing power of Google to draw on, but the open-source Leela Chess Zero project showed that the same principles could work if you drew on the computing power of an army of volunteers over a much longer period of time. Fat Fritz 1.0 was not based on a new adaptation of AlphaZero but on taking the Leela Chess Zero code and tools and training a network differently.

The drawback of Leela Chess Zero is that to run it well requires a high-powered graphics card, but a new development, NNUE (Efficiently Updatable Neural-Network-based evaluation functions), allowed CPUs to combine the speed of the traditional chess engine calculations with neural networks. Stockfish adopted that new technology to regain the lead in the computer engine race. 

Leela Chess Zero gave checkmate in the final game but Stockfish had won the 20th final of the TCEC with 5 games to spare

Then, as with Leela Chess Zero, Albert Silver pivoted to using the new strongest chess engine to “power” the latest version of Fat Fritz.

An open letter from Stockfish, LCZero and Lichess

The Stockfish, Leela Chess Zero and Lichess teams combined to pen the article Fat Fritz 2 is a rip off, which catalogues what they claim is a track record of deceptive claims by Albert Silver. It concludes:

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. It is also sad to see people appropriating the open-source work and effort of others and claiming it as their own.

Everyone is permitted and encouraged to modify and improve code from Stockfish/Leela while giving credit; that is the intent of open-source software. Everyone is allowed to copy Stockfish/Leela and sell them, provided the terms of the Stockfish/Leela license are met. But don’t pretend that the product being sold is something it isn’t.


The defence

One possible defence, or sales approach, for Fat Fritz 2 would be to point out that the strongest current chess engine (the free Stockfish) is bundled with a database, user interface and subscription to make a package that might be attractive, especially to newer, less technically aware chess aficionados. That wouldn’t allow you to hype it as a new invention that everyone needs to have, however.

ChessBase have now decided to double down on the original claims in a new article that obliquely addresses the criticism, beginning, “To say that Fat Fritz 2 has been making waves is an understatement”. “When you purchase Fat Fritz you are certainly not paying for the free Lc0 and Stockfish they come bundled with”, claims Albert Silver, while making a comparison between Stockfish as the “car” and his altered neural network as the “driver” that’s unlikely to make him any new friends in the open-source community.

On the Computer Chess Rating Lists website Fat Fritz 2 is currently still 2 points ahead of Stockfish 13 at the top of the BLITZ list (3718 to 3716), but behind on the 40/2 and the 40/15 lists - the latter is shown above 

The article focuses on the time and expense put in to training the network while playing down the importance of rating now that it’s become clear Fat Fritz 2 is behind or roughly equal to the latest Stockfish with its default neural network, instead claiming “a powerful and different perspective” based on training the network with Fat Fritz 1 rather than Stockfish evaluations.

The computer chess wars

Such qualitative claims are much harder to disprove, but the history of chess engines suggests they should always be treated with caution. When Rybka reigned supreme in the chess world its author Vasik Rajlich used to talk about how its intelligent search allowed it to operate at a lower depth than its rival engines. Later it was claimed Rybka had copied another engine’s code without attribution and manipulated the depth shown to hide the fact, with Rybka stripped of the four World Computer Chess Championship titles it won from 2007-2010.

Adapting existing engines has, until the appearance of AlphaZero, been an incomparably quicker means of creating a new and competitive engine, and with open-source software that approach is legitimate though sometimes ethically questionable if the resulting engine is then sold for profit. Houdini 6, another high-flying engine, is currently unable to compete in the TCEC computer championships after being accused of being a Stockfish clone.

In the case of Fat Fritz 2 the use of Stockfish isn’t hidden, though it is heavily downplayed in the ChessBase messaging. The arguments are sure to rumble on, with chess engine experts assessing the real added value or not of the new product. Caveat emptor, as they say – just how much stronger do you really need your 3500+ rated chess assistant monster to be anyway!


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