Stockfish Absorbs NNUE, Claims 100 Elo Point Improvement

In arguably the biggest development in computer chess since AlphaZero was introduced, Stockfish (the strongest traditional chess engine) is absorbing the fledgling Stockfish+NNUE project.

Update: see also our news report on the launch of Stockfish 12.

Stockfish+NNUE (NNUE stands for Efficiently Updatable Neural Network) combines the brute-force computing approach of traditional Stockfish with the advanced evaluation capabilities of neural network chess engines such as AlphaZero and Leela Chess Zero. Stockfish+NNUE can look at more than 50,000,000 positions per second which is half the speed of traditional Stockfish, but 500x the speed of Leela Chess Zero. The nuts and bolts are quite simple; the evaluation function of traditional Stockfish, which determines how desirable a position is, is stripped out and replaced with a neural network evaluation function.

In recent months, Stockfish+NNUE has made remarkable claims regarding Elo gains relative to traditional Stockfish. Chess.com tested these claims in a 600-game tournament featuring Stockfish+NNUE, Leela Chess Zero, and Stockfish 11. In our tournament in the Chess.com/ccc environment, these engines performed quite comparably. Leela Chess Zero won the tournament with a +3 score; Stockfish+NNUE finished second with a 50% score, and Stockfish 11 finished last with -3.

Statistically, each of these engines performed nearly indistinguishably, but through these and other private tests, the Stockfish team was evidently convinced that Stockfish+NNUE was the way of the future. Stockfish announced on July 25th that Stockfish+NNUE will be absorbed into the mainline Stockfish project. There are several compelling ramifications of this merger:

• Stockfish 12 will likely have NNUE integrated.
• Stockfish 12 will still include a traditional engine for users who cannot run a neural network on their machines.
• It is anticipated that the Stockfish team will extend their wide platform support to the NNUE project, making it easy for any user to install the program.

Stockfish+NNUE has since been integrated into the fishtest testing environment which supports accurate evaluation of performance. In less than a month since the integration, Stockfish+NNUE has shown more than 100 Elo points of improvement relative to Stockfish 11. This level of improvement equates to approximately two years of traditional improvement in the span of a month, and Stockfish+NNUE is still gaining strength.

Chess fans have also shown enthusiasm for the aesthetic playing style of Stockfish+NNUE. In multiple games, the engine has shown a pleasing integration of deep calculation and subtle positional evaluation. Here are some of the most popular games played by Stockfish+NNUE in the last month.

Want to see Stockfish+NNUE in action? Check out the Computer Chess Championship at chess.com/ccc where the top engines are always battling.