Stockfish's strength relative to the best human chess players was most apparent in a handicap match with grandmaster Hikaru Nakamura (2798-rated) in August 2014. In the first two games of the match, Nakamura had the assistance of an older version of Rybka, and in the next two games, he received White with pawn odds but no assistance. Nakamura was the world's fifth-best human chess player at the time of the match, while Stockfish 5 was denied use of its opening book and endgame tablebase. Stockfish won each half of the match 1.5–0.5. Both of Stockfish's wins arose from positions in which Nakamura, as is typical for his playing style, pressed for a win instead of acquiescing to a draw.[142]
In December 2017, Stockfish 8 was used as a benchmark to test Google division DeepMind's AlphaZero, with each engine supported by different hardware. AlphaZero was trained through self-play for a total of nine hours, and reached Stockfish's level after just four.[143][144][145] In 100 games from the normal starting position, AlphaZero won 25 games as White, won 3 as Black, and drew the remaining 72, with 0 losses.[146] AlphaZero also played twelve 100-game matches against Stockfish starting from twelve popular openings for a final score of 290 wins, 886 draws and 24 losses, for a point score of 733:467.[147][note 3]
AlphaZero's victory over Stockfish sparked a flurry of activity in the computer chess community, leading to a new open-source engine aimed at replicating AlphaZero, known as Leela Chess Zero. By January 2019, Leela was able to defeat the version of Stockfish that played AlphaZero (Stockfish 8) in a 100-game match. An updated version of Stockfish narrowly defeated Leela Chess Zero in the superfinal of the 14th TCEC season, 50.5–49.5 (+10 =81 −9),[32] but lost the superfinal of the next season to Leela 53.5–46.5 (+14 =79 -7).[32][149] The two engines remain very close in strength to each other even as they continue to improve: Leela defeated Stockfish in the superfinal of TCEC Season 17, but Stockfish won TCEC Season 18, TCEC Season 19, TCEC Season 20, and TCEC Season 21, each time defeating Leela in the superfinal.
Derivatives
YaneuraOu, a strong shogi engine and the origin of NNUE. Speaks USI, a variant of UCI for shogi.[150]
Fairy Stockfish, a version modified to play fairy chess. Runs with regional variants (chess, shogi, makruk, etc.) as well as other variants like antichess.[151]
Lichess Stockfish, a version for playing variants without fairy pieces.[12]
Crystal, which seeks to address common issues with chess engines such as positional or tactical blindness due to over reductions or over pruning, draw blindness due to the move horizon and displayed principal variation reliability.[152]
Brainfish, which contains a reduced version of Cerebellum, a chess opening library.[153]
BrainLearn, a derivative of Brainfish but with a persisted learning algorithm.[154]
ShashChess, a derivative with the goal to apply Alexander Shashin theory from the book Best Play: a New Method for Discovering the Strongest Move.[155][156]
Fat Titz, a fork of CFish which name is a pun of Fat Fritz 2 with a HalfKAv2-2048x2-64-64-1 evaluation network, which contains 4 times the knowledge of Stockfish 14. It was trained partially on Lc0 data, which gives a unique positional style, while still preserving the tactical sharpness of Stockfish.[157]
Houdini 6, an alleged Stockfish 8 derivative without providing the sources on request, violating the GPL license.[158]
Fat Fritz 2, an alleged Stockfish 12 derivative without providing the sources on request, violating the GPL license.[158][159][160]
