Computer chess

Stockfish wins TCEC Season 23

There were no major surprises in the Superfinal of the Top Chess Engine Championship, as Stockfish came out the comfortable winner – the final +17 score even surpassed most viewers’ expectations. 

TCEC Season 23 so far: Full standings Premier Division / It’s Stockfish vs Leela for the TCEC Superfinal / Ethereal trailblazes TCEC League 1 / Minic convincingly wins TCEC League 2

As has been the case for several years, engines had improved to the point where one needed openings with heavy bias to separate them – that is, the opening book provides a comfortable advantage to one side, and the competitors take turns to play that side. Sometimes the bias is too high, and the result is a “busted” opening where both engines win when given the advantage. The Superfinal started with such a busted opening, but Stockfish drew first blood in the very next opening by winning with White while drawing with Black. When Stockfish notched another win in the fourth opening, it looked like the Superfinal might turn into a rout. But the seventh opening served up a surprise: after Leela had finished off a dodgy variation of the Sicilian Rossolimo with White, it was Stockfish’s turn to try to make something happen with the White pieces. Stockfish mounted a kingside advance that forced Leela’s king into the open, and it was looking like it would be a routine victory for Stockfish. And then this happened:

You mean the developers didn’t tell Stockfish that this is a trivial draw?

As strong as engines are, sometimes the human brain is still superior! Leela held out to win the opening and cut Stockfish’s lead to 1 point. There were murmurs among Leela’s fans. Could she do it?
Unfortunately for Leela’s fans, that bubble burst quickly. Stockfish won the eighth opening … then the ninth … then the twelfth … and then won some more. By the midway point, with Stockfish up +7, the only question was whether Stockfish would crush Leela as badly as it had crushed Komodo Dragon in the Season 22 superfinal. Stockfish had won by +19 then, so it was behind time, but the fact that some of more high-bias openings had still to be played gave Stockfish fans some hope. Stockfish quickly won another four openings, but then it misevaluated another blocked position to hand Leela her second won game pair. The chance was gone, and Leela emerged from the Superfinal battered, but still less battered than Komodo Dragon.

There was still the “bookmaker’s Superfinal”, between the two bookmakers of the Superfinal – GM Matthew Sadler & Jeroen Noomen. Jeroen took the early lead, but the 7th opening was GM Sadler’s, so he had the only Leela victory in the books. The two remained neck and neck as the superfinal progressed. The second Leela victory went to Jeroen, but several Stockfish wins went to GM Sadler to keep the score level. The score remained unclear all the way to the final game 100: if it ended decisive, Jeroen would win, while if it ended drawn, GM Sadler would win. Stockfish had the White pieces in a Czech Benoni, an opening that usually leads to blocked positions. Stockfish’s eval was soon above +2, but since Stockfish had misevaluated several such positions earlier in the Sufi, it was unclear if Stockfish could actually convert. For the longest time it looked like the game was just shuffling. Both Stockfish’s and Leela’s evaluations kept rising, but even the kibitzing GM Sadler could not tell what was happening. Finally, after 60 moves, Leela played 60…b5, opening lines and going down in defeat. If Leela were human, we would say she cracked under pressure, except Stockfish also predicted Black would play 60…b5. Go figure – this mysterious game defies human understanding, and I for one am curious what Leela’s decisive mistake was. 

In any case, Stockfish’s victory gave Jeroen the Superfinal win, 11 decisive openings to 10. It is Jeroen’s second consecutive victory – he also won the Season 22 bookmaker’s Superfinal 11-8. Jeroen has been making books for engine competitions for a long time, so he surely has an advantage in experience, but GM Sadler is catching up.
The next major event is the Subfinal – played between the loser of the Superfinal (Lc0) and the winner of the Infrafinal (Komodo Dragon). While Stockfish is clearly in a league of its own, this match is likely to be much more contested. The two engines last played each other in Season 22, with Lc0 emerging victorious (+24 -15 =61). Will Leela win again this season? Most viewers certainly think so, albeit by a smaller margin of +1 to +5. The Subfinal is slated to begin on 19 November. Catch the games live at

Article by Low

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