Computer chess

Lc0 wins TCEC Season 23 Subfinal

The TCEC Season 23 Subfinal is a bonus 3rd-place match between the loser of the Superfinal (Lc0) and the winner of the Infrafinal (Komodo Dragon), played with the same opening book as the Superfinal. It ended with a decisive victory for Lc0, +24 -13 =63. Most viewers predicted a Lc0 win by +1 to +5, so this +11 margin of victory is a surprise. Still, few could have predicted how the Subfinal would turn out based on the start of the match. 

Komodo won the very first opening, but Leela struck back by winning the second opening. A few tense draws later, Komodo won the fifth opening, but Leela again struck back immediately by winning the sixth opening. Komodo broke the streak by winning the tenth opening, then won the 13th opening as well with a pretty combination (diagram). When it further won the 15th opening, it looked like the Subfinal might turn into a rout. Even Stockfish was ahead by only +4 at this stage of the Superfinal.

TCEC Season 23 so far: Stockfish wins TCEC S23 / It’s Stockfish vs Leela for the TCEC Superfinal / Full standings Premier Division / Ethereal trailblazes TCEC League 1 / Minic convincingly wins TCEC League 2

White to play and win. 31. Qe3! is a pretty queen sacrifice, deflecting the Black queen from the f7-pawn. 31…Qxe3 32. Nxf7+ Kg7 33. Nf5+ Kf8 34. Rxg8+ Kxg8 and 35. Nxe3 leaves White a pawn up with a winning endgame. Note 33…Kxf7 does not help since 34. Rxd7+ Ke8 drops the g8-rook with check. 35. Rxg8+ Kxd7 36. Rg7+ followed by 37. Nxe3 is also winning for White.

But just as Leela seemed down, she turned on god mode. Leela won 10 of the next 22 openings before Komodo finally won the 35th opening. One “busted” opening and a traded win later, Leela won another five openings to round out the superfinal. The subfinal did turn into a rout, but in Leela’s favor. The dramatic comeback from -3 to +11 speaks to the dangers of small sample size as well. Even the full 100 games is not many compared to the tens of thousands of games that engine developers regularly play when testing their patches!

There was still the “bookmaker’s subfinal”, between GM Matthew Sadler and Jeroen Noomen. The two authored the superfinal book, with Jeroen running out the victor in that match by one decisive opening. This bookmaker’s subfinal also turned out very close, and in fact uncannily resembled the superfinal: the two bookmakers remained neck and neck through most of the subfinal, involved a dramatic game 100 (Leela failed to convert a position that Komodo evaluated at +4), and ended with the same score – Jeroen wins by one opening. This time though, GM Sadler has the consolation prize of getting more wins for one engine. Jeroen only scored two Komodo wins, but GM Sadler got five. Of course, this also means that Jeroen got 11 Lc0 wins, while GM Sadler only got seven.

Next up are several more bonus matches: there are some very long time control games (6+ hours per side per game), some DFRC games (Double Fischer Random Chess, where both sides start with a different-but-valid FRC position), and exhibition games by the top engines against the best of the chasing pack. The pause gives all competitors a time to catch their breath before Season 24. Find all bonus games at

Article by Low

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