Computer chess

TCEC Season 24 kicks off with Lc0 winning the Cup and Stockfish winning the Swiss

TCEC Season 24 is here, and with it the return of the Cup and Swiss (last played in Season 22). Unlike in previous seasons, the Cup and Swiss were played first. The different formats of these competitions meant that it is possible a lower-rated engine will sneak a win, especially in the Cup – small sample size is a notorious bugbear among engine developers, and it occasionally manifests in competition as well, most famously when Houdini dumped Stockfish out of the Season 14 competition.

Unfortunately for the romantics, the first round served up no “Cupsets”. Stoofvlees struggled a little against Marvin, which is always tense because Stoof has a history of “oofing” and losing, but it didn’t happen this time. Round 2 saw a mini-cupset as 14th seed Koivisto put 6th seed Slowchess to the sword, but this wasn’t too surprising either because Koivisto had suffered from a major bug in the previous season and was seeded unusually low as a result. The rest of the matches all proceeded like clockwork as the higher-seeded engines bested their opponents.
But if people were beginning to lose hope, the semifinals brought a real surprise when Stockfish lost an opening to Komodo Dragon. Komodo Dragon is a strong engine, but it has not won an opening against Stockfish for a long time, with the -19 +0 =31 minimatch loss to Stockfish in the Season 22 superfinal an especially bad memory. There was no fairytale comeback this time as Stockfish recovered to win 6-4, but the dropped minimatch did make it seem like the otherwise-untouchable Stockfish can be beaten. Was it a blip, or was it a sign that possibly, just possibly, that Stockfish would fail to win the Cup? In the other semifinal, Lc0 dusted Ethereal away 5-3 to take up the reins as Stockfish’s final opponent. Things started out badly for Leela as Stockfish drew first blood in games 3/4, winning a French Defense, no less. Historically it had generally been Leela who was better at that opening. But in game 7, Leela demonstrated superior positional understanding after Stockfish failed to sense the danger of allowing its Bishop to be forced into a corner.


(Caption) A critical moment in game 7. White’s position is clearly pleasant, and Leela’s +0.97 evaluation is very high by Leela’s standards, but the game isn’t over by any means. Stockfish chose 19…O-O, allowing 20. h6 Bh8. Black’s Bishop is in trouble and the continuation 21. f3 f5 22. g5 made it even more miserable. Although superficially it looks like Black can open a diagonal for the Bishop with …c5, White only has to respond with c3 to keep the Bishop trapped.
This sparkling win tied the match. A tense series of draws later, Leela won game pair 15/16 to win the match. The mighty Stockfish was beaten for the first time in what feels like forever, and Leela won its first title since Season 17.
Next up was the Swiss. This format rewards engines that can reliably beat lower-rated engines, and Leela’s developers submitted a version with dynamic contempt for the first time. This feature is intended to make Leela play for wins against lower-rated opponents, and had already been used to great success by Stockfish (back when Stockfish used handcrafted eval) and Komodo Dragon. For the lower-rated engines, the Swiss took special significance because it affected placing in the Leagues – 22 spots in the Leagues were decided by placing from the previous season, with four more spots going to the top-placing non-qualified engines from the Swiss. 
I could highlight a lot of things, but one thing overshadows them all. In the very first round, Cup winner Leela produced the following brilliancy against QL engine Cheese:

Ok, so I sacrificed a piece and a pawn for no compensation. What’s the problem?

Something had clearly gone wrong with Leela’s dynamic contempt. Needless to say Cheese won easily from this position, and although Leela also won with White to tie the match, it was already behind its rivals – both Stockfish and Komodo Dragon won their opening matches against strong opposition. When the same thing happened to Leela again in round 2 (this time against Drofa), it looked briefly like Leela might not finish in the top 3. Fortunately for Leela fans, the “Leelabug” doesn’t affect Leela’s play against strong engines, and the Swiss format even meant that Leela faced weaker opponents for the next few rounds. Leela steadily fought its way up the table to eventually finish 3rd. For the other members of the Big Three, Stockfish continued to show some weakness when it lost games – but not matches – to Revenge and Scorpio, although it did win the head-to-head against Komodo Dragon. That one point swing, combined with Leela’s dropped half-points against Drofa and Cheese, turned out to be just enough to give Stockfish the tournament victory – its final score of 15.5/22 was exactly one point ahead of Komodo Dragon and Leela on 14.5/22.

For other competitors, the qualifiers to League 2 turned out to be Velvet, Expositor, Wasp, and Uralochka. Velvet played some strong chess, including a 2-0 double kill over Expositor, to finish the highest of the four. Meanwhile, Expositor benefited heavily from the Swiss format when it was paired against the bottom three finishers, scoring 5/6. Its 11.5/22 score actually placed it above League 1 engine Berserk, who had the misfortune of being matched against all of the Big Three. The two 4k engines (so-called because their size is limited to 4 kilobytes) ice4k and 4ku finished solidly last, well off the pace. The 4k handicap is a tough one. 4ku’s developer even deleted the “print eval” line to save space, making 4ku games rather mysterious to watch.

Next up is League 2, where the four qualifiers will face several seasoned competitors, the biggest names being former Premier Division engines Igel and Fritz. Unsurprisingly, both engines are favorites for promotion. Testing is currently under way, and the league is slated to start right after. All games will be played live at

Article by Low

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