Stockfish is the winner of the season 25 superfinal. The final score is 27-23 with 50 draws. Leela won another game pair and reduced Stockfish's lead to 4, the same gap as in the previous superfinal. The other 4 game pairs were drawn, 3 of them with two white wins. In the last 18 game pairs Leela beat Stockfish 4-0 in game pair wins, while after the first 32 game pairs Stockfish led 9-1.
The superfinal started with 6 decisive games in 3 drawn game pairs. Stockfish won the first decisive game pair, and it slowly increased its lead. Of the first 14 game pairs Stockfish won 3, then Leela won its first game pair and Stockfish led by only 2 wins after 30 games. Stockfish then won several game pairs and increased its lead to 6 wins at the half way point. In some of the games Stockfish demonstrated superior endgame skills, for example in complex queen endgames. Stockfish extended its lead to a maximum of 8 wins after 32 game pairs, then Leela won its second pair of the match and reduced the lead to 7. From this point Stockfish did not win another game pair, it seemed that somehow Leela became stronger. Stockfish managed to get out of a game pair loss after a very late Leela inaccuracy in game 70. Leela slowly won more pairs, and the match ended with Stockfish ahead by 4 wins. Who knows what could have happened if more games were played.
Season 25 started with a Swiss event, however unlike in the previous season this event was not considered a replacement for the qualification. According to the current rules the top 6 engines of the Swiss event, not including those that are already included in one of the leagues, will play in the initial league stage. This season there were many engines from the last season that did not participate, either by the developer choice or since they are not in development any more. This included premier division participants Berserk and Koivisto. As a result almost all the engines that played in the Swiss event continued to the leagues.
The first league event was the entrance league, including 16 engines of which 5 were new to TCEC. The four engines that advanced were all new: Viridithas, Clover, DeepSjeng and Caissa. Caissa managed to secure 4th place despite a bug that caused it to crash on time twice, Booot finished a close 5th. In league 2 the same 4 engines continued their journey and advanced again, in a different order. Caissa's bug was fixed and it finished 2nd, DeepSjeng and Arasan battled for 4th place and only a win in a direct encounter in the last rounds ensured that DeepSjeng would advance. League 1 was a different story for the 4 newcomers, none were strong enough to advance a third time. RubiChess dominated the league and finished in 1st place. rofChade was in 2nd place at the half way point, with Igel a close 3rd. In the last RR Igel managed to move ahead of rofChade and in the end it was Igel that advanced, despite losing two games to rofChade in the league.
The premier division was a month long 6-DRR event. The top3 engines, Stockfish, Leela and KomodoDragon were still dominant as in the previous several seasons, but this season there were greater score gaps between them. In the 90 top3-bottom5 game pairs the top3 did not lose a single game pair, and drew only 17. Stockfish finished first with no game pair loss at all, beating Leela 3 times and KomodoDragon 5 times. Leela beat KomodoDragon once, but it won 5 more game pairs than KomodoDragon against the bottom5 engines and finished 2nd. The top3 standings were obvious quite early in the division, the main race that was interesting was between Ethereal and RubiChess for 4th place that led to the division playoff stage. Ethereal and RubiChess were very close to one another from the start, each had one head to head game pair win in the first DRRs. After 5 DRRs Ethereal managed to lead RubiChess by 1.5 points. Then in the last DRR Ethereal unexpectedly lost a game pair to Igel, RubiChess also beat Ethereal in their final direct encounter game pair and this secured 4th place for RubiChess.
TBD - season summary: premier division playoff, personal
Games 91-92 started with a sideline in the QGA, Alekhine defense. The white queen was out on the queen side and a black rook was on a7 protecting the b7 pawn. In game 91 the engines opened files on the queen side and Stockfish castled short. There were many early piece exchanges, the game reached a QRN vs QRN position with white a pawn up on move 23. Stockfish moves its queen forward and Leela exchanged queens.
A pawn advantage with all pawns on the same side was small but significant, evals increased slowly. When it could Leela exchanged pawns while keeping its pieces on the board. On move 57 Leela created a passer in the center, on move 60 it connected a second passer while giving Stockfish a passer on the king side.
Leela pushed its passers forward and Stockfish couldn't stop them. Leela gave a knight to queen one passer and mated.
In game 92 the engines repeated game 91 for 16 plies with transposition, opening files on the queen side. Stockfish went a pawn up but all pieces stayed longer on the board. On move 27 the engines exchanged knight for bishop, the first pieces off the board. By move 32 all rooks were exchanged.
Leela avoided exchanging queens, Stockfish's eval started to increase as it pushed pawns in the center. In a complex position Leela chose to give a knight for 3 pawns and expose the white king. On move 47 Stockfish saw a win from far away, the engines reduced to a QBB vs QB position.
Leela gave many checks with its queen and captured the last white pawn. Stockfish found safety for its king in the center with the pieces blocking the black queen. It captured two black pawns and slowly improved its eval though it was hard to understand on the board. The white pieces slowly closed in on the black king, Stockfish had mate in its PV when Leela gave its queen and the game ended in a tablebase win. There were two white wins in this game pair, Stockfish leads 25-20.
Games 93-94 started with the Sicilian Lowenthal variation, played in high level human chess. In game 93 the engines castled in opposite directions. There was a long PV agreement and many early exchanges, the game reached a QRR vs QRR position on move 23. Evals started to increase, after exchanging queens Leela captured a pawn.
Stockfish had a passer in the center while Leela had a pawn majority on the queen side. Leela captured a pawn on the queen side, the black king moved forward to support the passer. Leela captured the second black pawn on the queen side, creating 3 connected passers. It blocked the black passer and pushed its own forward. Stockfish lost material trying to stop the white passers, Leela queened and mated.
In game 94 the engines repeated game 93 for 21 plies, Leela chose a different defense and Stockfish's eval started to increase. Again the game reached a double rook ending on move 29, this time material was equal but Stockfish had doubled rooks on the g file, blocking and threatening the black king on the open h file.
Leela's eval was stable for a long time, Stockfish wasn't in a hurry with repeating checks every few moves. The engines exchanged a pair of pawns and Leela created a passer. The passer was isolated and had to be protected. There was a threat of mate that Leela had to deal with as well. On move 58 Leela's eval jumped after a long think, Stockfish forced a small but significant change in the black rooks' placement.
The white king managed to move forward to the center to block the passer. The queen side pawns stabilized, after many check repeats Stockfish exchanged a pair of rooks on move 82. Material was still equal, the engines traded pawns and both had only two pawns on the queen side. However, the black king was far on the king side and unable to help. Stockfish moved its king behind the black pawns and the game ended in a tablebase win. There were two white wins in this game pair, Stockfish leads 26-21.
Games 95-96 started with a 24-ply line in the French Winawer, retreat variation. The white queen captured two pawns on the king side and the black queen captured a pawn on the queen side. In game 95 the engines exchanged minor pieces after the start, Stockfish castled long and Leela moved its king forward without castling. Evals started to increase as Leela captured another pawn on the king side, Stockfish chased the white queen away and pushed the e pawn forward. As a result the e pawn became an advanced passer, while Leela had 3 connected passers on the king side. Stockfish captured a pawn and created a passer on the queen side, on move 31 the engines exchanged queens and the game reached a RRB vs RRN position.
Leela blocked the black passers with two pieces and slowly pushed its passers on the king side. The two black passers were stopped on the 2nd rank, Stockfish used its rooks to try to stop the white passers but Leela managed to queen one. Stockfish lost material and was mated.
In game 96 both engines castled long, Stockfish shifted its queen to the queen side. Stockfish pushed a passer on the king side but without support it stopped on the 5th rank. The engines mostly shuffled and evals came down, Stockfish gave a rook for a bishop and pushed the passer to the 7th rank. The engines traded RB for Q, Leela captured the passer, evals were low and the game was adjudicated. Leela wins the game pair, Stockfish leads 26-22.
Games 97-98 started with a sideline of the Indian defense, Wade-Tartakower variation. In game 97 Leela castled long, there were a few minor piece exchanges and Stockfish also castled long. The engines shuffled for a while and evals came down, then they exchanged a pair of rooks and reduced to a QRB vs QRN position. Leela captured a pawn and created a passer, Stockfish regained the pawn later. After exchanging queens evals were low and the game was adjudicated. In game 98 Stockfish castled long and Leela chose to castle short after the white queen captured the b7 pawn. Stockfish pushed pawns on the king side and Leela blocked them. Evals came down and the game reached a RRB vs RRN position. After exchanging a pair of rooks the game was adjudicated.
Games 99-100 started with a rare 22-ply sideline of the Caro-Kann, Panov attack. In game 99 there were no exchanges after the start, both engines placed their major pieces on the center files. After a knight exchange on move 22 Stockfish created a passer in the center. The engines shuffled for a while, then Leela created a passer in the center after a pawn exchange on move 35. On move 43 Leela captured the black passer and exchanged bishops.
Evals increased as the engines stabilized the pawns on the king side. Leela exchanged a pair of pawns on the queen side and then the engines started to shuffle. Evals drifted down as the 50-move counter got close to 0, Leela saw exchanges in its PV but Stockfish decided to initiate a queen exchange. Evals increased quickly, on move 106 the game reached a RB vs RB position.
Stockfish tried to stop the center passer and Leela captured the two black pawns on the king side. Stockfish couldn't avoid a queening followed by mate.
In game 100 both engines concentrated their pieces in the center, the exchanges started on move 18. Stockfish pushed the h pawn to h6 and exchanged it, on move 26 the game reached a QBB vs QBB position.
Stockfish's eval slowly increased but there were no exchanges except a pair of pawns on the queen side. The black king hid in the corner protected by the black queen and it seemed that Stockfish was shuffling. Slowly Leela's eval increased as well, Stockfish moved its king to the center and pushed the king side pawns.
Leela had to defend its weak a5 and h7 pawns, and the possibility of a pawn attack on the king side. In the end Stockfish found a way to attack from the queen side, it exchanged a pair of bishops and saw the win in its PV. Stockfish reduced to a queen ending two pawns up. The white king moved forward, Leela lost its queen, Stockfish queened a passer and mated. There were two white wins in this game pair, Stockfish leads 27-23.
