Season 23 qualification league

Season 23 started with a qualification league. It has been 4 months since the season 22 superfinal ended, during this time TCEC ran a cup, a swiss tournament, an FRC championship, a DFRC championship and other bonus events. 

The format of season 23 has changed a little, as change is one of TCEC's constants. This season there will not be leagues 3 and 4, after qualification there will be league 2 directly. Leagues 1 and 2 will each have 12 engines playing 2 DRRs, 4 engines are planned to advance to leagues 1 and 2 from lower leagues. The qualification league will be a swiss tournament with 18 engines and 9 double rounds. The engine placement in lower leagues is determined by their results in the swiss event of season 22, abandoning the league results. I assume this is due to the fact that some engines are rated above their league result, in season 22 there were only 2 engines advancing between leagues and some were left behind (Berserk is the most obvious example).

There are 18 engines in the qualification league trying to win one of 4 spots that move on to the next stage. The participating engines are:

played in season 22 league 2: ClassicAra, Winter
played in season 22 league 3: Weiss, Drofa, 
played in season 22 league 4: Zahak, Stash, MrBob, Amoeba, 
played in season 22 qualification league: BlackMarlin, Mantissa, Counter, Velvet, ChessFighter, Asymptote, Cheese, Bagatur, tomitankChess
new: Expositor 

Final standings

Velvet, BlackMarlin, Weiss and Winter advance to league 2. Velvet started strong including a double win against BlackMarlin, it kept the lead from the start to qualify. BlackMarlin recovered from its early losses, it won games at a steady pace to secure 2nd place. Many engines fought for the two remaining spots, luck was a significant factor. Winter and ChessFighter used the swiss system to get a double win in the last round against weak opponents, they joined Weiss in a 3-way tie. The other contestants had tougher opponents in the last round and stayed behind. For example ClassicAra lost a game in the last round against Velvet. Winter and Weiss qualified on a tiebreak, they had stronger opponents than the ones ChessFighter faced.

The qualifiers will play 2DDR in league 2 together with the following 8 engines:

played in season 22 league 1: Nemorino, Minic
played in season 22 league 2: Fritz, Halogen
played in season 22 league 3: Tucano, Arasan, Marvin, Wasp

Interesting games

games 3-4, ChessFighter - ClassicAra: Both engines won in white, which is unusual for engines with similar ratings and with no opening books. ClassicAra played weak moves as black in a Najdorf Sicilian, ChessFighter went 3 pawns up to win. In the reverse the engines left theory early in a QGD exchange variation, a late blunder by ChessFighter led to a winning queen ending with white a pawn up.

games 11-12, Velvet - MrBob: Velvet beat MrBob in both games, an unexpected result that perhaps hints that the actual rating difference is much higher than 2 (in favor of MrBob !), the official TCEC rating difference of these engines. The first game reached a RB vs BN ending which Velvet managed to convert despite being 3 pawns down. In the reverse MrBob chose a weak variant in a Giuoco Pianissimo opening, Velvet attacked the white king and traded Q for RB to win.

game 17, BlackMarlin - Winter: Winter was not accurate in the opening, BlackMarlin used its advantage to capture two pawns. The game reached an opposite color bishop ending, BlackMarlin captured another pawn and that was enough to win.

game 26, Weiss - ClassicAra: ClassicAra again was weak in the opening, it moved its queen forward and captured pawns, eventually it gave a knight for 3 pawns. Weiss attacked the black king and used its extra piece to force mate. 

game 28, Counter - Drofa: The game reached a B vs N ending, Counter was a pawn up but it seemed Drofa was holding. However something went wrong, Counter was able to give back the pawn and promote a passer to win. Drofa supposedly has a rating advantage of 100.

game 30, Mantissa - Stash: An attack on the black king resulted in a double rook ending with white 3 pawns up, a win for the lower rated Mantissa.

game 41, Stash - Winter: Stash may have missed a win in a RB vs RB ending up a pawn, Winter was able to hold the draw.

games 51-52, Amoeba - Mantissa: Mantissa missed a win as black in a N vs B ending and a pawn advantage. As white Mantissa had an early pawn advantage, after slowly exchanging most pieces Mantissa used a passer to gain material and win.

games 53-54, Velvet - BlackMarlin: Another double win for Velvet, this time against one of the stronger engines in the league. Velvet appears much stronger than its opponents in qualification. BlackMarlin chose to trade pieces as black that resulted in a RRB vs Q imbalance with black 3 pawns up. Velvet's rooks and bishops were strong, it managed to control the black pawns and reduce to a winning ending. As white BlackMarlin was a pawn up with a small advantage in a closed position. Late in the game the white queen came forward and captured 2 pawns, Velvet reacted with a deadly attack on the white king that resulted in material gain and mate.

Standings after 3 double rounds: Velvet +5, Expositor Weiss Zahak Mantissa +2, BlackMarlin +1, Counter MrBob Amoeba 0, Cheese ClassicAra Stash Drofa Bagatur Asymptote ChessFighter Winter -1, tomitankChess -6.

game 66, Amoeba - MrBob: MrBob missed a win in a QB vs QN position, it chose to capture a pawn and allowed Amoeba to exchange queens and reduce to a drawn ending.

games 67-68, BlackMarlin - Expositor: As white BlackMarlin trapped a black knight early and captured it for 2 pawns. The engines exchanged pieces until only R vs B remained, a won ending for white. The reverse game reached a QB vs QB position. At some point it seemed Expositor was winning, but it exchanged queens leaving a drawn opposite color bishop ending. 

game 69, Weiss - Mantissa: Mantissa captured a knight for a pawn, this opened a file through which Weiss attacked the black king. Weiss gained material and created an advanced passer to win. 

games 71-72, Zahak - Velvet: Another double win for Velvet. As black Velvet gave a knight for a pawn and opened a file in the center, the white king remained in the center and Zahak couldn't develop pieces on the king side. Eventually Velvet gained material and reduced to a winning RN vs R ending. In the reverse game Zahak moved its queen forward and couldn't find a way back. Velvet gave a rook for a bishop and attacked the black king, by the time the black queen made its way back the game reached a winning QB vs Q ending.

games 79-80, MrBob - Zahak: Both engines won as black, which is borderline weird. Zahak as black was a pawn down but the white king moved without castling. Zahak reduced to a RN vs RB position and captured two pawns, it used a passer to capture a piece and win. As white Zahak had an early R for N advantage with an advanced passer. Then it blundered when MrBob offered a rook, Zahak missed the deadly attack on its king that followed. The game reached a queen ending with black two pawns up, enough to win. 

games 85-86, Drofa - Winter: Both engines won as white. The first game reached a RN vs RB position, Drofa as white was two pawns up and Winter had a potential advanced passer as compensation. For a while it seemed Winter was holding but it made a mistake. Drofa exchanged rooks and controlled the black passer, Winter couldn't block all the white pawns supported by the king. In the second game Drofa ignored the danger to its king, the result of the attack was an opposite color bishop ending. Winter was only a pawn up but it had connected passers supported by the king, enough for a win.

game 88, BlackMarlin - Mantissa: Mantissa blundered in a B vs N ending. It captured the bishop for two pawns, then had to give the knight to stop a passer. BlackMarlin had enough pawns remaining to win. 

game 99, Zahak - ChessFighter: Zahak had an eval advantage, eventually ChessFighter cracked. The engines exchanged pieces until reaching a rook ending, material was equal but the white king and pawns were better placed. Zahak captured a pawn and created a passer to win.

game 102, ClassicAra - Expositor: The engines shuffled in a RBN vs RNN position, black had a passer on the 2nd rank but the game seemed to be heading for a draw. Then ClassicAra blundered and lost a pawn, this was enough for Expositor to create a second passer on the 2nd rank. Eventually Expositor queened a passer and won.

game 104, Stash - Drofa: Stash had a passer on the 7th rank. For a while Drofa held on, then it gave a rook for a knight to capture the passer. Stash reduced to a RB vs NN position, it left one last pawn to ensure a win. 

Standings after 6 double rounds: Velvet +7, BlackMarlin Weiss +3, Expositor Stash +2, Winter Zahak Amoeba +1, ChessFighter Asymptote Mantissa Drofa MrBob ClassicAra 0, Counter -1, Bagatur -3, Cheese -5, tomitankChess -11. Velvet was slowed down but is still leading with a large margin. The other 3 qualification spots are still open, all engines but the 3 at the bottom still have a chance. Any encounter with a weak engine may result in a double win, part of the instability of the swiss system. 

game 121, Expositor - Weiss: The game reached a Q vs RB position, Expositor slowly captured black pawns until there was only one left. There were 3 white pawns, the advantage should have been sufficient. However Expositor could not convert, I couldn't say why exactly but apparently one wrong pawn move had deep consequences. Expositor bought time with checks but in the end it couldn't stop Weiss from queening first and this was enough for a draw.

game 124, BlackMarlin - Zahak: BlackMarlin had an eval advantage that gradually increased as the engines exchanged pieces. The game reached a rook ending with white a pawn up, BlackMarlin captured two more pawns and the white passers secured a win.

game 125, Stash - Velvet: The game reached a RN vs RB position and seemed drawn. After move 45 evals increased, Stash managed to capture a pawn and get an advantage. Stash created a passer and pushed it to the 7th rank, Velvet blocked it and held for a long time. Stash captured the bishop for the passer and then Stash blundered by capturing a white pawn. Stash kept its last pawn and exchanged rooks to get a tablebase win. 

Top engines with 2 double rounds to go: Velvet +6, BlackMarlin +4, Weiss Stash +3, Expositor ClassicAra Mantissa +2. Velvet lost a game for the first time in the league, but it will surely qualify. ClassicAra and Mantissa got double wins as gifts from the swiss system and are back in the race for now.  

games 135-136, Counter - Expositor: Both engines won as white. In the first game Counter was up a rook for a bishop, the game reached a RR vs RB position. Expositor captured a pawn without considering Counter's next moves, Counter gave the exchange back and reduced to a winning rook ending two pawns up. In the second game Expositor had an eval advantage that started to increase after move 44, Expositor gained material and reduced to a QR vs QN position. The black king was exposed to attacks, Expositor moved its queen across the board to capture a pawn and reduce to a winning R vs N ending. 

game 137, Amoeba - ClassicAra: ClassicAra played the opening better and it had an eval advantage. The game reached a RB vs RB position, ClassicAra had a better pawn structure. After a long shuffle ClassicAra captured 3 pawns, then used passers to gain material and win.

game 142, BlackMarlin - Stash: Stash moved its king without castling, giving BlackMarlin the advantage. BlackMarlin attacked the black king and the game reached a QRB vs QNN position. Stash had to give its queen to stop a passer, game over.

game 144, Velvet - Mantissa: The engines mainly shuffled in a QRR vs QRR position, Mantissa was a pawn up but evals were close to 0. Mantissa blundered when it captured a pawn and left the 7th rank unguarded. Velvet moved its rooks forward, captured a pawn and created an advanced passer. Mantissa had no defense, it lost a rook for the passer, giving Velvet the win.

Top engines before last round: Velvet +7, BlackMarlin +5, Weiss ClassicAra +3, Stash Expositor MrBob Zahak Drofa +2. Velvet and BlackMarlin will advance, the race for the other 2 spots will be fierce. Three more engines have a +1 score and are scheduled to play weak opponents, with a chance of a double win. It is quite possible that +3 will be enough to qualify and tiebreaks will be needed.

Winter and ChessFighter won twice against their opponents to end with +3, Counter failed to win even once and stayed on +1. Drofa, Expositor, Stash, Zahak, drew their games and stayed on +2. Weiss - which played against Zahak - stayed on +3. 

game 160, BlackMarlin - MrBob: BlackMarlin slowly increased the pressure on the black king, eventually the attack started and the result was a rook ending with white two pawns up. BlackMarlin used passers to win.

game 162, Velvet - ClassicAra: The black queen moved forward and captured a pawn, Velvet used this to develop its pieces faster. ClassicAra had a trapped rook and its king was exposed to attack, Velvet gained material and mated.

MrBob and ClassicAra lost a game each and their scores are under +3. Weiss and Winter join Velvet and BlackMarlin, ChessFighter is in 5th place on a tiebreak. This seems justified since Weiss and Winter faced the two leaders while ChessFighter did not.