Season 15 superfinal, games 91-100

Leela wins the TCEC season 15 superfinal. The final score is 14-7 with 79 draws. This comes as no surprise, Leela was ahead by 6 points since game 86. In the last 10 games Leela extended its lead to 7 points.

Superfinal summary
In season 14 Leela started from the lower divisions, climbed up and reached the superfinal where it lost by 1 point to Stockfish. This season Stockfish won the premier division but Leela was stronger head-to-head. In the superfinal Stockfish was still hopeful after it equalized to 1-1 in game 12. However Leela started to win minimatches, by game 26 it led 5-1. Stockfish won two more games in openings where Leela also won, Leela extended its lead to 8-3 in game 40. Stockfish tried to stay in the match with two minimatch wins and the score was 8-5 at the half. After many draws Leela crushed Stockfish's hopes with an unusual double win, extending the lead to 5 points on game 62. In the remaining games Stockfish won twice in openings that Leela also won, and Leela won two more mimatches to win the match by 7 points.

Leela improved its game compared to the previous season. It made very few blunders in the endgames, it played all types of openings well, usually better than Stockfish. It appears to have a much better understanding of long term advantages, often claiming it is much better long before Stockfish can see a win. There were a few cases where this proved to be wrong, but mostly Leela's predictions were correct.

There will be more NN engines in the next season. There were two NN engines in the premier division in season 15, will there be another one next season? Are the NN engines going to take over the chess world? When will we see a NN superfinal? Can Stockfish improve and be competitive? Will Houdini be updated? Will there be a hybrid NN-AB engine? I can't wait to find out.

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Games 91-92 were a King's Gambit variant where the white king moves. In game 91 the white king walked to the king side, trapping its rook. Stockfish's eval dropped to 0 as it regained the pawn, the engines exchanged queens on move 20. Stockfish let its rook out of the corner, then all rooks were exchanged and only BN vs BB were left on move 26. Leela's eval kept the game going, the engines reduced to a same color bishop ending and the game ended on move 103 in a 50-move draw. Game 92 started similarly, the queens stayed on the board. The white king side rook got out of the corner but the engines did not exchange rooks, evals were negative and stayed low. Both engines focused on the isolated center pawns, the game was peaceful with a few exchanges. Evals came down and the game was adjudicated with many pieces still on the board.

The engines opened the center early in game 93. Neither engine was strong enough to attack and evals came down. Leela moved its queen forward on the queen side, this led to a queen exchange and the removal of all the pawns on the queen side. The game reached a BN vs BB position on move 40, Stockfish was a pawn up but with evals close to 0 the game was adjudicated quickly.

Leela's eval started around 1 in game 94, and was over 3 already on move 12. The engines castled in opposite direction, both Leela's rooks controlled half open files, one facing the black king. Leela chose to attack on the king side, it doubled rooks on the f file and broke through. After exchanging a pair of rooks Leela had a rook on the 7th rank and a formidable bishop on d5.  

Stockfish did not want to exchange bishops and to give Leela a passer. Nevertheless, Leela captured a pawn on the king side to create one passer, then it exchanged knights and created a second passer. After exchanging bishops only QR vs QR remained, both engines had central passers but Leela's passer was much stronger. It pushed the passer forward and Stockfish stopped it on the 6th rank.

Stockfish got rid of the passer on d6 but lost another pawn on the king side. The game reduced to a rook ending with two white passers on the king side and the black king too far to help. The game was quickly adjudicated.

Both kings stayed uncastled in game 95, Leela opened the queen side and Stockfish pushed pawns on the king side. Eventually Leela castled short, Stockfish opened the king side with its pawns, then gave a bishop to get two pawns facing the black king on the 6th rank. Leela exchanged two rooks for a queen to get rid of one pawn. After a while Leela lost a bishop and captured the second pawn, the game ended in a RR vs Q tablebase draw. In game 96 Stockfish's eval was 0 from the start. Both engines castled long and the king side was open. Queens were off early and the game reached a RNN vs RBN position on move 27. The engines shuffled and stabilized the pawn lines, then reduced to a rook ending on move 52. Leela went a pawn up and the game continued until reaching a tablebase draw on move 87.

Games 97-98 started with a 20-ply book in the King's Indian Defense. In game 97 the engines formed long pawn lines with no exchanges, Stockfish pushed pawns on the queen side and Leela on the king side. Stockfish opened a file on the queen side on move 25, Leela gave two pawns and started a king side attack. A knight sacrifice opened the king side for the black major pieces, then in a series of exchanges most of the pieces were gone and Leela found a perpetual check draw. In game 98 there were several exchanges after the start, the engines opened the queen side. Leela's eval was over 1, Stockfish's eval stayed low. Leela refused to open the king side, after the engines exchanged a pair of rooks they started to shuffle and Leela's eval came down. The game reached a QRN vs QRN position on move 51, Stockfish opened the king side and found a perpetual check draw.

Stockfish castled long in game 99, Leela opened a file on the queen side and attacked. The white king ran to the center, Leela sacrificed a bishop and added pieces to the attack. Then most pieces were exchanged off and only RBN vs RN remained on move 29. Leela was two pawns up, all the pawns were on the king side. Stockfish's eval dropped to 0, Leela captured a pawn and thought it had small advantage. The game ended only on move 97 in a tablebase draw. In game 100 both engines castled long, though the black king did not have good pawn support on the queen side. Leela attacked with its queen and captured a pawn, Stockfish exchanged queens and regained the pawn. Stockfish had a trapped bishop, it gave a pawn to release it. This gave Leela a passer in the center and its eval jumped over 3. Stockfish blocked the passer with its pieces, the game reduced to RN vs RN position. Leela's eval peaked over 5, it couldn't find a way to improve and the engines started to shuffle from move 44. On move 104 Leela gave a pawn back and opened the queen side, its eval already below 2. The engines continued to shuffle and exchange pawns until reaching a tablebase draw on move 139.

Season 15 superfinal statistics

A summary statistics table of previous stages and seasons. 

Draw rate, wins


Final draw rate was 79%.


Game termination

The three most common game termination causes were:

34% - TCEC draw rule
30% - SyzygyTB
21% - TCEC win rule

Moves per game


Median= 79
Average= 89.4

The games were longer than usual, similar to what we saw in the season 14 superfinal. There were 26 games longer than 100 moves, the longest was 262 moves (game 33, draw).

Time per game (hours)


Median= 4:20
Average= 4:09

Openings

There were variable length book openings in this stage chosen by Jeroen. The first letter of the ECO codes was distributed as follows:


The book determined the opening in most cases, some of the book sequences were very short and allowed more freedom, one game pair was bookless. 86% of the game pairs repeated the same ECO code twice, and 84% repeated the same opening variant. In 3 game pairs the first ECO letter was not repeated.

Reverse pairs, wins

 
Reverse pairs, same moves 

 

Pairs of reverse games diverged very quickly, 72% of the pairs diverged at most after 1 move. The longest repeated sequence of moves was 9 plys, this happened in three game pairs (games 5-6, Dutch, two draws ; games 19-20, Sicilian Rauzer Attack, two draws ; games 79-80, Ruy Lopez modern Steinitz defence, two draws).     

Season 15 superfinal, games 81-90

After 90 games Leela leads 13-7 with 70 draws. There were 5 decisive games in the last 10, two double white wins and one minimatch win for Leela. Leela is 6 points ahead with 10 games to go, Stockfish still has a mathematical chance but Leela is going to win.

The engines castled in opposite directions in game 81. Stockfish's eval was around 1 after the start, it pushed pawns on the king side with support from its rooks. The king side opened and Leela hid its king behind a white pawn, then on move 26 Stockfish's eval jumped over 7 with no warning.

Leela wasn't expecting Stockfish's 28th move, white abandoned two minor pieces but the black king was in a mating net. Leela had to exchange RB for Q but it was too late. Its king was still under threat and Stockfish controlled the 7th rank. The black pawns started to fall.

Stockfish captured the knight and was 4 pawns up when the game was adjudicated a few moves later.

In game 82 the pawns in the center and on the king side locked early. There was one open file on the queen side, all the pieces remained on the board with the first piece exchange on move 23. By then Leela's eval was over 4, though it wasn't clear why. After exchanging a pair of rooks Leela took over the open file, its eval over 6.

Stockfish had very little room to move. It refused to take the rook on c6 since this would create an advanced white passer. The engines continued to shuffle, Stockfish was almost paralyzed and Leela was patient and arranged its pieces. It put a rook on the back rank and waited for an opportunity. On move 61 Stockfish abandoned its b pawn.

A few moves later Leela captured the b pawn, and then captured a bishop. Stockfish's admitted defeat and the game was adjudicated. There were two white wins in this opening.

Games 83-84 started with a 28-ply book in the Sicilian Dragon, Yugoslav Attack variation. White castled long and black castled short. In game 83 Stockfish pushed pawns on the king side and it managed to open a file. Evals came down as the engines exchanged pieces, reaching a same color bishop ending on move 40. Leela thought it had a small advantage and the game continued for 20 more moves and reached a tablebase draw. In game 84 Leela went a pawn up, exchanges cleared the black pawns on the queen side. Leela had two connected passers on the queen side, Stockfish had a passer in the center and its eval remained low. On move 28 Leela gave a rook for a bishop, its eval over 1.5. The black king was a little exposed, Stockfish was active on the queen side and this prevented an attack on its king. On move 45 Leela gave another rook for a bishop, only QBB vs QRR remained with Leela two pawns up. The bishop pair was very strong, Leela could get some of the material back but chose to wait. After exchanging queens a white passer reached the 7th rank, Stockfish lost a rook to stop it. Both engines queened in the BB vs R ending, it cost Leela a bishop. The game reached a tablebase draw in a QB vs QR position on move 107. 

Queens were off early in game 85, evals started around 1 and slowly came down. Stockfish went up a pawn and created a queen side passer, the game reached a RRB vs RRB position on move 27. Evals came down to 0, Leela's eval went slightly negative when it regained the pawn. The engines reduced to a bishop ending on move 47, Leela went up a pawn and continued playing until reaching a tablebase draw on move 96.

In game 86 the action was on the queen side. Leela pushed pawns forward, after exchanging a pair of rooks the engines captured most of the pawns on the queen side, leaving one white passer. Leela's eval was around 1.5 as the engines continued to exchange pieces until reaching an opposite color bishop ending on move 40. Instead of being a drawish ending as usual, without warning Stockfish's eval jumped over 7 as it saw the loss coming.

The black bishop had to keep an eye on the a7 square, the black d pawn could not be protected. Leela created a passer on the king side and protected it with its bishop, now it saw the win as well. Stockfish was going to lose the bishop for one of these passers eventually and the game was adjudicated quickly.

There was one piece exchange early in game 87, all pawns remained on the board. Leela's king walked to the queen side, Stockfish castled short and then changed its mind and its king walked back to the center. Leela's eval turned negative as the engines shuffled, Stockfish pushed a pawn to a6. The first pawn was captured on move 45, Leela took the a6 pawn and went a pawn up, exposing the queen side to the white pieces. Leela's eval turned positive again and Stockfish's eval started to climb, it opened the king side and created an advanced passer in the center. When the passer reached the 7th rank Stockfish's eval was over 4, Leela was still feeling secure with an eval under 1.


The pin and attack on the a6 pawn together with the dangerous d7 pawn were too much to defend, as Leela soon realized. It gave Q for RB and its eval jumped over 8, the game was adjudicated before Stockfish won more material.

In game 88 there was one pawn exchange on move 16 and all the pieces remained on the board. Both engines castled short, Stockfish's eval was low and Leela's eval was around 1. On move 32 the engines started a series of exchanges, Leela's eval jumped when Stockfish gave a rook for a bishop. The game reached a RRN vs RNN position on move 39.

Evals continued to increase as Leela moved its pieces patiently. It took 10 moves for Leela to exchange knights and another 10 moves to capture a pawn. Leela's eval was over 10 by then.

The rook on h4 was trapped but not for long. The white king moved to the center, Stockfish had to move its pieces and the white rook could get out safely. Stockfish avoided exchanging rooks and it defended as long as it could. Eventually Leela moves a rook behind the black pawns and used the other rook to keep black king far on the other side. The game was adjudicated on move 99. Two white wins in this opening.

Games 89-90 started with a 19-ply book in the Benoni Defense. Evals were around 1 at the start of game 89. The first piece exchange was on move 24, in a series of exchanges all the queen side pawns were gone, Leela gave a rook for a bishop and the game reduced to a RRN vs RBN position on move 34. The knights were exchanged, evals came down and the engines started to shuffle. Leela's eval turned negative, all pawns were gone except for two black pawns. Leela continued playing, the game was finally adjudicated on move 180 with the 50-move draw counter almost at 0. In game 90 there were again only a few exchanges from the start. Leela controlled an open file on the queen side, Stockfish gave a rook for a bishop and pawn and Leela's eval jumped over 3. After exchanging queens the game reduced to a RRB vs RBN position on move 40. The white rooks were strong but the bishop was trapped on the king side. Leela went a pawn up and its eval close to 10 but it couldn't find a way to close the game. The engines shuffled, then exchanged rooks, Stockfish got a pawn back and the engines shuffled some more. The white bishop stayed trapped and Leela's eval slowly came down. On move 155 Leela finally gave its bishop up for a pawn and the game reached a drawn 7-man R vs BN position. The game was adjudicated on move 211. Was that a narrow escape for Stockfish? Or was it a bad evaluation by Leela?

Season 15 superfinal, games 71-80

After 80 games Leela leads 10-5 with 65 draws. The last 10 games were all draws, including the bookless pair of games. Only 20 games are left, Leela is going to win almost surely.

In games 71-72 black started a pawn up in the Vienna Gambit. Stockfish's eval was 0 almost throughout game 71. Leela had an advanced passer on the king side, after a series of exchanges the white king side pawns were gone and Leela had a second passer. Stockfish's king walked to the queen side to get out of trouble. Leela gave a piece in a series of exchanges, its two passers reached the 6th and 7th ranks in a QRB vs QR position on move 29. Stockfish gave checks and then blocked the passers, Leela's eval came down and the game was adjudicated. In game 72 Stockfish had the same passer on the king side but its eval came down to 0 very quickly. Leela castled long and gave a rook for a bishop to remove the black passer. Most of the king side pawns were gone and Stockfish moved its king to the queen side. The engines shuffled for a while, then Stockfish gave two pawns and opened the queen side. Leela's king was under attack, after a series of exchanges the game reduced to a drawn queen ending. Leela continued playing until adjudication on move 81. 

Games 73-74 started with white a pawn up and with a queen side passer in the Benko Gambit. In game 73 evals started over 1. Stockfish had to move its king early, it walked to the king side. The white passer got to the 5th rank and Leela blocked it with pieces. Evals came down slowly, the engines exchanged most of the minor pieces. After a while the center and king side were blocked and the pawns on the queen side couldn't advance. The engines shuffled and the game was adjudicated early on move 38. In game 74 both engines castled short, Leela kept its queen side pawns back and its eval remained over 1.5. After a series of exchanges only QN vs QB remained on move 30, Leela's eval jumped over 2 while Stockfish's eval dropped to 0. Leela went up 3 pawns but could only give checks, or stop and allow Stockfish to give checks. Leela's eval slowly came down, it lost its knight and then the engines exchanged queens. The game ended in a tablebase draw.

In game 75 both engines castled long, most pieces and pawns were on the board and evals slowly came down. Stockfish pushed a pawn on the queen side and threatened the black king with its queen. The attack was not strong enough, evals were close to 0 and Stockfish ended the game with a perpetual check. In game 76 Stockfish pushed pawns on the queen side, Leela opened a file on the king side and exchanged rooks. The black king remained in the center, the white king castled long, both were vulnerable to attack. The engines exchanged most pieces and only QB vs QB remained on move 39. Stockfish's eval was at 0, Leela played on. On move 64 the queens were exchanged to reach an opposite color bishop ending. The game ended in a tablebase draw only on move 142.

Games 77-78 were bookless. Stockfish started game 77 with e4, and it turned into an Italian opening. All pawns remained on the board and evals stayed low. Leela concentrated its major pieces on the king side while Stockfish opened the queen side. After a series of exchanges the game reached a RNN vs RBN position on move 28. Stockfish could have finished the game with a repetition but continued and the engines reduced to a rook ending. The game ended in a tablebase draw. Leela started with d4 in game 78 and the game continued in a closed Catalan opening. On move 16 Leela's eval started climbing though it was a pawn down. It was over 1.5 for a few moves and then came down slowly. Stockfish remained calm, it gave the pawn back and exchanged pieces. The game reached a QN vs QB position on move 37 and Stockfish's eval was 0. Leela avoided exchanging queens until move 69, its eval came down and the game was adjudicated not long afterwards.

In game 79 the engines played behind the pawn lines, there was one piece exchange and one pawn exchange in the first 25 moves. On move 32 Leela's eval turned negative over 1, the king side pawns locked in a pawn line with a black pawn on g3 and the h file open. The engines started to shuffle, Leela trying to break in and Stockfish defending the entrances from within. The engines exchanged a pair of rooks on move 63, Leela's eval started to climb. The remaining rooks were gone on move 82 and the shuffle continued. Stockfish's eval suddenly jumped over (negative) 1.5 on move 111, evals climbed over 2. Then on move 125 Stockfish's eval dropped to 0, did Leela miss something? The engines started exchanging pawns and pieces and the game reduced to a queen ending, Stockfish found a perpetual check draw. In game 80 again there were few exchanges in the opening. Stockfish's eval stayed low, the engines opened a file on the queen side and Stockfish controlled it. A pair of rooks was exchanged on move 40, after some shuffling Leela's eval came down. The game reached a RB vs RB position and was adjudicated a few moves later.

Season 15 superfinal, games 61-70

After 70 games Leela leads 10-5 with 55 draws. Leela added two wins, winning a game pair from both sides. This is quite unusual for top level engines, Stockfish made a serious error of judgment in the opening as white. WIth 30 games left it is likely that Leela is going to win this match.

In game 61 evals were around 1 at the start. Leela thought Stockfish's 18th move was a blunder and its eval jumped over 1 (negative) while Stockfish's eval remained positive. Two moves later Leela had a protected passer on the king side.

The engines continue to shuffle with all pieces still on the board. Stockfish did not see any danger until move 33, by then Leela's eval was over 4. On move 37 the first pieces were exchanged and Leela added a second passer on the king side.

Leela opened a file on the queen side and threatened the white king. Stockfish was careful not to exchange pieces that would weaken its defense. After some shuffling Leela gave a rook for a knight, exchanged queens, captured a pawn and created a third passer in a RRB vs RRN position.

The black pawns started to move and Stockfish couldn't stop them and its counter attack was weak. Stockfish gave a rook and bishop to capture all three passers, the game was adjudicated in a R vs RNN position.

In game 62 the engines opened the center and the black king was forced to move early. Leela's eval climbed over 2 while stockfish's eval remained low. Stockfish moved its king to the queen side in a slow long castling, then the engines locked the queen side pawns. After opening a file on the king side Stockfish's major pieces faced the white king but were not strong enough to attack. Leela's eval jumped over 3, after exchanging queens it went a pawn up and created a king side passer.



Stockfish exchanged a bishop for a knight and got the pawn back. Both engines applied pressure on their opponents' weak pawns, in the end it was Leela who had the upper hand. After exchanging a pair of rooks it won the f5 pawn and connected a second passer on the king side in a RBN vs RNN position.



Stockfish captured the c4 pawn as well, material was equal but Leela had a winning advantage. It could block the black passer while pushing its own, Stockfish had to give a knight to stop it. The game was adjudicated when the second white passer pushed forward. An unexpected double win for Leela in this opening.

Evals were low throughout game 63. There were many early pawn exchanges and open files. The two kings were vulnerable to attack. In the end Stockfish gave two rooks for minor pieces and finished with a repetitive check draw. In game 64 Stockfish's eval was 0 right from the start. The center was blocked, Leela exposed the black king with a knight sacrifice, and got the material back a few moves later. Only QB vs QB remained on move 36, both kings were exposed and the queens gave checks. Leela gave a bishop to get a pawn to b7, Stockfish continued to give checks until the game was adjudicated on move 138.

In games 65-66 the engines castled in opposite directions. Leela was active on the queen side in game 65, it sacrificed a knight to clear the white pawns and exposed the white king. Evals stayed low as Stockfish moved its king to the center and Leela created a queen side passer. In a series of exchanges Stockfish gave two minors for a rook and only RR vs RB remained on move 46. All the remaining pawns were passers but the engines couldn't force a queening. The game reached a tablebase draw on move 74. In game 66 Leela was active on the king side but could not break the black defense. Evals stayed low, when Leela sent all its major pieces forward Stockfish atacked the white king with its queen and found a perpetual check draw.

In games 67-68 again the engines castled in opposite directions. In game 67 Stockfish pushed pawns on the king side, after a long series of exchanges only RRB vs RRN remained on move 27. Leela was a pawn up, Stockfish moved both its rooks to the 7th rank and forced a perpetual check draw. In game 68 Leela pushed pawns on the king side and again there were early exchanges, this time the engines kept the queens on the board. Stockfish's eval came down to 0, Leela's eval stayed around 1. Stockfish went a pawn up on move 34, Leela pressed the king side but did not find a way in. The engines exchanged a pair of rooks and Leela avoided exchanging another pair. Stockfish ended the game with a rook sacrifice and perpetual check.

Games 69-70 started with a 24-ply book in the Semi-Slav Defense, with black two pawns up and a development advantage for white. There were many early exchanges in game 69, Stockfish forced the black king side rook to move and Leela kept its king in the center. Evals came down as queens were exchanged, the game reached a RB vs RN position on move 29. Stockfish equalized material, both engines had a passer they pushed forward. Once the pawns stopped moving the game was adjudicated. In game 70 Leela's eval jumped over 1 after a few moves and started climbing. The black king stayed in the center and the queens did not come off. The game reached a QRB vs QRN position on move 38 with equal material, Leela's eval was over 2. After exchanging the rooks Stockfish's dropped to 0. Leela had a queen side passer, Stockfish threatened mate and Leela gave checks until it could make a pawn move safely. The passer stopped on the 7th rank and Leela continued with checks. It could queen the pawn but the resulting QB vs Q ending would be a draw. This only delayed the inevitable, the game finally reached a tablebase draw on move 202.

Season 15 superfinal, games 51-60

After 60 games Leela leads 8-5 with 47 draws. Last 10 games were all draws, no change in the score.

Games 51-52 started with black a pawn up. In game 51 Stockfish exchanged pieces and its eval remained low. The white major pieces were active in the center and Leela had to defend the back rank. Stockfish put a rook on the 7th rank and the engines started to shuffle. On move 64 the engines started to exchange pieces and the game reached a queen ending. Leela played on and didn't lower its eval for a long time, the game ended in a tablebase draw on move 143. In game 52 the engines locked the queen side and didn't push the remaining pawns. They exchanged pieces gradually until reaching a rook ending on move 42. Stockfish went a pawn up and its eval was around 1, but the ending was a draw. The game was adjudicated only on move 144. 

All knights were off early in game 53, evals started at 1 and slowly came down. The engines kept all the pawns until move 25. The king side was blocked, the engines opened a file on the queen side and Stockfish's major pieces controlled it. After exchanging queens only RRB vs RRB remained on move 32. Leela went a pawn up and Stockfish ended the game with perpetual check. In game 54 the engines opened the queen side and center. On move 25 Leela exposed the black king, yet Stockfish's eval went down to 0 and stayed there. After exchanging queens only RBB vs RBB were left on move 34, Leela's eval came down as well. The engines reduced to a same color bishop ending, Leela kept playing until it lowered the eval for the draw rule on move 116.

Stockfish started game 55 with an eval over 1. Leela's eval clilmbed over 2 as the engines exchanged queens and Stockfish captured a rook for a knight early in the game. There were only a few piece exchanges as the engines' pawns advanced, evals came slowly down and were under 1 when the game reached a RRB vs RBB position on move 38. Leela created connected passers in the center that compensated for the material, evals dropped to 0. Pawn moves kept the game going, Stockfish gave a rook to get rid of one black passer and create two pairs of white passers on both sides. Leela gave a bishop to get rid of the pair on the king side. The game reduced to a 7-man RB vs RB position and ended by the 50-move rule on move 186. In the reverse game 56 Leela's eval remained over 1 for a long time. There were many early exchanges and the game reached a RRB vs RRB position on move 22. The engines shuffled with a few pawn moves, Stockfish's eval remained low. A series of exchanges reduced to a B vs R ending on move 85, Leela with a pawn on the 7th rank. Leela's eval came down and the game ended in a tablebase draw.

Games 57-58 started with a 22-ply book in the Benoni defense. Evals started over 1 in game 57. The engines cleared the queen side pawns in a series of exchanges, Stockfish's eval dropped to 0. The engines continued to exchange pieces and only RN vs RN were left on move 34. Stockfish was a pawn up but Leela's eval also came down. The game reached a drawn rook ending and was adjudicated. In game 58 Leela's eval jumped over 2 when Stockfish sacrificed a bishop and knight for 4 pawns. All the king side white pawns were gone and the white king was exposed and under attack. It ran to the center, Stockfish opened the queen side and threatened to add rooks to the attack. A series of exchanges stopped the attack and left a QRB vs QB position on move 35, Stockfish was 4 pawns up and Leela had only one pawn left. Stockfish's eval was 0, it gave checks and attacked the white pieces with the queen. On move 71 Leela gave material back and the game reduced to a bishop vs pawns ending, the game reached a tablebase draw.

Games 59-60 started with black two pawns up in a Danish Gambit. There were many early exchanges in game 59, only RRN vs RRB remained on move 24, Stockfish one pawn down. Stockfish pushed a pawn to h6 and placed a rook on the 7th rank, its eval dropped to 0. On move 45 the game reached a double rook ending with equal material, Leela played on and the game ended in a tablebase draw on move 90. In game 60 there were again many early exchanges and the game reached a RB vs RN position on move 23, Stockfish a pawn up and evals close to 0. Leela equalized material, the engines continued to capture pawns and the game ended in a tablebase draw on move 44.

Season 15 superfinal, games 41-50

After 50 games Leela leads 8-5 with 37 draws. Stockfish won two minimatches and Leela's lead is down to 3 points. In both wins the game was heading for a draw and the change in evals came very late when Leela was short on time. 

There were no exchanges for the first 24 moves in game 41, Leela's eval came down and Stockfish's eval stayed around 1. The engines opened a file on the queen side, then Leela gave a pawn on the king side. Stockfish had a passer but all the white pawns were blocked. The engines shuffled for a while, Leela gave a second pawn but Stockfish couldn't break the black defense. After a series of exchanges Leela regained the material and the position opened. Stockfish ended the game with perpeteual check. In the reverse game 42 there were many early exchanges and only RBN vs RNN were left on move 24. Leela's eval was around 1.5 for a while, after some shuffling the engines reduced to a rook ending. Leela's eval came down and the game ended in a tablebase draw.

The black king moved early in game 43, it was exposed on the king side but Stockfish didn't try to attack. Leela was surprised by Stockfish's move 19 and its eval jumped over 2.5, in a long series of exchanges the engines reduced to a QB vs QB position with Stockfish a pawn up.

There were a few pawns moves and exchanges, and then a long shuffle. It appeared that the game was heading for a draw, evals remained constant or slowly decreasing. Leela was getting short on time and perhaps this caused the blunder, it let Stockfish exchange queens on move 79. Leela's eval jumped over 2.5, but Stockfish saw mate after a few moves.

Leela realized it was losing much too late. There was no way to prevent Stockfish from taking the a pawn, and Leela couldn't stop both white pawns.

In game 44 the black king side was open and Stockfish castled long. Leela's eval was around 2, queens were off early and there were several exchanges until move 23. The engines started to shuffle with pawn moves resetting the 50-move counter. On move 120 the engines exchanged a pair of rooks and only RBB vs RBN remained. The next exchange was on move 163, the engines slowly reduced to a B vs N ending. The network crashed on move 201, and since it was a drawn 7-man position the game was not resumed but manually adjudicated a draw.

There were a few exchanges early in game 45, but none from move 8. The engines formed pawn lines and stayed behind them, Leela pushed a pawn to a3, Stockfish pushed its king side pawns. Evals stayed under 1, from move 40 the pawn moves stopped and the engines started to shuffle. Stockfish's eval started to increased on move 77, Leela was short on time and its moves may have been less than perfect. On move 84 pawn moves ended the shuffle,  Stockfish had a passer in the center and a knight on g6, evals were over 3.

A series of exchanges opened the position, when it was over Stockfish was up two pawns with two advanced passers.

Leela tried to counter attack with its queen and rook through the queen side. This left the black king vulnerable, Stockfish protected its king and attacked the king side. The game was adjudicated with mate in the PV. 

In game 46 Leela moved its king without castling, it pushed its king side pawns and had a space advantage. There were no pawn exchanges until move 25, Leela's eval was over 3 when Stockfish gave a pawn and the engines opened two files on the queen side. The engines shuffled for a while, on move 52 the queens were exchanged and Leela's eval was over 4.5. All the pawns were blocked, Leela gave a rook for a bishop and two pawns to create a queen side passer in a RB vs RR position. Stockfish's eval dropped, it allowed Leela to capture a pawn and found a perpetual check draw.

Evals came down quickly in game 47 and they remained low throughout the game. The engines gradually exchanged pieces and the game reached a RB vs RN position on move 35. The game was soon adjudicated. In game 48 the black king side rook moved early and Stockfish kept its king uncastled. Leela's eval kept climbing, peaking over 2. The engines exchanged several pieces and played mostly behind the pawn lines. On move 39 the pawns stopped moving, Leela couldn't find a way to improve and its eval slowly came down. The game ended with the 50-move draw.

Evals were around 1 at the start of game 49. There were no pawn exchanges for the first 23 moves, then the engines exchanged pawns and opened the queen side. By move 34 all the pawns on the queen side were gone and evals came down. The game reached a RBN vs RBN position and ended in a tablebase draw. In game 50 again the engines waited before exchanging pawns. After a long series of exchanges that ended on move 29 Stockfish was a pawn up, there were open files on both sides of the board controlled by Leela's rooks. Stockfish's eval came down to 0 as the engines shuffled for almost 50 moves. The engines exchanged the remaining pieces and reached a rook ending. Both had passers, first Stockfish gave a rook queened and then Leela did the same. Leela's eval was over 2 but the queen ending was a draw, the game was adjudicated on move 158.

Season 15 superfinal, games 31-40

After 40 games Leela leads 8-3 with 29 draws. Leela extends its lead to 5 points. Five of the last 10 games were decisive, including two openings with two white wins. Stockfish showed it can win games on its own without a Leela blunder, but only when it gets a big enough advantage that Leela can beat it in the reverse. The winning styles are very different, Stockfish goes for the kill when it can, Leela sees the win from afar and slowly strangles the opponent.

In games 31-32 black started a pawn up with a small avantage. In game 31 the network crashed again and the game was resumed. Evals stayed low, Stockfish's eval was 0 almost throughout the game. Stockfish gave a second pawn early, it had a strong bishop pair that prevented Leela from castling. The game reached a RB vs RN position on move 32 and Stockfish regained the material. Leela continued to play until reaching a tablebase draw on move 87. In game 32 Leela's eval was 0, Stockfish castled early and kept its pieces longer on the board. The engines gradually exchanged pieces without attacking. Stockfish captured two more pawns and gave a rook for a bishop, the game reached a QRB vs QBN position on move 37. Stockfish found a perpetual check several moves later.

In game 33 the engines opened the queen side and exchanged many pieces early with evals coming down to 0. After some shuffling the engines exchanged all rooks and the game reached a QB vs QN position on move 43. Leela thought it had a small advantage and played on, the engines gradually exchanged pawns until only one was left on move 89. Leela kept the 7-man position alive with pawn moves until finally reaching a tablebase draw on move 262. In game 34 again there were many early exchanges, Stockfish's eval came down to 0. All rooks were gone on move 27 and the game reached a QB vs QB position on move 33. This time Leela lowered its eval and finished the game with perpetual check.

Games 35-36 started with a 22-ply book in the Sicilian Najdorf variation, with the engines castling in opposite directions. In game 35 Stockfish pushed pawns on the king side, Leela gave a rook for a knight and opened a file on the queen side. Stockfish pushed a pawn to h6 and the engines started to shuffle on move 26. The black king walked to the center, away from the king side cage. Evals started to climb only on move 90 (!!), on move 100 Stockfish opened a file on the queen side with evals over 1.



The white king was exposed and moved away. Stockfish's attack on the queen side did not succeed. Evals started to jump when Leela countered and pushed the b pawn forward. After a series of exchages only QRB vs QBB were left and Stockfish had a passer.


Leela gave a few checks with its queen but had to block the passer. Stockfish found an opportunity to put its rook on the back rank, when it took the h7 pawn the game was over.

In game 36 Leela surprised Stockfish on move 19 with c4, opening a file on the queen side with the black major pieces facing the white king. Leela's eval jumped over 2 as it moved its knight to c6.



It took Stockfish a long while to see it was in trouble. The c6 knight was a monster and Stockfish could not force it away with pawns or major pieces. The only way was to use the black knight, and it took Stockfish many moves to get its knight to an attacking position. Leela was willing to exchange knights since this would create an advanced passer. When Stockfish tried instead to block a file with the knight Leela surprised it again and gave a rook for the black knight.



Leela got its passer and kept its knight. The white king moved forward for support, and after taking the black b pawn Leela had connected passers advancing on the queen side. Stockfish lost a bishop to capture one, the other reached a7. It was a matter of time before the game was adjudicated. Two white wins in this opening, with two very different games.

In game 37 the engines opened a file on the queen side and exchanged all the rooks. Evals slowly came down to 0 and the game was adjudicated by the draw rule on move 50. In game 38 the same file opened on the queen side but Leela kept its rooks. Its eval climbed over 2 as it opened a second file, Stockfish's eval was over 1 as the engines shuffled for a while. Leela surprised when it gave a rook for a knight on move 42.

The pawn structure looked ugly, but Leela's knight was an advantage and the black bishop could not attack the isolated white pawns. Leela captured the d6 pawn and its eval climbed over 4. The game reached a QBN vs QRB position and evals continued to climb. Stockfish gave the material back and Leela went two pawns up in a QB vs QB ending.

The ending was not trivial, Stockfish's last hope was to reach a drawn opposite color bishop ending. Leela was willing to trade queens if its queen was on c6 since this would create connected passers. In the end Leela managed to capture the b6 pawn and then exchange queens. Three pawns were enough for the win.

In game 39 the center was blocked, Leela opened a file on the queen side and moved its major pieces there. The engines exchanged a pair of rooks and Stockfish's eval jumped over 2. It saw an opportunity for a king side attack with the black queen too far to retreat. Leela saw the danger 3 moves too late.


Stockfish sacrificed a bishop and a pawn, opening the king side and threatening mate. It then followed with a rook sacrifice that forced Leela to lose a queen. The game reached a QB vs RBN position on move 31.

Stockfish gave the bishop but captured the black knight a few moves later. Leela had no defense against the white queen, after capturing a few pawns the f pawn became a passer and the game was adjudicated with mate in the PV.
A spectacular win for Stockfish.

Leela's eval jumped over 2 early in game 40. Leela pushed a pawn to h6, then in a series of exchanges the queens were off. Leela had a bishop pair and the black bishop was almost trapped, yet material was equal and Stockfish was calm.


Evals increased slowly, Leela pushed pawns on the king side and moved its king forward. Stockfish would not exchange its knight for a bishop while its own bishop was trapped. Leela opened a file on the queen side and forced a bishop for knight exchange. After exchanging a pair of rooks Leela went a pawn up.


The remaining bishops were of opposite color so Leela had to be careful to keep its rook. It managed to push the black king back and then moved the white king forward. Leela started capturing black pawns, it was 3 pawns up when the game was adjudicated. Again two white wins in this opening in two different styles.

Season 15 superfinal, games 21-30

After 30 games Leela leads 5-1 with 24 draws. Leela gets two more wins, so far the only way Leela loses is by blundering in an endgame. We have not seen a weak opening for Leela until now.

Stockfish had a small eval advantage from the start of game 21. The engines opened the king side and Leela was able to get a pawn to f3, its eval turned slightly negative. The white king walked to the center, Leela shifted to the queen side and opened a file. Stockfish attacked the back rank with a rook, after a series of exchanges only RR vs RB remained. Leela was two pawns up with a queen side passer, Stockfish closed the game with perpetual check. In game 22 Stockfish's eval was 0 from the start and remained almost constant the entire game. There were only a few exchanges after the start, the engines developed behind pawn lines. A series of exchanges that started on move 28 opened the position, both engines had passers on the queen side. Leela pushed a passer to a6 and there it was blocked. After a while the engines reduced to a QRN vs QRB position and captured all the queen side pawns. Leela continued to play and the game ended in a tablebase draw on move 97.

Games 23-24 started with a 24-ply book in a closed Ruy Lopez. In game 23 evals started around 1 and slowly came down. Stockfish was strong in the center and Leela pushed pawns on the queen side. A series of exchanges cleared most of the pawns on the queen side by move 34, by move 47 only QRN vs QRB remained. Stockfish went a pawn up and reduced to a rook ending, 3 vs 2 pawns on the king side. It took almost 50 moves for Stockfish to lower its eval for the draw rule.

In game 24 Leela's eval started at 1 and slowly increased. There were almost no exchanges until move 28, Stockfish's eval stayed under 1. A series of exchanges opened the position, material was equal yet Leela's eval was over 2, Stockfish was still not convinced.

The engines exchanged a few pawns and evals continued to increase. The game reached a BN vs BN position on move 53 and Leela went a pawn up for the first time after taking the h4 pawn. Stockfish had connected passers in the center but Leela's passers on the two flanks were more dangerous. 

The kings went to the king side and the pieces to the queen side. Stockfish was too busy defending that it could not push its passers. Leela exchanged bishops and captured another pawn, the resulting knight ending was a win for white.

Games 25-26 started with a 21-ply book in the Queen's Indian defense, Fianchetto variation. In game 25 the engines opened the queen side early and Leela was up a pawn. Evals slowly came down, the engines mainly shuffled for a while. On move 30 the exchanges resumed and Stockfish equalized material. On move 43 the game reached a QBN vs QBN position and the game was adjudicated a few moves later.

The network crashed in game 26 and it was later resumed. It started identically to the reverse game up to move 17. Stockfish was a pawn up and had a queen side passer, Leela had the bishop pair advantage and more space. The engines started to shuffle on move 20 with a few pawn moves in between. On move 38 Leela gave a second pawn, and this led to a series of exchanges and a QRB vs QNN position.

For a while evals did not change. The engines exchanged pawns and queens, all remaining pawns were on the king side except for one isolated black pawn on the king side. The black knights were too far away and busy defending their king. Leela captured the a pawn, its eval jumping over 6.5.

The white pieces were stronger than the black knights. The game was adjudicated after Leela moved its king forward.

There were many early exchanges in game 27, by move 21 only RRB vs RRN remained. Stockfish was up a pawn while Leela pushed a pawn to h3 and had a rook on the 2nd rank. Stockfish captured a second pawn but evals were close to 0, the white king was in danger and Leela's control of the 2nd rank compensated for the material. The engines reduced to a rook ending and continued to play, the game ended in a tablebase draw on move 87. In game 28 both engines captured a rook in the corner early. The white king had to move but Leela's eval was over 1.5 and increasing. The white king walked to the queen side, Leela's eval peaked over 4 and the game reached a QRB vs QRN position on move 32. Stockfish gave a pawn and created an advanced passer, its eval dropped to 0 while Leela's eval remained around 3. Leela could not improve its position, it won a second pawn but eventually lowered its eval as Stockfish found a perpetual check draw.

In game 29 the engines opened the queen side early, both kings castled short and were not involved in the game. Evals started around 1 but came down as the engines fought for control of the open files. The rooks were first to go, then the queens and the game reached a drawn BBN vs BBN position. The network crashed again in game 30 and the game was later resumed. Stockfish's eval was close to 0 throughout the game. There were many early exchanges that opened the position and led to a R vs BB imbalance. Leela was two pawns up and it thought it had an advantage, Stockfish held the position without a problem. The game reached a R vs BB ending on move 45, Leela kept playing untile reaching a tablebase draw on move 166.

Season 15 superfinal, games 11-20

After 20 games Leela leads 3-1 with 16 draws. Stockfish equalized after a Leela blunder in an endgame. Leela responded with two wins. So far only Leela has shown it can turn an advantage to a win. It is still early in the match but Stockfish does not appear to be strong enough to win.

Games 11-12 started with a 17-ply book in the French Tarrasch closed variation. Evals fell to 0 very quickly in game 11, the engines gradually exchanged pieces and Leela's eval turned slightly negative. On move 30 Stockfish gave a rook for a knight and pawn, Leela refused to exchange queens and the game reached a QRB vs QRR position. The engines captured pawns, then in a series of exchages reduced to a B vs R drawn ending.

In game 12 Stockfish's eval fell to 0 very quickly, Leela kept a small eval that stayed under 1. On move 32 Leela captured a rook for a knight, Stockfish had a passer it pushed to the 3rd rank. The game reached a QRB vs QBN position, the engines shuffled and Leela's eval slowly came down. On move 62 the queens were exchanged and Stockfish created a second advanced passer. The game was heading for a draw when suddenly Stockfish's eval jumped over 4.

Leela should have blocked the d pawn with its rook. Instead Leela had to block with the king, it couldn't take the pawn because it would lose the rook in a fork. It took Leela 6 moves to realize it was losing. The two passers were too strong and Leela couldn't stop both without losing material.

Games 13-14 started with a 22-ply book, a popular Symmetrical English variation. In game 13 Stockfish had an eval of around 1. It opened the king side but did not develop an attack on the black king. Leela opened a file on the queen side, the engines exchanged a pair of rooks and evals came down. The engines exchanged pieces until the game was adjudicated in a QBN vs QBN position. In game 14 Leela pushed pawns on the king side, Stockfish gave a bishop and cleared the white king side pawns, exposing the white king. The engines continued to exchange pieces and pawns and evals came down. On move 43 only RN vs R were left, Leela continued to play another 30 moves but Stockfish held the draw.

Game 15 started with a network crash and was restarted. Stockfish had an eval over 1 from the start, it kept its king in the center even though the engines cleared the center pawns. The black queen was very active, Leela went a pawn up. By the time Stockfish moved its king to safety evals came down. Stockfish regained the pawn, after queens were exchanged the position became very drawish with pawns only on the king side. The game ended in a tablebase draw.

Leela started game 16 with an eval over 1 that increased slowly. All the pawns remained on the board, the engines mostly played behind the pawn lines. On move 21 Stockfish remained without a knight, a disadvantage in a closed position. Stockfish's eval was under 1, only when Leela's eval reached over 3 did Stockfish start to think it was in trouble.

The black king side rook was trapped, and the c6 pawn was a long term weakness which Leela could attack with multiple pieces. The engines shuffled for a while, then Leela found a plan to improve its position. The doubled pawns on the d file had to be protected, Leela moved its king to the center so it could take the role of defender.

Leela could attack more freely with its pieces, it captured the h4 pawn, the first pawn to be taken in the game. A few moves later the queens were exchanged leaving only RRN vs RRB, Leela's eval was winning, Stockfish still wasn't convinced. Indeed, there was a very long shuffle, only on move 120 Leela went two pawns up. Stockfish finally agreed it was losing, the game was adjudicated on move 143.

Games 17-18 started with a 22-ply book in the King's Indian defense, Bayonet attack. Evals were over 1 from the start of game 17, the center was blocked and Stockfish had a significant space advantage on the queen side. Stockfish gave a pawn and opened the queen side, then moved its knights forward. Leela had two connected passers on the queen side while Stockfish put a protected knight on e6. Leela gave a rook to get rid of the white knight, simplifying the position. After some shuffling Stockfish gave the material back and captured the black passers. The engines reduced to a BB vs BN position, Stockifsh was a pawn up but could only shuffle. The game was adjudicated on move 111 with the 50-move draw getting close.

In game 18 Stockfish pushed pawns on the king side and opened a file, while Leela pushed pawns on the queen side all the way to a6. Leela's eval was over 2 after it opened a file on the queen side and exchanged several pieces. Stockfish's eval also increased slowly, it had less space and its pieces surrounded the black king defensively, trying to hold the position.

Leela applied pressure and tried to find a way to break black's position. After exchanging the LS bishops Leela gave a pawn temporarily and forced a queen exchange. Evals jumped over 5 as Stockfish's defense collapsed.

All the black pawns were on dark squares, targets for Leela's bishop. Stocksih couldn't protect them all and Leela went two pawns up. Then it was just a matter of pushing pawns forward to get the win.

In game 19 Stockfish castled long while Leela kept its king uncastled and pushed pawns on the queen side. Evals came down to 0 by move 20 and stayed there for the rest of the game. Leela pushed a pawn to a3 but could do no more on the queen side. Stockfish opened the king side but did not attack the black king. The engines continued to exchange pieces, Stockfish ended the game with perpetual check. In the reverse game 20 both engines castled long. There were many early exchanges, with RN vs RB remaining on move 25. The game reached a N vs B ending which continued for more than 50 moves before reaching a tablebase draw.

Season 15 superfinal, games 1-10

The season 15 TCEC cup is over, Leela is the winner, keeping its title from last season. In the semifinals Leela beat Houdini with two wins in 7 games, and Stockfish beat KomodoMCTS with the same score. It is interesting to note that KomodoMCTS beat its elder brother Komodo in the quarter-final by a single win in the last game of eight played. In the final Leela won the third game and Stockfish won the 8th game to even the score. Then Leela won the next game and held the draw in the reverse, winning the first tiebreak minimatch and the cup.

In the premier division Stockfish was in front, it had a better score than Leela against the lower ranked engines. However in their head to head games Leela managed to beat Stockfish once out of 6 games. Recall that last season in the superfinal Stockfish won but by the smallest of margins. It is very hard to predict the result of this superfinal and I hope we have another close and competitive match.

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After 10 games Leela leads 1-0 with 9 draws. The first 9 games were drawn, including one pair of King's Gambit games where Stockfish is considered to have an advantage.

Stockfish started game 1 with an eval close to 1. The center was blocked, the engines castled in opposite directions but the position remained closed with only one pair of pawns exchanged. Leela's eval became negative, the queen side was blocked and the engines shuffled from move 23. On move 49 Leela gave a rook for a bishop and created a king side passer, its eval jumping over 1. It pushed the king side pawns a little and after a few more piece exchanges its eval peaked over 2.5. However, Leela could only continue shufflling, in the end Stockfish gave back the material and found a perpetual check draw. In the reverse game Leela's eval was over 1, the engines exchanged pieces and the position was more open. Stockfish's eval dropped to 0 as Leela captured pawns. After exchanging all rooks Stockfish was down 3 pawns, its counterattack on the white king was strong enough to win a bishop. Leela captured all the black pawns and Stockfish forced a perpetuak check draw.

Games 3-4 started with a 30-ply book sequence in the Sicilian Scheveningen variation. In game 3 Stockfish started with a small eval advantage that fell to 0 after the engines opened the queen side. Stockfish had a queen side passer, the engines exchanged pieces until reaching an opposite color bishop ending and a draw. In game 4 Leela's eval went over 1 after it created a passer on the king side. Stockfish hid its king behind the passer and was relatively safe. The engines exchanged pieces and shuffled for a while, evals came down after Stockfish opened the center. The exposed white king gave Stockfish enough compensation, the game was adjudicated a draw in a QRN vs QRB position.

Queens were off early in game 5, Stockfish was a pawn up. The game reached a RRB vs RRB position on move 28 and the engines shuffled until adjudication close to the 50-move rule. In game 6 Leela had an eval around 1 from the start. Stockfish kept its king in the center, Leela attacked on the queen side and forced the black king to move. The game reached a R for BN imbalance on move 24, Leela attacked the exposed black king with its queen and rook. Stockfish's eval remained at 0 despite what appeared to be a scary situation. On move 40 the engines traded Q for RB and the game reached a RRB vs QBN position, Leela's eval dropped. The rest of the game was mostly shuffling, Leela managed to capture all the black pawns but not more. In the end Stockfish found a way to force stalemate on move 144.

Games 7-8 were a KGA variant. In game 7 queens were off early and Stockfish's eval was 0. There were many early exchanges, on move 32 Stockfish gave a rook for a knight and only RBN vs RRN were left, Stockfish two pawns up. Then Stockfish gave a knight for a pawn, Leela was down to one pawn and Stockfish had 4 passers on the queen side. The white passers compensated for Leela's material advantage, Leela captured two passers but could only stop the other two. After a long shuffle the engines exchanged down to a tablebase draw. In game 8 Stockfish had 2 connected passers facing the white king on move 11. Both evals were negative over 1 when the engines played out a PV agreement that started with Leela giving a rook and pawns for a bishop. Stockfish's eval dropped despite the material advantage, Leela gave another rook for a knight and attacked the black king. Stockfish gave up its passers and an exchange to get out of trouble. Leela's eval dropped and Stockfish ended the game with perpetual checks.

In game 9 the network crashed on move 20 and the game was later resumed. Stockfish had an eval advantage of around 1 from the start. Stockfish won a pawn with doubled rooks on the f file, Leela forced a queen exchange. The queen side was blocked, Leela used the open file to exchange all rooks and the game reached a same color bishop ending on move 34. Evals dropped to 0 and the game was adjudicated.

Leela had an eval over 1.5 after the opening in game 10. It opened the a file and pushed a rook to a7, Stockfish countered by pushing pawns on the queen side and creating an advanced passer. Leela won a piece, Stockfish's eval was around 1 as it thought the passer was worth the material. Then Leela offered a rook exchange on move 24:

Stocfish exchanged on c6, allowing Leela to create a passer. Leela thought this was a mistake and its eval jumped over 3, it anticipated an exchange on c8.  As expected Leela gave a knight to remove the black passer. It was up a pawn with two passers on the queen side, yet Stockfish thought it was relatively safe. It anticipated to get rid of the c6 passer through a queen exchange, and failed to see Leela's move 29:

Stockfish's eval jumped after this move. Leela gave back a pawn but kept its passers, only then the queens were exchanged. Several moves later Stockfish had to give a knight to get rid of the passers, leading to a RBN vs RB position.

The ending was far from trivial, with all the pawns on the king side. However Leela did not blunder, perhaps taking longer than was necessary. Eventually it forced a bishop exchange and Stockfish agreed the RN vs R position was a win.

Season 15 premier division statistics

A summary statistics table of previous stages and seasons. 

Draw rate, wins


Final draw rate was 75%.


Game termination

The three most common game termination causes were:
38.7% - TCEC draw rule
24.4% - TCEC win rule
20.2% - SyzygyTB

KomodoMCTS crashed once in the division.

Moves per game


Median= 69.8
Average= 80.8

There were 38 games longer than 100 moves, the longest was 217 moves (Game 112, Komodo - Leela, draw).

Time per game (hours)


Median= 2:59
Average= 2:51

Openings

There were 16-ply book openings in this stage chosen by Cato. The first letter of the ECO codes was distributed as follows:


The engines had almost no freedom to choose the opening variant. Indeed, all of the game pairs repeated the same ECO code twice, and repeated the same opening variant.

Reverse pairs, wins


Reverse pairs, same moves   


Pairs of reverse games diverged very quickly, 40.5% diverged immediately out of book, 75% of the pairs diverged at most after 1 move. The longest repeated sequence was 11 plys (Komodo - Houdini, games 134 and 162, Sicilian Kan variation, two draws).