The 20th season of the Top Chess Engine Championship began on 1 December 2020 and ended on 1 February 2021. The defending champion was Stockfish, which defeated Leela Chess Zero in the previous season's superfinal. The season 20 superfinal was a rematch between the same two engines. Stockfish once again came out ahead, winning by 6 games (+14 -8 =78).[1][2][3]
TCEC underwent some major changes for season 20. There were a plethora of new engines participating in the Qualification League for the first time. Furthermore, two major new rules changes went into effect:[4]
Engines are no longer disqualified if they crash three times, although crashes are still treated as losses.
The "TCEC win rule", which stated that games are adjudicated as won if both engines showed a +10 eval or greater for 5 consecutive moves, is abolished. Instead, games are played till mate, 3-fold repetition, the 50-move rule is reached, a Syzygy[5] 6-piece tablebase position is reached, or if the "TCEC draw rule" kicks in.[6]
The decision to treat crashes differently also meant that the number of crashes was no longer used for tiebreaks. Furthermore, r-mobility was lifted to the second tiebreak condition.[4]
Finally, the advent of NNUE in the previous season led to several engines that implemented NNUE showing quantum leaps in strength. To keep the participant lineup unique, TCEC implemented guidelines for engines that utilize NNUE: one is allowed to use basic NNUE code freely, but modifications are encouraged, and all training data should be generated from the engine's own search and eval code.[4]
Qualification League
When the qualification league started, several engines had not responded to the invitations sent by TCEC. As a result, the number of promoting engines increased from the original two to four.[7][8]
The qualification league contained mostly engines playing in TCEC for the first time. New entrant Seer wound up winning the league easily, scoring 8 wins and conceding none. Fellow new entrant Halogen finished second, a point behind, after losing the head-to-head against Seer. Koivisto had the most polarized performance: in its 12 games, it won five times but crashed the remaining seven games. The result left it half a point behind A0lite and just out of the promotion spots.
Koivisto's polarized performance led TCEC to implement an exceptional ruling: although updates are not usually allowed between the lower leagues, if an engine promotes, it would be allowed to fix any crash-causing bugs.[4]
Engines play each other twice, once as white and once as black.
White \ Black
AZL
CHS
HAL
KOI
MRB
SEE
STA
A0lite
—
1–0
0–1
0–1
0.5–0.5
0.5–0.5
1–0
Cheese
0.5–0.5
—
1–0
0–1
0–1
0–1
0.5–0.5
Halogen
1–0
1–0
—
1–0
1–0
0–1
1–0
Koivisto
1–0
0–1
0–1
—
0–1
0–1
1–0
MrBob
0.5–0.5
1–0
0.5–0.5
1–0
—
0.5–0.5
1–0
Seer
0.5–0.5
1–0
0.5–0.5
1–0
1–0
—
1–0
Stash
0–1
1–0
0–1
0–1
0–1
0–1
—
Source: [9][2] Legend: Blue = left column team win; Yellow = draw; Red = top row team win.
League 4
Similar to the qualification league, the number of engines promoting from League 4 was increased to three because of the lack of response from Gogobello.[7][10]
QL winner Seer won League 4 with a dominating performance, scoring 8 wins and only losing one game. It further scored a win against 2nd-placed Weiss, as well as defeating tail-enders A0lite, Bagatur, and FabChess 2-0. Weiss remained in contention until the very last round in spite of its loss to Seer. It won 9 games, but also lost three, to finally finish half a point behind. Fourth-place finisher Monolith scored a win against Weiss and third-place finisher Tucano, but nonetheless finished behind Tucano because of Tucano's strong performance against the bottom half of the field: Tucano scored +5 against the four bottom-placed engines, while Monolith was only even. With three promotion slots only, Tucano squeezed past Monolith, half a point ahead.
Engines play each other twice, once as white and once as black.
White \ Black
AZL
BAG
CHE
FAB
HAL
MON
MRB
SEE
TUC
WEI
A0lite
—
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0–1
0.5–0.5
0.5–0.5
Bagatur
0.5–0.5
—
0–1
0–1
0.5–0.5
0.5–0.5
1–0
0–1
0–1
0–1
ChessFighter
0.5–0.5
0.5–0.5
—
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0–1
FabChess
1–0
1–0
0.5–0.5
—
0.5–0.5
1–0
0.5–0.5
0–1
0.5–0.5
0–1
Halogen
1–0
0.5–0.5
0.5–0.5
1–0
—
0.5–0.5
0.5–0.5
0.5–0.5
1–0
0–1
Monolith
0–1
1–0
0.5–0.5
1–0
0.5–0.5
—
0.5–0.5
0.5–0.5
0.5–0.5
1–0
MrBob
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
0.5–0.5
—
0–1
0.5–0.5
1–0
Seer
1–0
1–0
0–1
1–0
0.5–0.5
0.5–0.5
0.5–0.5
—
0.5–0.5
1–0
Tucano
1–0
1–0
0.5–0.5
1–0
1–0
0–1
1–0
0.5–0.5
—
0.5–0.5
Weiss
1–0
1–0
1–0
0.5–0.5
0.5–0.5
0.5–0.5
1–0
0.5–0.5
1–0
—
Source: [11][2] Legend: Blue = left column team win; Yellow = draw; Red = top row team win.
League 3
Seer vs. Weiss
a
b
c
d
e
f
g
h
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
a
b
c
d
e
f
g
h
In this Exchange Caro-Kann, Weiss had gradually outplayed Seer, but was not able to gain a decisive advantage. The two engines eventually reached this position, which is a 7-piece endgame tablebase draw.[12] However, the engines only had access to 6-piece tablebases, and Seer was further handicapped by not supporting tablebases. It blundered with 60.Nxb6+??, which simplified into a losing Rook and pawn versus rook endgame. The game was immediately adjudicated as a win for Weiss per the adjudication rules.
Wielding new NNUEevaluation functions, Minic and Nemorino finished comfortably above the competition in first and second place. The two engines tied in points - Nemorino had one fewer win, but also lost no games - and the head-to-head score was even as well. The tie was eventually broken by r-mobility, the first time this criterion was used. Nemorino wound up ahead.[13] r-mobility also broke the tie between third-placed Seer, fourth-placed Tucano and fifth-placed Combusken, this time causing Seer to promote. The three engines, along with six-placed Weiss, were very close with each other throughout the league. Seer threw away one game against Weiss (diagram), who also scored a win against Combusken. However, Weiss lost both games against Nemorino, and was only even against Counter, Topple and Pirarucu. This caused Weiss to eventually finish sixth, half a point behind its other three rivals.
League table
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
Nemorino
18
7
11
0
12.5
Advance to League 2
2
Minic
18
8
9
1
12.5
3
Seer
18
5
11
2
10.5
4
Tucano
18
5
11
2
10.5
5
Combusken
18
7
7
4
10.5
6
Weiss
18
6
8
4
10
7
Counter
18
2
11
5
7.5
8
Topple
18
4
7
7
7.5
9
Pirarucu
18
1
10
7
6
10
Marvin
18
0
5
13
2.5
Relegate to L4
Source:
League 2
After an update before the start,[7][10] League 3 winner Nemorino won League 2, scoring 8 wins and only losing one game. Second place was closely contested, with eventual third-place finisher Vajolet getting whitewashed by both Nemorino and fourth-placed Pedone, but dominating the bottom half of the table. Both Vajolet and Defenchess managed to finish a point ahead of Pedone, but r-mobility tiebreaks had to be used to decide Defenchess as the promoting engine.
At the other end of the table, in a surprising reversal of fortunes, fellow promoted engine Minic, which had finished level on points with Nemorino in League 3, suffered 6 losses with only one win to finish in the relegation zone. r-mobility tiebreaks then condemned Minic to relegation, with the third promoted engine Seer finishing just above Minic in spite of not winning any games.
League table
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
Nemorino
18
8
9
1
12.5
Advance to League 1
2
Defenchess
18
6
10
2
11
3
Vajolet
18
8
6
4
11
4
Pedone
18
7
6
5
10
5
Winter
18
5
9
4
9.5
6
Fritz
18
3
12
3
9
7
Arasan
18
4
9
5
8.5
8
Seer
18
0
13
5
6.5
9
Minic
18
1
11
6
6.5
Relegate to L3
10
Wasp
18
1
9
8
5.5
Source:
League 1
A new, high-bias[14] opening book led to a very bloody League 1, as engine after engine took turns to win an opening. By the end of the league there had been 13 game pairs in which engines traded wins. Nonetheless, there were also a lot of decisive game pairs, and every engine lost at least one opening.[15] When the dust settled Ethereal finished first, two points ahead of rofChade and Igel. rofChade lost both openings to Ethereal, but beat Nemorino and Defenchess twice. Comparatively, Igel won both openings against Ethereal, but also lost both openings to Xiphos, eventually missing out on promotion to rofChade on r-mobility tiebreaks.
At the other end of the table, Defenchess was quickly left behind, but the last relegation spot was closely contested, with only one point separating four engines. Ultimately Nemorino was relegated.
League table
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
Ethereal
28
11
13
4
17.5
Advance to Premier Division
2
rofChade
28
11
9
8
15.5
3
Igel
28
9
13
6
15.5
4
SlowChess
28
8
12
8
14
5
RubiChess
28
9
10
9
14
6
Xiphos
28
7
14
7
14
7
Nemorino
28
6
14
8
13
Relegate to L2
8
Defenchess
28
3
11
14
8.5
Source:
Premier Division
A high-bias[14] opening book, similar to that used in League 1, meant that Premier Division also saw many decisive games. However, many of the stronger engines were able to defend when they had the black pieces while winning game after game with the white pieces. Defending champion Stockfish cruised to an early lead when it not only defended with black successfully, but even won two games with black, against Scorpio and rofChade. It further won an opening against season 19 runner-up Leela Chess Zero in round two. As the division progressed, however, Leela kept up a relentless chase: at the end of the third round robin, it had won all but two games it played with white, and narrowed the gap further by winning an opening against Stockfish. Leela finally caught Stockfish in the fourth round robin when Stockfish lost with black against Stoofvlees. Behind for the first time, Stockfish kept things interesting with a streak of wins against rofChade, Scorpio, Ethereal, and the reverse game against Stoofvlees. It came down to the final game between Stoofvlees and Leela, which Leela needed to hold with the black pieces. This would have let it tie Stockfish on points, and win the division because it had better r-mobility tiebreaks. It successfully managed to draw, allowing Leela to win the Premier Division for the second time (after TCEC Season 17).
For the other contestants, traditional powerhouse Komodo played with a NNUE evaluation function for the first time. The effect was immediately apparent, as after flirting with relegation in previous seasons it finished solidly third this season, well behind Stockfish and Leela but ahead of AllieStein and Stoofvlees. Komodo was also the only engine other than Stockfish to win a game with the black pieces. AllieStein was the only engine to play without an update from the previous season, but still managed to finish fourth. Scorpio and Ethereal fought a close battle to avoid relegation. Scorpio blundered a few mate-in-ones due to an apparent bug in its code, but scored a head-to-head win, and was also able to draw two more games against AllieStein than Ethereal. It finally finished sixth, 1.5 points clear of the relegation zone. rofChade, the only engine playing without neural network-based eval, finished solidly last without winning a single opening.
League table
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
Leela Chess Zero
56
24
28
4
38
Advance to Superfinal
2
Stockfish
56
24
28
4
38
3
KomodoDragon
56
20
25
11
32.5
4
AllieStein
56
15
32
9
31
5
Stoofvlees
56
15
24
17
27
6
ScorpioNN
56
4
34
18
21
7
Ethereal
56
6
27
23
19.5
Relegate to L1
8
rofChade
56
4
26
26
17
Source:
Superfinal
Stockfish won the superfinal. Jeroen Noomen, who writes the opening book used, decided to order the openings such that the most balanced openings (as determined by older versions of Stockfish/Leela on weaker hardware)[16] are played first. As a result, the first half of the superfinal saw draw after draw, with Stockfish narrowly in the lead after winning games 33 and 49. At the start of the second half, Leela narrowed the score by winning games 52 and 56. There followed a tense period where the superfinalists traded wins, but beginning from game 71 Stockfish scored six wins without reply to take a big, and as it turned out decisive, lead. Stockfish mathematically won the superfinal after drawing game 95. The final score was +14 -8 =78 in favor of Stockfish.[17]
Notable games
In an article published shortly after the superfinal, GM Matthew Sadler called the superfinal a 'captivating' affair which Stockfish won because it was better at defending inferior positions. The following are selected from the many games analyzed by GM Sadler.[18]
Games 5/6
Games 5/6 book exit
a
b
c
d
e
f
g
h
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
a
b
c
d
e
f
g
h
The book exit position. White has a very dangerous attack after 13.Nh3 Re8 14.Qh4!, but it took older versions of Stockfish a lot of time to appreciate White's attacking potential after Bxg6 followed by doubling on the h-file with Queen and Rook. 14...Bf3 15. Rg1 e5 16. Be3 threatening Ng5 and White won material.
Because the openings were ordered in ascending bias, it was very unexpected for an opening pair this early to yield a 1-0, 1-0 result. In analyzing the book exit position (diagram), GM Sadler found that older versions of Stockfish could not tell how dangerous White's position is until it had searched to a high depth.
Position after 15.Bc3. Giri–Tari continued 15...Bf6, while Stockfish played 15...Qd6.
This game featured a topical line of the Ruy Lopez Berlin. A few days prior to this game being played, Anish Giri had won an impressive game against Aryan Tari at the Tata Steel Chess Tournament 2021 in this very line.[19] In this game however, Stockfish had no trouble defending with Black, making for an important theoretical game.
Position after 24...Bd7. White's position is clearly preferable, but it is not obvious how to make progress.
Stockfish drew first blood in this game, winning with White and holding the reverse. After 24 moves (diagram) the position was looking rather locked. Stockfish was spending most of its time on f5, while Leela predicted White will play b4-b5, neither being a plan that looked very convincing to human eyes. Stockfish evidently agreed that f5 would not make progress and started shuffling, while Leela forced things with 34...a4 followed by 35...Nb3. This pawn sacrifice gave Stockfish a queenside majority that it eventually converted into a win.
Position after 78...Kh8?! This move places the King on a vulnerable square opposite White's rooks. After 79. Bd3 Bb5 80. Bc2 Bd7? 81. Nf5! crashes through. From afar Leela had counted on 81...gxf5 82. g6 f4 83. Qg2 Rfg8, pinning the pawn and seemingly grinding White's attack to a standstill, but 84. Qg5! breaks this defense with decisive material gains.
Stockfish won its second opening after Leela missed a crucial tactical sequence (diagram). In human terms, it tunnel-visioned at a single move, then played quickly when its opponent responded in an unexpected way, completely missing its opponent's upcoming reply.[20]
Position after 14...f5. Leela played 15.b4, a move that GM Danny Gormally opined no human would ever consider. The point is that after 15...Nxb4 16. h6 Bh8 17. Bxc5 dxc5 18. a3, if Black plays 18...Na6, its knight suddenly has no scope.
In one of the best games of the superfinal, Leela plays the inhuman move 15.b4!? with a long-term plan to blockade Black's minor pieces. Stockfish sacrificed a piece for four pawns to break the blockade, but Leela successfully played against the weaknesses this manoeuvre created to convert the endgame.
Position after 28. Qxd5. White threatens Ra7 and Qf7 to crush Black's second rank.
Game 59
a
b
c
d
e
f
g
h
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
a
b
c
d
e
f
g
h
The key position which Stockfish had evaluated many moves earlier as winning. The point is that both of Black's pieces are stuck (or White moves the Rook intending Rc8), while Black can never play ...Ke7 because Nc6+ wins or ...Kh6-g5 because Nf7+ wins. As a result, Black slowly runs out of moves.
This opening was the Taimanov variation of the Modern Benoni, a line known to be difficult for Black. Nonetheless, Stockfish was able to hold the reverse game successfully while out-assessing Leela to win this game. Both engines saw the same main line after 28. Qxd5, but Stockfish evaluated the key position after 37. Ne5 as a cast-iron win, an evaluation that turned out to be correct.
Position after 21...Bf6. Leela anticipated completing its development with 22...Bb7, but Stockfish stopped that plan with 22. a3, threatening axb4 if Black plays ...Bb7.
Game 77
a
b
c
d
e
f
g
h
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
a
b
c
d
e
f
g
h
Position after 29...gxh6. With the attack raging, Stockfish takes time off to play 30. Kb2, avoiding ...Qf4 ideas with queen trades.
In a powerful attacking game, Leela opened lines against its king in an effort to complete its development, and Stockfish sliced through.
Position after 21. Rb3. Having earlier played 19. a5 to stop Black from opening lines on the queenside, Leela now prevents ...c4-Nd3 while simultaneously freeing its c1-bishop to move. Optically the a5-pawn is vulnerable to ...Qxa5, but it turns out that if Black dares to play that move, White can unleash a devastating kingside attack while the Queen is deflected.
In this game featuring the Ruy Lopez Zaitsev variation, Leela comes up with an unusual plan to stop Black's queenside play while simultaneously pursuing its kingside attacking ambitions (diagram).
Nolot is a chess test suite with 11 very difficult positions from real games. They were compiled by Pierre Nolot for the French chess magazine Gambisco and posted on the rec.games.chess Usenet group in 1994. Some of these positions were particularly hard to solve for chess engines at the time.
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The English Defence is a chess opening characterised by the moves:
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The 16th season of the Top Chess Engine Championship began on 15 July 2019 and ended on 13 Oct 2019.
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↑ This draw rule adjudicates the game as drawn if, at Black's move 35 or later, the evals from both playing engines are between [-0.15, +0.15] for 10 plies without a pawn advance or capture.
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