Why can such powerful programs commit strategic blunders?

[Kanizsa]

In the the fifty-third game on the seventeenth move,

https://tcec-chess.com/#div=sf&game=53&season=25

Leela makes a mistake 17. Bh4?, thinking that the best plane is to exchange the dark-squared bishop with 17. Bf2-h4 Bh8-f6, and 18. Bxf6.

It's a positional blunder. The correct plane is the one initially suggested by Stockfish, which is to play 17.h4! and then exchange the other pair of light-square bishops via 18. Bf1-Bh3.

After weakening the white squares, then White can play on the queenside by advancing the pawns. In my humble opinion, Leela could have won.

[pgn][Event "TCEC Season 25 - Superfinal"] [Site "https://tcec-chess.com"] [Date "2023.10.21"] [Round "53.1"] [White "LCZero 0.31-dag-e429eeb-BT3-2790000"] [Black "Stockfish dev-20231010-00263636"] [Result "1/2-1/2"] [Annotator "archive"] 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. f3 O-O 6. Be3 Nc6 7. Nge2 e5 8. d5 Ne7 9. g4 Ne8 10. Nc1 f5 11. g5 c5 12. Nd3 b6 13. Qd2 Bh8 14. O-O-O h5 15. gxh6 f4 16. Bf2 Kh7 17. Bh4 Bf6 18. Bxf6 Nxf6 19. Rg1 a6 20. Ne2 Bd7 21. Nexf4 exf4 22. Nxf4 Rg8 23. Bh3 Nh5 24. Be6 Nxf4 25. Bxg8+ Nxg8 26. Qxf4 Qf6 27. Qxf6 Nxf6 28. h4 Kxh6 29. b3 Kg7 30. Kd2 Rh8 31. Rh1 b5 32. a3 Kf7 33. Rb1 Ra8 34. Ke3 Rc8 35. Kd2 Ra8 36. Ke3 Re8 37. Ra1 Rc8 38. Rac1 a5 39. Rb1 Rb8 40. Kd2 Ra8 41. Kd3 Rb8 42. Kc3 Rf8 43. Kd2 Ra8 44. Kc3 Rf8 45. Rh2 Ra8 46. Kd3 Rb8 47. Rbb2 Nh5 48. Rhf2 Kf6 49. Ke3 b4 50. a4 Rh8 51. Rg2 Ng7 52. Rh2 Kf7 53. Rbg2 Nh5 54. Rg5 Nf6 55. Rhg2 Rh6 56. f4 Bh3 57. Rg1 Rxh4 58. e5 Bf5 59. exf6 Kxf6 60. R5g3 Rh2 61. R1g2 Rh1 62. Rb2 Rf1 *[/pgn]

[CornfedForever]

In the the fifty-third game on the seventeenth move,

https://tcec-chess.com/#div=sf&game=53&season=25

Leela makes a mistake 17. Bh4?, thinking that the best plane is to exchange the dark-squared bishop with 17. Bf2-h4 Bh8-f6, and 18. Bxf6.

It's a positional blunder. The correct plane is the one initially suggested by Stockfish, which is to play 17.h4! and then exchange the other pair of light-square bishops via 18. Bf1-Bh3.

After weakening the white squares, then White can play on the queenside by advancing the pawns. In my humble opinion, Leela could have won.

[pgn][Event "TCEC Season 25 - Superfinal"] [Site "https://tcec-chess.com"] [Date "2023.10.21"] [Round "53.1"] [White "LCZero 0.31-dag-e429eeb-BT3-2790000"] [Black "Stockfish dev-20231010-00263636"] [Result "1/2-1/2"] [Annotator "archive"] 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. f3 O-O 6. Be3 Nc6 7. Nge2 e5 8. d5 Ne7 9. g4 Ne8 10. Nc1 f5 11. g5 c5 12. Nd3 b6 13. Qd2 Bh8 14. O-O-O h5 15. gxh6 f4 16. Bf2 Kh7 17. Bh4 Bf6 18. Bxf6 Nxf6 19. Rg1 a6 20. Ne2 Bd7 21. Nexf4 exf4 22. Nxf4 Rg8 23. Bh3 Nh5 24. Be6 Nxf4 25. Bxg8+ Nxg8 26. Qxf4 Qf6 27. Qxf6 Nxf6 28. h4 Kxh6 29. b3 Kg7 30. Kd2 Rh8 31. Rh1 b5 32. a3 Kf7 33. Rb1 Ra8 34. Ke3 Rc8 35. Kd2 Ra8 36. Ke3 Re8 37. Ra1 Rc8 38. Rac1 a5 39. Rb1 Rb8 40. Kd2 Ra8 41. Kd3 Rb8 42. Kc3 Rf8 43. Kd2 Ra8 44. Kc3 Rf8 45. Rh2 Ra8 46. Kd3 Rb8 47. Rbb2 Nh5 48. Rhf2 Kf6 49. Ke3 b4 50. a4 Rh8 51. Rg2 Ng7 52. Rh2 Kf7 53. Rbg2 Nh5 54. Rg5 Nf6 55. Rhg2 Rh6 56. f4 Bh3 57. Rg1 Rxh4 58. e5 Bf5 59. exf6 Kxf6 60. R5g3 Rh2 61. R1g2 Rh1 62. Rb2 Rf1 *[/pgn]

"Positional Blunder" is far to extreme a characterization. To me it makes all the sense in the world. The Bf2 is doing nothing...the Nd3 is doing nothing...exchange the bishop and bring the Nd3 to f2 and then g4. Of course aiming for the played 21. Nef4 sac may be fine as well...but only because White was able to exchange the dark squared bishops - opening the position up like this would not be advisable if Black had elected to keep his dark squared bishop.

[Kanizsa]

In TCEC competitions these c4/d5/e4 vs. c5/d6/e5 structures have almost always been won by white. it's a big surprise that Leela didn't succeed.

[CornfedForever]

In TCEC competitions these c4/d5/e4 vs. c5/d6/e5 structures have almost always been won by white. it's a big surprise that Leela didn't succeed.

But that does not mean it was a "positional blunder".

[Kanizsa]

At depht 44
https://www.chess.com/analysis?tab=analysis

Torch is considering best line 17 h4! followed by 18. Ah3 !

17. h4 Ng8 18. Bh3 Bxh3 19. Rxh3 Nxh6 20. Rh2 Nf6 21. Rg1 Nf7 +0,79

Secondary lines are 17 Bh4 17 Be1 17 Qe2 17 a4 with similar ratings. Is there a good correspondence player here to judge whether it is better 17. Bh4 ore 17. h4 and 18 Ah3 ?

[BrendanJNorman]

In the the fifty-third game on the seventeenth move,

https://tcec-chess.com/#div=sf&game=53&season=25

Leela makes a mistake 17. Bh4?, thinking that the best plane is to exchange the dark-squared bishop with 17. Bf2-h4 Bh8-f6, and 18. Bxf6.

It's a positional blunder. The correct plane is the one initially suggested by Stockfish, which is to play 17.h4! and then exchange the other pair of light-square bishops via 18. Bf1-Bh3.

After weakening the white squares, then White can play on the queenside by advancing the pawns. In my humble opinion, Leela could have won.

[pgn][Event "TCEC Season 25 - Superfinal"] [Site "https://tcec-chess.com"] [Date "2023.10.21"] [Round "53.1"] [White "LCZero 0.31-dag-e429eeb-BT3-2790000"] [Black "Stockfish dev-20231010-00263636"] [Result "1/2-1/2"] [Annotator "archive"] 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. f3 O-O 6. Be3 Nc6 7. Nge2 e5 8. d5 Ne7 9. g4 Ne8 10. Nc1 f5 11. g5 c5 12. Nd3 b6 13. Qd2 Bh8 14. O-O-O h5 15. gxh6 f4 16. Bf2 Kh7 17. Bh4 Bf6 18. Bxf6 Nxf6 19. Rg1 a6 20. Ne2 Bd7 21. Nexf4 exf4 22. Nxf4 Rg8 23. Bh3 Nh5 24. Be6 Nxf4 25. Bxg8+ Nxg8 26. Qxf4 Qf6 27. Qxf6 Nxf6 28. h4 Kxh6 29. b3 Kg7 30. Kd2 Rh8 31. Rh1 b5 32. a3 Kf7 33. Rb1 Ra8 34. Ke3 Rc8 35. Kd2 Ra8 36. Ke3 Re8 37. Ra1 Rc8 38. Rac1 a5 39. Rb1 Rb8 40. Kd2 Ra8 41. Kd3 Rb8 42. Kc3 Rf8 43. Kd2 Ra8 44. Kc3 Rf8 45. Rh2 Ra8 46. Kd3 Rb8 47. Rbb2 Nh5 48. Rhf2 Kf6 49. Ke3 b4 50. a4 Rh8 51. Rg2 Ng7 52. Rh2 Kf7 53. Rbg2 Nh5 54. Rg5 Nf6 55. Rhg2 Rh6 56. f4 Bh3 57. Rg1 Rxh4 58. e5 Bf5 59. exf6 Kxf6 60. R5g3 Rh2 61. R1g2 Rh1 62. Rb2 Rf1 *[/pgn]

You are right.

it would be very counterintuitive for a human master to willfully trade his "good" bishop for the opponent's "bad" one with Bh4.

The h4/Bh3 plan is screaming to be played.

That being said, from my experience, there's always a reason when strong engines do weird things.

I wonder what Leela was thinking/avoiding/hoping for?!?!

[Werewolf]

In the the fifty-third game on the seventeenth move,

https://tcec-chess.com/#div=sf&game=53&season=25

Leela makes a mistake 17. Bh4?, thinking that the best plane is to exchange the dark-squared bishop with 17. Bf2-h4 Bh8-f6, and 18. Bxf6.

It's a positional blunder. The correct plane is the one initially suggested by Stockfish, which is to play 17.h4! and then exchange the other pair of light-square bishops via 18. Bf1-Bh3.

After weakening the white squares, then White can play on the queenside by advancing the pawns. In my humble opinion, Leela could have won.

[pgn][Event "TCEC Season 25 - Superfinal"] [Site "https://tcec-chess.com"] [Date "2023.10.21"] [Round "53.1"] [White "LCZero 0.31-dag-e429eeb-BT3-2790000"] [Black "Stockfish dev-20231010-00263636"] [Result "1/2-1/2"] [Annotator "archive"] 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. f3 O-O 6. Be3 Nc6 7. Nge2 e5 8. d5 Ne7 9. g4 Ne8 10. Nc1 f5 11. g5 c5 12. Nd3 b6 13. Qd2 Bh8 14. O-O-O h5 15. gxh6 f4 16. Bf2 Kh7 17. Bh4 Bf6 18. Bxf6 Nxf6 19. Rg1 a6 20. Ne2 Bd7 21. Nexf4 exf4 22. Nxf4 Rg8 23. Bh3 Nh5 24. Be6 Nxf4 25. Bxg8+ Nxg8 26. Qxf4 Qf6 27. Qxf6 Nxf6 28. h4 Kxh6 29. b3 Kg7 30. Kd2 Rh8 31. Rh1 b5 32. a3 Kf7 33. Rb1 Ra8 34. Ke3 Rc8 35. Kd2 Ra8 36. Ke3 Re8 37. Ra1 Rc8 38. Rac1 a5 39. Rb1 Rb8 40. Kd2 Ra8 41. Kd3 Rb8 42. Kc3 Rf8 43. Kd2 Ra8 44. Kc3 Rf8 45. Rh2 Ra8 46. Kd3 Rb8 47. Rbb2 Nh5 48. Rhf2 Kf6 49. Ke3 b4 50. a4 Rh8 51. Rg2 Ng7 52. Rh2 Kf7 53. Rbg2 Nh5 54. Rg5 Nf6 55. Rhg2 Rh6 56. f4 Bh3 57. Rg1 Rxh4 58. e5 Bf5 59. exf6 Kxf6 60. R5g3 Rh2 61. R1g2 Rh1 62. Rb2 Rf1 *[/pgn]

You are right.

it would be very counterintuitive for a human master to willfully trade his "good" bishop for the opponent's "bad" one with Bh4.

The h4/Bh3 plan is screaming to be played.

That being said, from my experience, there's always a reason when strong engines do weird things.

I wonder what Leela was thinking/avoiding/hoping for?!?!

I agree, unless there was something concrete in the search with the dark-square bishop trade, it looks horrible. However, of course it understands about bad bishops, so there must have been some reason.

[tissatussa]

Strange .. my SF 16 NNUE shows at depth 48 (!) that Bh4 is best move :

48 +0.87 1.Bh4 Bf6 2.Bxf6 Nxf6 3.Nf2 Rh8 4.Bh3 Kxh6 5.Bxc8 Rxc8 6.h4 Kg7 7.Nh3 Rh5 8.Ng5 Qd7 9.a3 Rch8 10.Kb1 a6 11.Ne6+ Kf7 12.Qh2 Nh7 13.Ne2 Nf6 14.N2xf4 exf4 15.Qxf4 b5 16.Ng5+ Kg7 17.Rc1 Re8 18.Ne6+ Kf7 19.Rhg1 bxc4 20.Ng5+ Kg7 21.Rc2 Reh8 22.Rh1 Rxg5 23.Qxg5 Kf7 24.Rxc4 Rh5 25.Qg3 g5 (3654.23)

48 +0.80 1.h4 Ng8 2.Bh3 Bxh3 3.Rxh3 Nxh6 4.Rhh1 Nf7 5.Rdg1 Qc8 6.Qc2 Bg7 7.Be1 Bh6 8.Qg2 Rg8 9.Kd2 Nf6 10.Bf2 Bf8 11.b4 Be7 12.Rb1 Kg7 13.Rhg1 Kf8 14.a4 g5 15.hxg5 Rxg5 16.Qf1 Rxg1 17.Qxg1 Nh7 18.bxc5 Nhg5 19.Qg2 bxc5 20.Rh1 Qa6 21.Nb2 Rb8 22.Nb5 (3654.23)

48 +0.77 1.Kc2 Ng8 2.h4 Bf6 3.Be1 Nxh6 4.Qh2 Rh8 5.Bh3 Bxh3 6.Qxh3 Nf7 7.Bf2 Qc8 8.Qxc8 Rxc8 9.b4 Kg7 10.a4 a5 11.bxa5 bxa5 12.Rb1 Bd8 13.Kd2 Kf8 14.Ke2 Ra8 15.Nb5 Kg8 16.Na3 Rh6 17.Rb7 g5 18.Rg1 Kf8 19.hxg5 Nxg5 20.Nxe5 dxe5 21.Bxc5+ Kg8 (3654.23)

48 +0.76 1.Be1 Bf6 2.h4 Ng8 3.Kc2 Nxh6 4.Qh2 Nf7 5.Bh3 Bxh3 6.Qxh3 Rh8 7.Bf2 Kg7 8.b4 Qc8 9.Qxc8 Rxc8 10.a4 a5 11.bxa5 bxa5 12.Rb1 Bd8 13.Kd2 Kf8 14.Ke2 Ra8 15.Nb5 Be7 16.Na3 Bd8 17.Kd2 Rh6 18.Nb5 Rb8 19.Ke2 Rh8 20.Na7 (3654.23)

47 +0.64 1.Qe2 Ng8 2.h4 Nxh6 3.Bh3 Bxh3 4.Rxh3 Nf7 5.a4 Bf6 6.Rg1 Ng7 7.b4 Nh5 8.Rhh1 Qc8 9.Kc2 Qa6 10.Kb3 Qc8 11.Qf1 Nd8 12.Kc2 Nb7 13.b5 Na5 14.Qg2 Qe8 15.Nb2 Nb7 16.Ncd1 Rg8 17.Qh3 Qc8 18.Qxc8 (3654.23)