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Using stockfish_8_x64 with Arena 3.5.1, I see a weird pick by Stockfish. Start here: Position

White, Stockfish, to move. It plays bb6-g1,losing the bishop and the win.

Also strange: the same engine does not make this weird move in the Tarrasch program. Playing with KBN against K, I've seen Stockfish often blunder away the advantage in these endgame setups.

Any clue how this could be happening, or why it would happen only with Arena?

EDIT: SmallChess answer is correct. The problem was a residual, non-zero value in the position "number of half-moves" setting. See my reply to his posting for more information.

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    @Larry: I think you have multiple accounts; that's why you're not able to accept the answer. Some moderator should be able to merge your accounts and then you should be able to accept.
    – ShreevatsaR
    Feb 16, 2018 at 23:17
  • @ETD Can you please help?
    – SmallChess
    Feb 17, 2018 at 0:39
  • Thank you @SmallChess, I was able to resolve this on my own. I'm good at IT, not so good at chess :( BTW I downloaded your program, it is a masterwork. Thank you.
    – Larry
    Feb 17, 2018 at 3:29

2 Answers 2

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This is not a blunder, expected behaviour from the engine. Everything worked as intended. Try to copy the FEN string out, and you'll know.

Although the position looked winning, White didn't have enough moves to force checkmate before the 50 moves rule. Stockfish, knowing the position was a dead draw immediately asked for simplification.

The position you had is 100% equal, there was no way to win it. It's a draw. What Stockfish did was not wrong.

If the Tarrasch program didn't send out the 50 moves counter to Stockfish, you'd never get the problem on Tarrasch. If that was the case, it was an error in Tarrasch because the 50 move counter is part of the game.

Solution?

Setup a new board with the same position but reset the 50 moves counter.

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    Mind boggles. B+N checkmate doesn't take 50 moves so how did it get into this situation in the first place?
    – Joshua
    Feb 16, 2018 at 14:33
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    @Joshua If you played the moves on the board but unable to checkmate yourself. On your 49th move, you turned on Stockfish for advice. Too late for checkmate!!
    – SmallChess
    Feb 16, 2018 at 14:42
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    This seems plausible but how do you and the 19 upvoters know this is the case here?
    – JiK
    Feb 16, 2018 at 23:19
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    @JiK Did you see the answer below? "Apparently there was a move counter from a previous game or analysis that got somehow applied to the new position".
    – SmallChess
    Feb 17, 2018 at 0:36
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    @SmallChess. Yes, the problem I was having with Arena was that I was setting up a new position after analyzing a variation. There is a field called "number of half-moves" which is preset to the number of moves in the variation you just finished. In order to really start with a clean slate, you either need to clear that field, or delete the entire variation before doing the new setup. Thank you (stack accounts now merged!)
    – Larry
    Feb 17, 2018 at 3:24
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enter image description hereThere must be an issue with your copy of Arena 3.5.1 because Stockfish found a mate in 20. I'm using Scid for my GUI. I have attached the moves.

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  • No there was no issue. OP had been making moves, there was not enough moves left for checkmate.
    – SmallChess
    Dec 10, 2018 at 23:49

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