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Its a bit of a wordy question as there is a lot of background to provide context for thinking and analysis so for that I apologise you can skip the background if you want but it might provide some additional context that might be useful.

The Background: Came across an interesting position today which has resulted in me going down a rabbit hole of analysis.

Generally Whites position is looking reasonably strong and is up in material. The d6 pawn is safe from capture because mate would quickly ensue with Rxc8. The black c8 bishop is out of play as long as Black doesn't move his king. The biggest issue is for White at the moment is the knight on g5. It is preventing Blacks king escaping through f7 but otherwise is fairly useless in this position.

The position arises on move 30 with Black playing h6. The knight having no escape squares is going to fall. To make matters worse the knight has no capture possibilities or defence possibilities.

Ultimately I played Ne6 , the pawn took leading to promotion and ultimately Blacks resignation. I decided to do some analysis on this position and found that the engines really didn't like this position and repeatedly provided the wrong moves unless until they were shown the move. I could clearly see Ne6 in the lines but for some reason it would suggest Raa8 over Ne6 but the second Ne6 was played it would read that as the better and stronger move.

Question: Why do some engines fail to locate best move options even though it analyses the position and only updates it as the correct move after it has been played? Is this common in NNUE based engines?

Example: Komodo Dragon, Stockfish 12 and Lc0 all saw 2. Raa8 (Scenario B) as a better move than Ne6 (Scenario A) until it moved passed that move and then saw Ne6 as the stronger move

Scenario A

[Date "Nov 2020"]
[White "Yours Truly"]
[Black "Random Hero"]
[Result "1-0"]
[Event "Scenario A"]
[FEN "1Rb3k1/3p2pp/2rP4/4p1N1/4P3/5P1P/6P1/R5K1 w - - 0 30"]

1. h3 h6 2. Ne6! dxe6 3. d7 Kf7 4. dxc8Q Rxc8 5. Rxc8 1-0

Scenario B

[Date "Nov 2020"]
[White "Yours Truly"]
[Black "Random Hero"]
[Event "Scenario B"]
[FEN "1Rb3k1/3p2pp/2rP4/4p1N1/4P3/5P1P/6P1/R5K1 w - - 0 30"]

1. h3 h6 2. Raa8 hxg5 3. Rxc8+ Rxc8 4. Rxc8+ Kf7
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  • Additional note. I recognize after doing analysis that 1.h3 loses a forced mate sequence.
    – Dheebs
    Nov 13, 2020 at 4:59
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    I use NNs to analyse variants and I have to say it happens a lot that they miss a move. I also remember Fabiano Caruana saying that working with NNs is hard and you need to question them a lot, as they will give you amazing lines, but then miss mate in 3.
    – B.Swan
    Nov 13, 2020 at 9:36
  • I don't think 2. Ne6 is actually stronger than 2. Raa8. The main point is that both moves lead to positions that are completely and utterly and irretrievably won for White. Whether an engine calls one of them +11 and the other +15, or the other way around, is irrelevant. (But anyway, Black doesn't particularly need to play 2 ... dxe6 after 2. Ne6, do they? That looks like the quickest way to lose. Black has a lot of other moves that resist for longer, and then the position is a bit more messy than the clean position you give at the end of your 2. Raa8 line.)
    – James Martin
    Nov 13, 2020 at 9:37
  • @B.Swan or a perpetual, this is always something you have to hunt around for and verify on your own (or with SF or something- both is always good)
    – pulsar512b
    Nov 16, 2020 at 4:43

2 Answers 2

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This is just an issue every engine has (although they may miss different moves). In this case, it just pruned (ie, didn't consider/didnt consider enough) Ne6. It's not common per se, but has to be kept in mind

Also note Lc0 is not NNUE, it is just a normal neural network not NNUE which is special.

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I think NNUE engines are right because after Ne6 black can play Rxd6 instead of taking knight(that exchanges all of his major pieces). he loses bishop after Rxd6 but his rook will remain and he has some chance to go on even if that is 1%. but after Raa8 hxg5(the only move because khight on g5 supports king's scape way and there is checkmate threat) Rxc8 Rxc8 white will have a rook but black won't have any major piece and all of his remaining pieces will be pawns and king so his winning chance is less than the previous line I think

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