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Here's a chess puzzle that gave a confusing response. https://chesspuzzle.net/Puzzle/52112

[FEN "5R2/1p6/p4Ppk/3r4/7p/1P6/P7/1K6 w - - 0 1"]

1. Rh8+ Kg5 2. f7 Rf5 3. f8=R

If you play f8=Q in this final position, you will not fail the puzzle, but be told "There is a better move..." (f8=R).

It seems inevitable that the black captures the promoted piece, and the result is the same after white recaptures.

Incidentally, f8=Q was played in the game. https://www.chess.com/games/view/15563079

What is more curious, is that I have played other puzzles on this site that allow multiple moves in a position for continuation, and my assumption therefore was that it only suggests a "better move" when your move doesn't change the theoretical outcome of the game, but gives significantly lower advantage (i.e. still decisively winning, but slower). This would suggest that chesspuzzle.net has evaluated f8=Q to be an inaccuracy of some sort.

Is there any reasoning to be found for this? Or is it just a strange computer evaluation?

I have tried inputting the position on the analysis board on lichess.org, and it flip flops between the two moves as the depth builds, but eventually starts showing f8=Q with a consistent advantage. (The opposite of chesspuzzle.net)

This seems strange also, as in my perhaps naïve reasoning explained initially, both moves should be identical.

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  • It's not a better move. Both are completely winning and the site you're solving puzzles in probably allows for only one correct solution (which should be f8=Q by the way)
    – David
    Nov 4 '20 at 7:07
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    If you promote f8=Q then you lose a queen after Black's next move; if you promote f8=R then you only lose a rook :) Nov 4 '20 at 20:24
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If anything, promoting to a queen is the better choice, as it forces Black to exchange their rook. For promoting to a rook, Stockfish NNUE actually slightly prefers moving the f5-rook away instead of playing ...Rxf8 (although any continuation gives White a large winning advantage). So at the very least Black is given a choice.

But even if we were certain it's best for Black to play ...Rxf8 if White promotes to a rook, there would still be no reason to say that promoting to a rook is better than promoting to a queen - both lead to the same outcome. What possibly happened is that the site's computer calculated one line further/differently, and arrived at an evaluation that was higher in the line for promoting to a rook. Although since you said the site allows multiple solutions to sometimes be accepted, this seems like a bug in their software.

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    Other than avoiding a stalemate, I can't think of a good reason to choose a rook over a queen
    – corsiKa
    Nov 4 '20 at 19:37
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    There are positions where the best move is a Rook promotion to get stalemated! (The Rook ends up pinned or entombed, while a Queen would be able to move.) As far as I know this has never happened in actual play, though a few composed studies with this feature look quite gamelike. Of course in the present case there's no reason to prefer f8=R to f8=Q. Nov 4 '20 at 23:09
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For an engine, it makes no difference if you promote to a rook or a queen. In both cases, your advantage is huge. There is no real resistance for black, but his best chances, if any, are to give his rook for the promoted piece. 3.f8R Rxf8 and 3.f8Q Rxf8 result in the same position. No difference.

But chess puzzles have their own etiquette. And one of the unwritten rules: the less the value of the promoted piece, the more beauty. Underpromotion to a knight or a bishop would not add more beauty but are a blunder, because the black passed pawns are strong now. So a rook or queen it has to be. Both are winning, and the underpromotion is in puzzle terms the „better“ move.

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    I doubt that the 'chess puzzles have their own etiquette' is relevant in this case. The observation is true in general for composed problems, which this almost certainly is not. Also, unless the selected engine has been configured for solving chess compositions
    – user24765
    Nov 4 '20 at 6:15
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    ... it seems more likely that the engine used by this site find a difference between the two variations, and bases its suggestion on that.
    – user24765
    Nov 4 '20 at 6:36
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    If anything, the chess puzzle etiquette should be that there is only one winning choice, and if there are several, then it's not a "true" puzzle (or your own analysis is wrong). GM Ben Finegold said as much in this video, when analyzing "the hardest mate in 1 puzzle ever". The analysis of the move in question starts at about 31:30
    – Arthur
    Nov 4 '20 at 8:21
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It seems as if the puzzle is based on an actual game played between Jorden van Foreest and Martin Petrov in April 2019.

If you click on the button to show the solution, it shows the full game, of which the puzzle is but a fragment. It looks to me as they want you to solve the puzzle by following the game as played, not by coming up with another solution.

The curious thing is that 365Chess.com has the promotion as a Queen, while ChessPuzzle.net has it as a Rook. Analysis on Chess.com puts promotion to a Queen at 0.94, while promotion to a Rook is 0.96, even though Black's next move is Rxf8 in both cases.

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    Not always -- sometimes there's a combination that was missed in the game, or the ChessPuzzle site gives a different defense from what happened in the game (or continues where the actual game ended by resignation or draw agreement). Nov 4 '20 at 23:12
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It is a quirk/bug of the software used by that site to pick moves expected in puzzles.

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