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I was analyzing a game on chess.com and it showed me that, in the below scenario, it is better to promote to a rook instead of promoting to a queen. It also shows that it has a mate in 6 with the queen, but it also a mate in 4.

I am not sure why chess.com thinks it is mate in 6 and why the rook move is better. I am only ~1200 so I could be missing something obvious. Do you have any ideas?

The game can be seen below, or here on chess.com.

Chess.com analysis

[fen "2k3r1/ppp4P/3rp1Q1/8/3P4/4q3/PPP5/1K1R2R1 w - - 0 1"]
1. d4 d5 Bf4 Nc6 Nf3 Bf5 e3 Nf6 Bd3 Bg6 Nc3 e6 Ne5 Be7 h4 Nxe5 Bxe5 Qd7 g4 Bxd3 Qxd3 O-O-O g5 Ne4 Nxe4 dxe4 Qxe4 Bd6 Bxg7 Rhg8 Bf6 Be7 Bxe7 Qxe7 O-O-O Qd6 Qxh7 Qa6 Kb1 Qd6 Qxf7 Rgf8 Qh5 Rxf2 g6 Rff8 g7 Rg8 Rhg1 Rd7 Qh8 Rdd8 Qh7 Qd5 h5 Qf3 h6 Qxe3 Qg6 Rd6 h7 Rgd8 g8=Q Rxg8 hxg8=Q+ Rd8 Qxd8+ Kxd8 Qh7 Qc1+ Rxc1 c5 Rg8#
3
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    Seems like the analysis engine prefers to throw away less material, but promoting to a queen and exchanging it for the rook on d8 leads to the same mate. – Maciej Stachowski Mar 29 at 11:33
  • Stockfish engine depth issues. More time, higher depth it will see better. – SmallChess Mar 29 at 11:45
  • @MaciejStachowski The engine line 34. Q6e8 also attempts to sacrifice a queen for the rook and if it had the clear choice between #6 and #4, the engine would pick the #4 no matter the material. But it just doesn't see the #4 yet - likely unfortunate pruning at work (which will be fixed by going to higher depths as SmallChess notes). – Annatar Mar 29 at 12:55
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There's no reason why the rook promotion is better. Don't listen to engines in positions where there's such a big imbalance

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Chess engines prefer to checkmate you as soon as they can, and if they can checkmate you in 10 moves, they look if they can also checkmate you in 9 moves. That's also why chess.com thinks rook promotion is better, because it saw a sooner mate. But if you see a mate in 3 and don't see the mate in 1, just play the mate in 3

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