Why does Fritz prefer underpromotion in this position? How can underpromotion to bishop evaluate so much higher than normal queen promotion?

Can this be a bug in the Fritz 14 chess engine?

Strange evaluation


It's because 1. exf8=Q+ Kxh7 2. Rxd7?? would be stalemate. I think it is a even a theoretical draw after 1. exf8=Q+ Kxh7 2. Qg7+. Therefore 1. exf8=B+! (with check!) is better, since white then can keep an extra piece and win easily.

[FEN "5r2/3qPbkB/8/7P/8/8/8/1K1R4 w - - 0 1"]

1. exf8=Q+ (1. exf8=B+!) Kxh7 2. Rxd7? (2. Qg7+)
  • 4
    Indeed, after 1.exf8Q+ Kxh7 2.Qg7+, it's a draw, according to the tablebases. It's the famous endgame R+p vs. B where white would be winning with a pawn on h2, h3 or h4, but not on h5!
    – Maxwell86
    Jun 18 '15 at 15:11
  • 4
    @Maxwell86 This endgame has also been discussed in an earlier question chess.stackexchange.com/questions/2801 Jun 18 '15 at 15:13
  • I don't want to seem dumb but what's wrong with kx(whatever is on f8)? I don't see how it varies either, because a queen can do all the bishop can!
    – Alec Teal
    Jun 19 '15 at 6:34
  • 2
    After 1.exf8=Q+ Kxf8 2.Rxd7 black is simply lost, that's what's wrong with it. Jun 19 '15 at 8:06

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