How is this not a blunder?

Hi,

Here is a position in a game where my opponent made a move that directly allowed me to checkmate him. The computer analysis said it was a mistake, though. So, a move that allows a big material gain is a blunder (see here), but a move that enables your opponent to chekmate you isn't? I don't get it.

Thanks

I'm not an expert on the inner workings of the engines, and yeah, I'd call that a blunder.  My best guess is that no matter what he does, you're winning a piece, so it's not blowing an advantage or anything, just highly accelerating defeat.

EDIT:  though looking at the diagram more closely, I see that you're already down a piece.  Most curious indeed.

He did not accelerate defeat per se, as I wouldn't have been able to defeat him if he hadn't made that move.

chess.com's engine has problems.

Where's the problem in that?

c8=B and c8=Q lead to the same position one move later, yet chess.com's engine didn't realize this and claimed I blundered. I lose a queen instead of a bishop, but any modern engine should realize this doesn't make any difference.

"Blunder - you should have played c8=B."

You can't be serious, shepi13--it really said that?! LOL