Shortest-proof-game challenge

@6352

Is the diagram upside down or not?
Is the white king at b1 or at g8?

The white king is at b1, according to the coordinates shown in the diagram. You said the last move must have been a promotion on g8. EvinSung tried to tell you, using the fewest possible number of words, that you overlooked the possibility of the white bishop being original and the last move being cxb5#.

I wonder who will be first to find a 25-move or shorter proof game for this fairly simple position.

@6354

So the position is:

Yes. EvinSung is constrained by a tight quota on words posted per day and thus is forced to make very short posts, which are sometimes cryptic.

A start:

18.5 moves

I had a hunch daStrwbrry would be first, and will likely be able to shorten the 50.5 move game I found for this position.

After this is taken care of, the next challenge will be to shorten daStrwbrry's game at #6359 to the SPG, 17.5 moves.

Indeed I do shorten your 50.5. Was stuck on 45.5 moves for quite a while, but finally found 44.5:

I feel like 43.5 moves could be possible though since there is a wasted move with the bishops.

Can’t find this 17.5 move SPG yet. I’ve tried various ideas such as no black promotion to rook promotion, but just got more 18.5s. What am I missing?

If no one finds it, I'll post a proof game. But I'll give a hint which I hope doesn't make it too simple. Both missing trapped bishops might not have been captured by knights.

I do not like typing

After several failed attempts, I finally found a 17.5 mover! That was an incredibly tough challenge for me. Capturing the stuck bishops was one of the hard parts to optimise, and balancing the white moves and black moves was tricky too.

Another “avoid a draw by insufficient material” position, this time with more bishops.

Yknow what, have another bishop position.

Good work, and you didn't need my hint about non-knight captures, which might have led you to this version.