It is known what the fastest stalemate is. But what but is the fastest possbile stalemate in which castling must be the last move?

33 ply is my best result so far.

[FEN ""]

1. e4 h5 2. Qxh5 d6 3. Qxh8 Bh3 4. Qxg7 Bxg2 5. Qxg8 Bxf1 6. Qxf7+ Kd7 7. Qxf8 Kc6 8. Qxd8 e5 9. Qxd6+ Kb5 10. Qxc7 Bd3 11. Qxb8 Bxc2 12. Qxa8 Bd1 13. Qxa7 Kc4 14. Qxb7 Kd3 15. Nf3 Kc2 16. Qb3+ Kxc1 17. O-O

Castling for a pin isn’t the only possible motivation though. Here is another 33 ply sequence that shows motivation in protecting pieces and guarding squares.

[FEN ""]

1. f4 e5 2. fxe5 d6 3. exd6 Qg5 4. dxc7 Ke7 5. cxb8=Q Qxg2 6. Qxc8 Qxh2 7. Qxa8 Qxg1 8. Qxa7 g5 9. Qxb7+ Ke6 10. Qxf7+ Ke5 11. Qxf8 Qh2 12. Qxg8 Qh5 13. Qxh8+ Kf4 14. Qxh7 Kg4 15. Bg2 Kg3 16. Qxh5 g4 17. O-O
  • In the second example, castling doesn't need to be the last move, as Kf1 would work. – D M May 22 at 11:19

I’ve found a way to reduce the count down to 31 ply in the time since posting my question.

[FEN ""]

1. f4 e5 2. fxe5 d6 3. exd6 Qg5 dxc7 Qxg2 5. cxb8=Q Qxh2 Qxa7 Qxg1 7. Qxa8 Qg2 8. Bxg2 g5 9. Qxb7 g4 10. Qxc8+ Ke7 11. Qxf8+ Ke6 12. Qxf7+ Ke5 13. Qxg8 Kf4 14. Qxh8 h5 15. Qxh5 Kg3 16. O-O

Since White has only two non-capturing moves-the first pawn move and castling and Black uses all of their moves without a waiting one, I think that 31 ply may be optimal, unless some way to shave off a pawn capture can be down.

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