Stockfish 15.1 crashes for: position fen 3qk1nb/1p3p2/p1r1p1p1/3p4/3P4/2N1BP2/PPPQ1P2/R3K3 w Qk - 0 15

[Mercutio1243]

Stockfish 15.1 crashes for: position fen 3qk1nb/1p3p2/p1r1p1p1/3p4/3P4/2N1BP2/PPPQ1P2/R3K3 w Qk - 0 15

[Disservin]

Your fen isn't a valid FEN. position fen 3qk1nb/1p3p2/p1r1p1p1/3p4/3P4/2N1BP2/PPPQ1P2/R3K3 w Qk - 0 15 Black cannot castle king side. There's a bishop in the corner not a rook.

[Mercutio1243]

Thank you, this is a good pointer!

Why is Stockfish then accepting the following fen:
position fen 2rqk1nb/1p3p2/p1B1p1p1/3p4/3P4/2N1BP2/PPPQ1P2/R3K3 b Qk - 0 14

[Disservin]

Probably because the king is in check and chess rules say we cant castle, so we dont try to generate the move and after the king moves we clear all castling squares for the king.

[PGG106]

How the engine reacts to illegal fens should be considered undefined behaviour, you can't rely on the engine consistently saving itself from a crash.
If you want fen validation your best bet currently it to validate the fen before passing it to sf.

[Mercutio1243]

Ok, thank you very much for the super fast response! I fully agree, the error is with the generation of the fens that I am using (not your problem).

For future reference below all the previous fens from that game that I am using were also faulty but did not result in an error. But again, not your issue as PGG106 is pointing out (but maybe interesting for someone else at some point).

rnbqk1nr/ppp1ppb1/6p1/3p4/8/2N5/PPPPPPP1/R1BQKBNR w KQkq - 0 5 -- here still okay
rnbqk1nR/ppp1ppb1/6p1/3p4/8/2N5/PPPPPPP1/R1BQKBN1 b Qkq - 0 5 -- here the rook gets taken but castling rights are preserved for black
rnbqk1nb/ppp1pp2/6p1/3p4/8/2N5/PPPPPPP1/R1BQKBN1 w Qkq - 0 6
rnbqk1nb/ppp1pp2/6p1/3p4/8/2N1P3/PPPP1PP1/R1BQKBN1 b Qkq - 0 6
rnbqk1nb/pp2pp2/6p1/2pp4/8/2N1P3/PPPP1PP1/R1BQKBN1 w Qkq c6 0 7
rnbqk1nb/pp2pp2/6p1/2pp4/8/2N1PN2/PPPP1PP1/R1BQKB2 b Qkq - 1 7
r1bqk1nb/pp2pp2/2n3p1/2pp4/8/2N1PN2/PPPP1PP1/R1BQKB2 w Qkq - 2 8
r1bqk1nb/pp2pp2/2n3p1/2pp4/3P4/2N1PN2/PPP2PP1/R1BQKB2 b Qkq d3 0 8
r2qk1nb/pp2pp2/2n3p1/2pp4/3P2b1/2N1PN2/PPP2PP1/R1BQKB2 w Qkq - 1 9
r2qk1nb/pp2pp2/2n3p1/1Bpp4/3P2b1/2N1PN2/PPP2PP1/R1BQK3 b Qkq - 2 9
r2qk1nb/pp2pp2/2n3p1/1B1p4/3p2b1/2N1PN2/PPP2PP1/R1BQK3 w Qkq - 0 10
r2qk1nb/pp2pp2/2n3p1/1B1p4/3P2b1/2N2N2/PPP2PP1/R1BQK3 b Qkq - 0 10
2rqk1nb/pp2pp2/2n3p1/1B1p4/3P2b1/2N2N2/PPP2PP1/R1BQK3 w Qk - 1 11
2rqk1nb/pp2pp2/2n3p1/1B1p4/3P2b1/2N1BN2/PPP2PP1/R2QK3 b Qk - 2 11
2rqk1nb/pp3p2/2n1p1p1/1B1p4/3P2b1/2N1BN2/PPP2PP1/R2QK3 w Qk - 0 12
2rqk1nb/pp3p2/2n1p1p1/1B1p4/3P2b1/2N1BN2/PPPQ1PP1/R3K3 b Qk - 1 12
2rqk1nb/pp3p2/2n1p1p1/1B1p4/3P4/2N1Bb2/PPPQ1PP1/R3K3 w Qk - 0 13
2rqk1nb/pp3p2/2n1p1p1/1B1p4/3P4/2N1BP2/PPPQ1P2/R3K3 b Qk - 0 13
2rqk1nb/1p3p2/p1n1p1p1/1B1p4/3P4/2N1BP2/PPPQ1P2/R3K3 w Qk - 0 14
2rqk1nb/1p3p2/p1B1p1p1/3p4/3P4/2N1BP2/PPPQ1P2/R3K3 b Qk - 0 14
3qk1nb/1p3p2/p1r1p1p1/3p4/3P4/2N1BP2/PPPQ1P2/R3K3 w Qk - 0 15 -- here stockfish 15.1 crashes

Feel free to mark the issue as closed