Interesting rare bug in Sapling 1.27 revealed

[rwbc]

Note that those games were not played by me, but happened in a very new serious tournament/rating site, which tries to do what openrank
by Andrew (Ethereal/Torch) wanted to acchieve, but it never was started.
https://cope-chess.live/

Sapling tried to castle when castling rights should have gone, also no castling Rook available anymore at that side.
Evals reflect something is wrong before the illegal castling is made.
(Reproduced the evals with a release binary for the 2nd game here)
Edit: In both cases the castling Rook was killed by a pawn promoting to a Q, thus I believe Sapling just ignores that scenario?
This also explains why it is extremely rare and was never reported so far I guess.

[Event "All-Round Bullet | Series 15"]
[Site "COPE Chess"]
[Date "2026.08.08"]
[Round "416"]
[White "Altair v7.0.0"]
[Black "Sapling 1.2.7"]
[Result "1-0"]
[FEN "rnb1k2r/pppp1ppp/5n2/4p1q1/1bB1P3/2P2Q2/PP1PNPPP/RNB1K2R b KQkq - 0 1"]
[SetUp "1"]
[GameId "125175"]
[TournamentId "43"]
[GameNumber "55"]
[Termination "engine error: 63 returned illegal bestmove: e8g8"]
[Opening "EPD 456916"]
[UTCDate "2026.08.08"]
[UTCTime "15:15:34"]
[TimeControl "60+0.6"]
[PlyCount "20"]

1... Bd6 {-1.58/21 2.13s engine 63}
2. d4 {+1.25/21 2.983s engine 67} Qg6 {-1.78/22 1.923s engine 63}
3. Nd2 {+1.38/20 2.704s engine 67} Nc6 {-1.64/19 1.293s engine 63}
4. Bd3 {+1.11/23 2.543s engine 67} Be7 {-1.64/19 2.384s engine 63}
5. Nf4 {+1.53/22 1.983s engine 67} exf4 {-2.00/24 2.326s engine 63}
6. e5 {+1.28/23 1.533s engine 67} Qg4 {-1.45/23 2.27s engine 63}
7. exf6 {+1.64/18 1.199s engine 67} Qxf3 {-1.53/24 1.875s engine 63}
8. fxg7 {+1.99/20 1.119s engine 67} Qxd3 {-1.82/24 1.295s engine 63}
9. **gxh8=Q+** {+3.12/19 1.714s engine 67} Bf8 {-1.14/22 1.41s engine 63}
10. Qf6 {+3.44/20 1.853s engine 67} Be7 {+0.00/24 1.772s engine 63}
11. Qxf4 {+3.76/21 1.69s engine 67}
1-0

[Event "All-Round Bullet | Series 20"]
[Site "COPE Chess"]
[Date "2026.08.11"]
[Round "130"]
[White "Sapling 1.2.7"]
[Black "Sirius v9.0"]
[Result "0-1"]
[FEN "r1b1kbnr/pp1n1ppp/1q2p3/2pp4/5P2/1PN1PN2/PBPP2PP/R2QKB1R w KQkq - 0 1"]
[SetUp "1"]
[GameId "144102"]
[TournamentId "47"]
[GameNumber "6"]
[Termination "engine error: 63 returned illegal bestmove: e1c1"]
[Opening "EPD 2181017"]
[UTCDate "2026.08.11"]
[UTCTime "11:51:24"]
[TimeControl "60+0.6"]
[PlyCount "30"]

1. g4 {+1.24/19 1.81s engine 63} Ne7 {-0.45/26 10.677s engine 44}
2. Bg2 {+1.49/21 2.187s engine 63} h5 {-0.24/24 2.504s engine 44}
3. g5 {+1.67/21 2.407s engine 63} h4 {-0.35/22 2.113s engine 44}
4. Qe2 {+1.91/21 1.633s engine 63} a6 {-0.48/23 5.517s engine 44}
5. Rg1 {+1.90/22 2.197s engine 63} Qc7 {-0.23/22 3.059s engine 44}
6. a4 {+1.47/18 2.264s engine 63} Nb8 {-0.49/23 7.711s engine 44}
7. Nd1 {+1.43/19 1.982s engine 63} Nbc6 {-0.26/21 1.571s engine 44}
8. Nf2 {+1.65/20 1.31s engine 63} Qb6 {-0.37/22 1.647s engine 44}
9. Ng4 {+2.03/19 2.143s engine 63} h3 {-0.17/23 3.923s engine 44}
10. Bf1 {+2.11/18 1.581s engine 63} c4 {-0.40/22 2.095s engine 44}
11. Bc3 {+1.93/19 1.827s engine 63} cxb3 {+0.21/19 1.012s engine 44}
12. a5 {+1.98/19 1.109s engine 63} Nxa5 {+0.48/19 1.071s engine 44}
13. Bxa5 {+7.08/20 1.359s engine 63} b2 {+0.59/21 2.145s engine 44}
14. Qd1 {+7.04/21 1.984s engine 63} **bxa1=Q** {+0.25/24 5.058s engine 44}
15. Qxa1 {+9.90/18 1.332s engine 63} Qc5 {+0.17/22 0.731s engine 44}
0-1

Guenther (rwbc)