Weird Castle

As a chess960 enthusiast, I like this.

However, fun fact: in this specific position, 0-0 is impossible in chess960 as well, because how in the world would the d8 rook have moved from a8 or b8 without the king moving? Pawns can't go backwards either, so the a-side rook should still be trapped. (Remember king has to start somewhere between rooks.)

Anyway, yeah, we should be able to set up impossible positions.

Bug: Lichess uses KQkq for castling rights in Ches960 FENs even in positions where there are multiple rooks on one side of the king. Which rook is the original king's rook?

What #12 said is a typical problem for 960 FENs, which I've read about somewhere, maybe in this wikipedia article:

en.wikipedia.org/wiki/Chess960#Coding_games_and_positions

"The solution implemented by chess engines like..."

It's the solution implemented by chess engine communication protocols like UCI (used by most engines these days, including Stockfish) and Winboard (the other main protocol).

#11 Technically the rook on d8 (or on h8!) could be an underpromoted pawn.

#12 Agreed, the UI has a bug, although I think X-FEN input does work.

#14 Nice to know, thanks. Lichess could solve the bug like that, right?

#15 You are right, the position with right to castle is actually possible because of underpromotion :D and some white minor piece having snapped off the original rook from a8 or b8.