user30060: Is vanilla Stockfish perfectly capable of understanding Chess960 games (including the way castling works), or do you need to use something like Fairy-Stockfish to get the best results?
It seems like normal Stockfish should work, but I want to be sure.

user15223: Normal Stockfish can handle FRC positions. You just need to tell it that it is a FRC game - which shouldn't be a problem since it should be apparent from the FEN you give it.
As can be seen from the code, normal Stockfish has a "isChess960" boolean which, as its name implies, tells the engine if the position is a Chess960 position and calls the relevant functions if true.

user4418: Vanilla Stockfish uses the castling rule given in article 3.8.2 of the FIDE Laws of Chess. In that rule the king is always on the e file and the rook either on the a file or h file.
In Chess960 the files the rooks and kings can be on are different. If they are on the "correct" files then castling will work. If not then castling will not work.

30060: <p>Is vanilla Stockfish perfectly capable of understanding Chess960 games (including the way castling works), or do you need to use something like Fairy-Stockfish to get the best results?</p>
<p>It seems like normal Stockfish should work, but I want to be sure.</p>
 15223: <p>Normal Stockfish can handle FRC positions. You just need to tell it that it is a FRC game - which shouldn't be a problem since it should be apparent from the FEN you give it.</p>
<p>As can be seen from the <a href="https://github.com/official-stockfish/Stockfish/blob/02ef1f4496965b5ad8c26ac6bc18245eaffae2ea/src/position.h#L380" rel="nofollow noreferrer">code</a>, normal Stockfish has a &quot;isChess960&quot; boolean which, as its name implies, tells the engine if the position is a Chess960 position and calls the relevant functions if true.</p>
 4418: <p>Vanilla Stockfish uses the castling rule given in article 3.8.2 of the <a href="https://handbook.fide.com/chapter/E012018" rel="noreferrer">FIDE Laws of Chess</a>. In that rule the king is always on the e file and the rook either on the a file or h file.</p>
<p>In Chess960 the files the rooks and kings can be on are different. If they are on the &quot;correct&quot; files then castling will work. If not then castling will not work.</p>
