FEN extensions - ddugovic/Stockfish GitHub Wiki

Some variants require an extension of the FEN standard. In the following, the formats that are supported by Stockfish are summarized:

Crazyhouse

The pieces in hand are stated in square brackets after the board representation with no space in between, e.g., rnb1kbnr/ppp1pppp/8/3q4/8/8/PPPP1PPP/RNBQKBNR[Pp] w KQkq - 0 3, where white and black each have one pawn in hand. The starting position is represented by rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR[] w KQkq - 0 1. The same format is used for loop chess and bughouse. The lichess format, where pieces in hand are separated by a slash, e.g., rnb1kbnr/ppp1pppp/8/3q4/8/8/PPPP1PPP/RNBQKBNR/Pp w KQkq - 0 3, is supported on the input side, but the output format always is with square brackets.

Three-check chess

The number of checks required to win for white and black, separated by a plus sign, are stated after the en passant field, e.g., rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 3+3 0 1 for the starting position. Inputs in the scid/lichess format, e.g., rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 +0+0, are supported, too.