ToledoChess [0] has a few implementations of this in different languages. Some highlights: 2KB of JavaScript with castling, en passant, promotion, search and e

ToledoChess [0] has a few implementations of this in different languages. Some highlights: 2KB of JavaScript with castling, en passant, promotion, search and even a GUI 326 bytes of assembly, without the special rules I don't think the author has a UCI-compliant one, but it should be easier than the GUI. There are forks of the JS one that might do it. [0] https://nanochess.org/chess6.html https://nanochess.org/chess6.html