Yes, totally agree. I think we could be lenient on moves validity - let's accept that the program will work correctly only if the opponent plays legal moves, an

Yes, totally agree. I think we could be lenient on moves validity - let's accept that the program will work correctly only if the opponent plays legal moves, and it also doesn't need to castle. Maybe it is also not necessary that it will detect mate or stalemate, we can assume that some external arbiter will end the game. But what I think a chess program absolutely needs to implement to be called a chess program is: - should be able to play with both white and black - support all normal moves obviously - must allow opponent to play any legal move (castling, en passant, promote pawn to any figure) - must not keep its own king in check