What about moves validity, and detection of mate/stalemate? Would it be ok that the program assumes, that the opponent is playing legal moves only? Also, should

What about moves validity, and detection of mate/stalemate? Would it be ok that the program assumes, that the opponent is playing legal moves only? Also, should it be required to detect mate and stalemate, and stop playing at that point? Also, there is a 50-moves rule and 3rd position repetition rule, but I believe this doesn't need to be implemented, since you are not required to invoke the rule.