At what percentage would you allow an AI to consider a game unwinnable? While an AI that behaves erratically when the odds are low might be worth allowing it to be considered forfeit worthy, but the thing about humans is we make mistakes. Therefore an ideal AI that can continue to execute reasonable moves should have a lower percentage threshold where it decides to forfeit. See this match[0] for an example of a spectacular comeback that I feel an AI might have considered forfeit worthy if not well defined.