Welcome to my secret message.
One thing we weren't sure about was how much the person has to say what they intend to do before you can interrupt. Like if they had the spell that lets you swap Future Cards, do they have to say which cards they are going to flip before you interrupt? For the one that lets them buy 3 more potions than what is allowed, do they have to say which 3?
Yeah, the intent is that they have to say exactly what they're doing with the spell.