Arcane Proxy

If another effect causes Arcane Proxy's power to be less than the mana value of the target card as the ability tries to resolve, the target is illegal. The card won't be exiled, and you won't get to cast a copy.

If an effect copies a prototyped spell, that copy (as well as the token it becomes on the battlefield) will have the same characteristics as the prototyped spell. Similarly, if an effect creates a token that's a copy of a prototyped permanent or causes another permanent to become a copy of it, the copy would have the same characteristics as the prototyped permanent.

When casting a prototyped spell, use only its prototype characteristics to determine whether it's legal to cast it. For example, if Blitz Automaton is exiled with the last ability of Chandra, Dressed to Kill, you would be able to cast it for 2 generic mana red mana (because it's a red spell), even though you wouldn't be able to cast it as a colorless spell for its normal cost.