Arcane Proxy

You cast the copy while the ability is resolving and still on the stack. You can't wait to cast it later in the turn.

If you don't want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are checked.

When cast as a prototyped spell, that spell has the mana cost, power, and toughness characteristics shown in its colored, secondary text box rather than the normal values of those characteristics. Its color and mana value are determined by that mana cost. The permanent that spell becomes as it resolves has the same characteristics. If the spell leaves the stack in any other way, or the permanent it becomes leaves the battlefield, it immediately resumes using its normal characteristics.