Mysterious Stranger

You cast the copy while Mysterious Stranger’s last ability is resolving and still on the stack. You can’t wait to cast it later in the turn.

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

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.