Historical Rules · 23 July 2021
Rule 604.5
Official source text from the 23 July 2021 Rules release
Some static abilities apply while a spell is on the stack. These are often abilities that refer to countering the spell. Also, abilities that say “As an additional cost to cast . . . ,” “You may pay [cost] rather than pay [this object]’s mana cost,” and “You may cast [this object] without paying its mana cost” work while a spell is on the stack.