Historical Rules · 19 January 2018
Rule 112.6c
Official source text from the 19 January 2018 Rules release
An object’s ability that allows a player to pay an alternative cost rather than its mana cost functions in any zone in which its mana cost can be paid (which, in general, means it functions on the stack). An object’s ability that otherwise modifies what that particular object costs to cast functions on the stack.