Historical Rules · 18 November 2022
Rule 702.160a
Official source text from the 18 November 2022 Rules release
Prototype is a static ability that appears on prototype cards that have a secondary set of power, toughness, and mana cost characteristics. A player who casts a spell with prototype can choose to cast that card “protoyped.” If they do, the alternative set of its power, toughness, and mana cost characteristics are used. See 719, “Prototype Cards.”