Historical Rules · 25 January 2019
Rule 702.116a
Official source text from the 25 January 2019 Rules release
Surge is a static ability that functions while the spell with surge is on the stack. "Surge [cost]" means "You may pay [cost] rather than pay this spell's mana cost as you cast this spell if you or one of your teammates has cast another spell this turn." Paying a spell's surge cost follows the rules for paying alternative costs in rules 601.2b and 601.2f-h.