Toofer, Keeper of the Full Grip

When you cast Divination from your hand, you’re losing a card (Divination itself), but after it resolves, you’ve gained two cards in your hand. Overall, you’ve gained a card. Opponents haven’t gained any cards, so you’ve gained card advantage. Conversely, when you cast Prosperity for X=4, you lose a card (Prosperity itself), but you gain four cards, for a net gain of three. However, even with just one opponent, that player will draw four cards, so that ends up being a total net loss of one card for you, which means Prosperity with X=4 was not card advantage.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Abilities that cause you to gain card advantage won’t cause Toofer’s ability to trigger, even if those abilities trigger from casting spells. Similarly, card advantage from other sources, like combats where opponents lose more creatures than you do, won’t cause Toofer’s ability to trigger.