Gwenom, Remorseless {3}{B}{B}
Legendary Creature — Symbiote Spider Hero
Deathtouch, lifelink
Whenever Gwenom attacks, until end of turn, you may look at the top card of your library any time and you may play cards from the top of your library. If you cast a spell this way, pay life equal to its mana value rather than pay its mana cost.
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Cards that do something similar expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Yennett, Cryptic Sovereign 88%
$0.35#10884
"Whenever this card attacks, reveal the top card of your library. You may cast it without paying its mana cost if its mana value is odd. If you don't cast it, draw a card."
Hans Eriksson 88%
$0.13#17784
"Whenever this card attacks, reveal the top card of your library. If it's a creature card, put it onto the battlefield tapped and attacking defending player or a planeswalker they control. Otherwise, put that card into your hand. When you put a creature card onto the battlefield this way, it fights this card."
Sapling of Colfenor 87%
$7.08#6640
"Whenever this card attacks, reveal the top card of your library. If it's a creature card, you gain life equal to that card's toughness, lose life equal to its power, then put it into your hand."
The Lunar Whale 86%
$0.33#11209
"As long as this card attacked this turn, you may play the top card of your library."
Rulik Mons, Warren Chief 85%
$0.17#12851
"Whenever this card attacks, look at the top card of your library. If it's a land card, you may put it onto the battlefield tapped. If you didn't put a card onto the battlefield this way, create a 1/1 red Goblin creature token."
Raphael, Most Attitude 83%
$0.21#18375
"Whenever this card attacks, until end of turn, you may play a card exiled with this card."
Ma Chao, Western Warrior 82%
$80.20#26861
"Whenever this card attacks alone, it can't be blocked this combat."
Shredder, Shadow Master 81%
$0.94#6431
"Whenever this card attacks a player, for each other opponent, create a token that's a copy of this card tapped and attacking that player, except it isn't legendary. Sacrifice those tokens at end of combat."
Etali, Primal Storm 81%
$0.43#260
"Whenever this card attacks, exile the top card of each player's library, then you may cast any number of spells from among those cards without paying their mana costs."
The Seventh Doctor 80%
$0.27#10814
"Whenever this card attacks, choose a card in your hand. Defending player guesses whether that card's mana value is greater than the number of artifacts you control. If they guessed wrong, you may cast it without paying its mana cost. If you don't cast a spell this way, investigate."