Veiled Sentry {U}
Enchantment
When an opponent casts a spell, if this permanent is an enchantment, it becomes an Illusion creature with power and toughness each equal to that spell's mana value.
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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.