X on the Stack

Judge Level 1

Fireball with X=1 — and suddenly Spell Snare counters it.

Cards involved

Fireball Fireball
Spell Snare Spell Snare
Step / 4

Fireball with X=1

You cast Fireball (XR) with X=1, paying 1R. X is locked in while casting — the value applies as long as the spell is on the stack.

Rules: 107.3b

107.3bIf a player is casting a spell that has an X in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

What happens?

Your opponent holds Spell Snare.

"Counter target spell with mana value 2." Can they counter your Fireball?

Pick an answer to continue.

The mana value uses the announced value of X while the spell is on the stack (107.3b, 202.3). Everywhere else — hand, graveyard, library, exile — X=0: there, Fireball has mana value 1.

Rules: 202.3 107.3b

202.3The mana value of an object is a number equal to the total amount of mana in its mana cost, regardless of color.

107.3bIf a player is casting a spell that has an X in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

Spell Snare targets

Perfectly legal: right now your Fireball is a spell with mana value 2.

Countered

The Fireball goes to the graveyard — where its mana value is 1 again. Rule of thumb: X only "lives" on the stack.

Rules: 107.3b

107.3bIf a player is casting a spell that has an X in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

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Stack

Fireball

X damage — announced with X=1

Spell Snare

Counter target spell with mana value 2

The stack is empty.