Spell Burst — ruling 1

For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.

Spell Burst : règles, fonctionnement et interactions (MTG)

Spell Burst is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Buyback (You may pay an additional as you cast this spell. If you do, put this card into your hand as it resolves.) Counter target spell with mana value X.

How it works : Spell Burst

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
For spells with in their mana costs, use the value chosen for X to determine the spell's mana value.
No other card is named in this ruling.