Sifter Wurm — ruling 1

Once Sifter Wurm's triggered ability begins to resolve, no player may take other actions until it's done. Notably, opponents can't try to change your library after you scry but before you reveal the top card of your library.

Sifter Wurm — ruling 2

For cards in your library with {X} in their mana costs, X is considered to be 0.

Sifter Wurm : règles, fonctionnement et interactions (MTG)

Sifter Wurm 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

Trample When this creature enters, scry 3, then reveal the top card of your library. You gain life equal to that card's mana value.

How it works : Sifter Wurm

Original English text — Wizards of the Coast / Scryfall

#1November 10, 2020· Wizards of the Coast
Once Sifter Wurm's triggered ability begins to resolve, no player may take other actions until it's done. Notably, opponents can't try to change your library after you scry but before you reveal the top card of your library.
No other card is named in this ruling.
#2November 10, 2020· Wizards of the Coast
For cards in your library with in their mana costs, X is considered to be 0.
No other card is named in this ruling.