Circle of Protection: Blue — ruling 1

A source of damage is a permanent, a spell on the stack (including one that creates a permanent), or any object referred to by an object on the stack. A source doesn’t need to be capable of dealing damage to be a legal choice.

Circle of Protection: Blue — ruling 2

Can be used even when there is no damage to prevent. It prevents the next damage (if any) from the source this turn.

Circle of Protection: Blue : règles, fonctionnement et interactions (MTG)

Circle of Protection: Blue 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

: The next time a blue source of your choice would deal damage to you this turn, prevent that damage.

How it works : Circle of Protection: Blue

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
A source of damage is a permanent, a spell on the stack (including one that creates a permanent), or any object referred to by an object on the stack. A source doesn’t need to be capable of dealing damage to be a legal choice.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
Can be used even when there is no damage to prevent. It prevents the next damage (if any) from the source this turn.
No other card is named in this ruling.