Coastal Breach — ruling 1

Causing an opponent to lose the game after you’ve announced that you’re casting a spell with undaunted and determined its total cost won’t cause you to have to pay more mana.

Coastal Breach — ruling 2

Effects that reduce what you pay to cast a spell don’t affect its mana value.

Coastal Breach : règles, fonctionnement et interactions (MTG)

Coastal Breach 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

Undaunted (This spell costs less to cast for each opponent.) Return all nonland permanents to their owners' hands.

How it works : Coastal Breach

Original English text — Wizards of the Coast / Scryfall

#1November 8, 2016· Wizards of the Coast
Causing an opponent to lose the game after you’ve announced that you’re casting a spell with undaunted and determined its total cost won’t cause you to have to pay more mana.
No other card is named in this ruling.
#2November 8, 2016· Wizards of the Coast
Effects that reduce what you pay to cast a spell don’t affect its mana value.
No other card is named in this ruling.