Heartfire — ruling 1

You must sacrifice exactly one creature or planeswalker to cast this spell; you can’t cast it without sacrificing one, and you can’t sacrifice additional permanents.

Heartfire — ruling 2

Players can respond to Heartfire only after it’s been cast and all its costs have been paid. No one can try to destroy the permanent you sacrificed to stop you from casting this spell.

Heartfire : règles, fonctionnement et interactions (MTG)

Heartfire 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

As an additional cost to cast this spell, sacrifice a creature or planeswalker. Heartfire deals 4 damage to any target.

How it works : Heartfire

Original English text — Wizards of the Coast / Scryfall

#1May 3, 2019· Wizards of the Coast
You must sacrifice exactly one creature or planeswalker to cast this spell; you can’t cast it without sacrificing one, and you can’t sacrifice additional permanents.
No other card is named in this ruling.
#2May 3, 2019· Wizards of the Coast
Players can respond to Heartfire only after it’s been cast and all its costs have been paid. No one can try to destroy the permanent you sacrificed to stop you from casting this spell.
No other card is named in this ruling.