Halo Forager — ruling 1

Halo Forager's enters-the-battlefield ability triggers and goes on the stack without a target. If you pay {X}, the second "reflexive" triggered ability will trigger. You'll choose the target instant or sorcery card for that second ability at that time, based on the value you chose for X. X can be 0.

Halo Forager — ruling 2

You choose whether or not to cast the target instant or sorcery card as the reflexive triggered ability resolves. If you do, you do so as part of the resolution of the triggered ability. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.

Halo Forager — ruling 3

If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the card.

Halo Forager — ruling 4

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Halo Forager : règles, fonctionnement et interactions (MTG)

Halo Forager 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

Flying When this creature enters, you may pay . When you do, you may cast target instant or sorcery card with mana value X from a graveyard without paying its mana cost. If that spell would be put into a graveyard, exile it instead.

How it works : Halo Forager

Original English text — Wizards of the Coast / Scryfall

#1April 14, 2023· Wizards of the Coast
Halo Forager's enters-the-battlefield ability triggers and goes on the stack without a target. If you pay , the second "reflexive" triggered ability will trigger. You'll choose the target instant or sorcery card for that second ability at that time, based on the value you chose for X. X can be 0.
No other card is named in this ruling.
#2April 14, 2023· Wizards of the Coast
You choose whether or not to cast the target instant or sorcery card as the reflexive triggered ability resolves. If you do, you do so as part of the resolution of the triggered ability. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.
No other card is named in this ruling.
#3April 14, 2023· Wizards of the Coast
If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the card.
No other card is named in this ruling.
#4April 14, 2023· Wizards of the Coast
If the card has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
No other card is named in this ruling.