Hagi Mob — ruling 1

A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.

Hagi Mob — ruling 2

If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.

Hagi Mob — ruling 3

If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.

Hagi Mob — ruling 4

If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.

Hagi Mob : règles, fonctionnement et interactions (MTG)

Hagi Mob 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

Boast — : This creature deals 1 damage to any target. (Activate only if this creature attacked this turn and only once each turn.)

How it works : Hagi Mob

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.
No other card is named in this ruling.
#2February 5, 2021· Wizards of the Coast
If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.
No other card is named in this ruling.
#3February 5, 2021· Wizards of the Coast
If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.
No other card is named in this ruling.
#4February 5, 2021· Wizards of the Coast
If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.
No other card is named in this ruling.