The Alchemist's Fire item description states the following (PHB, p. 148):
This sticky, adhesive fluid ignites when exposed to air. As an action, you can throw this flask up to 20 feet, shattering it on impact. Make a ranged attack against a creature or object, treating the alchemist's fire as an improvised weapon. On a hit, the target takes 1d4 fire damage at the start of each of its turns. A creature can end this damage by using its action to make a DC 10 Dexterity check to extinguish the flames.
I can think of three possibilities here and I'm not entirely sure which one is how it works:
- The action mentioned is the Attack action, and you throw the flask as an improvised weapon when you make the attack.
- The action mentioned is the Use An Object action, and part of using the object instructs you to throw it and make an attack roll.
- Neither of the above. It's its own action, similar to activating a magic item.
Many class abilities only apply if you take the Attack action, most notably the Extra Attack class feature. On the other hand, Rogue (Thief) can take Use An Object as a bonus action. It's generally regarded that a healer's kit works with the Use An Object action, but a healer's kit doesn't ask you to make an attack.
Which one of these is it? I've seen conflicting answers to this and I'm curious how it would work by both Rules as Written and Rules as Intended.