The cantrip Magic Stone states:
You touch one to three pebbles and imbue them with magic. You or someone else can make a ranged spell attack with one of the pebbles by throwing it or hurling it with a sling. If thrown, it has a range of 60 feet. If someone else attacks with the pebble, that attacker adds your spellcasting ability modifier, not the attacker’s, to the attack roll. On a hit, the target takes bludgeoning damage equal to 1d6 + your spellcasting ability modifier. Hit or miss, the spell then ends on the stone.
If you cast this spell again, the spell ends early on any pebbles still affected by it.
The casting time of this spell is a bonus action, but the spell description doesn't say whether the attacks made with the stones require an action, or a bonus action. Upon reading this spell a couple of times now, I could even interpret it as "casting the cantrip imbues three stones AND you can make a ranged spell attack", all in the same bonus action, because that's literally what the spell says.
Is this interpretation correct? If not, how come? And how about the throwing the remaining two pebbles: do they require an action to throw?
Best Answer
An action is required to hurl a magic stone
The magic stone spell description states:
So the question is what does making a ranged spell attack require? The section on "Actions in Combat" (specifically the "Attack" section) states:
This shows us that making a ranged attack, which includes ranged spell attacks, requires your action. If they wanted it to require a bonus action they would have specifically said so as they do with the spiritual weapon spell:
Additionally, we have confirmation of this interpretation from the lead game designer Jeremy Crawford in an unofficial ruling (a tweet):
It might use the "Hurl A Magic Stone" action and not the Attack action
Most things that use up your action don't use one of the pre-listed actions of the PHB but use their own action. As such, it may be that hurling a magic stone does not actually use the Attack action but the "Hurl A Magic Stone action". This would impact some aspects of the spell (such as if some other feature that activates when taking the Attack action, would activate).
Another thing affected is how this spell interacts with Extra Attack. The fact that the stone can be hurled by hand or by using a sling somewhat points towards this using the Attack action, but it's going to be up to your GM whether this spell uses the Attack action, or the "Hurl A Magic Stone" action; and thus, whether or not somebody with Extra Attack can hurl multiple stones in a turn.
The Attack action does not typically make ranged spell attacks except when explicitly being allowed to do so such as with the Sun Soul Monk's Radiant Sun Bolt feature:
The magic stone spell never mentions granting a new use for the Attack action and so, as far as I can tell, hurling a stone to gain the benefits of magic stone does not use the Attack action at all.