Does Magic Stone require an action or a bonus action for attacking with it?

Question

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?

Answer 1

An action is required to hurl a magic stone

The magic stone spell description 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 [...]

So the question is what does making a ranged spell attack require? The section on "Actions in Combat" (specifically the "Attack" section) states:

[...] With this action, you make one melee or ranged attack. See the "Making an Attack" section for the rules that govern attacks [...]

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:

[...] When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon [...]

[...] As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it [...]

Additionally, we have confirmation of this interpretation from the lead game designer Jeremy Crawford in an unofficial ruling (a tweet):

Q. The stones that get magiced from the spell 'Magic Stone', does it take an action or a bonus action to chuck them?

A. The magic stone spell imbues magic in pebbles as a bonus action. Hurling them requires a separate action.

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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:

You gain a new attack option that you can use with the Attack action. This special attack is a ranged spell attack with a range of 30 feet [...]

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.

Answer 2

Throwing the stone is a normal attack.

Spells that cause you to make an attack as part of their effect say "make an attack", imperative case. For example, fire bolt, guiding bolt, eldritch blast, and chromatic orb all say "Make a ranged spell attack against the (target / creature)", and shocking grasp says "Make a melee spell attack against the target."

(Green-flame blade, because it's a special snowflake, uses a different pattern: "As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range." This isn't imperative but it's very clear about the attack being part of the casting of the spell.)

This is different: the spell doesn't tell you to make the attack. It tells you that you can make a spell attack by throwing the stone. That's the key: you must throw the stone (or shoot it from a sling) to make the spell attack happen. This requires taking an Attack action, most likely.

It's a common pattern for a bonus-action spell to enhance a normal attack, so that you can cast the spell and then immediately attack as your action. Other spells using this pattern include shillelagh, divine favor, magic weapon, and all of the paladin "smite" spells.

Answer 3

The RAW here is... well... nonfunctional.

You or someone else can make a ranged spell attack with one of the pebbles by throwing it or hurling it with a sling.

This line in particular is weird.

Normally if this ranged spell attack was included as part of the same action used to cast the spell, it would say "As part of the action used to cast this spell" like Booming Blade does. It doesn't, so it's not part of the same action.

Normally if this ranged spell attack could be done as an action or bonus action, it would say "You can use your action to make..." like Flame Blade does. It doesn't, so you can't do it as an action or bonus action.

Normally if this ranged spell attack would be done using the basic weapon rules, it would say "It counts as a simple melee [or ranged] weapon..." like Shadow Blade does. It doesn't, so it doesn't use the weapon rules.

So by the letter of the RAW, you can't ever actually attack with the stones except by using them as improvised weapons maybe? That's nonsensical and nonfunctional.

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Since spells are meant to be useful,^{[citation needed]} we can infer how it should work.

It doesn't specify that that attack is made without an action, so we can assume it needs some sort of action to be used. Jeremy Crawford confirms this unofficially, but doesn't go so far as to specify what action is to be taken.
Magic Stone also doesn't specify that which specific action must be taken, so we can assume a generic action will suffice.
With that in mind, the Attack action seems a likely candidate. It's a generic action that allows you to make melee or ranged attacks, that any creature can take. One could also argue in favor of a Hunter Ranger's Volley action allowing these attacks, or any other feature that allows a generic ranged attack to be made.

Since there are no current ways to take the Attack action as a bonus action, Magic Stone requires an action (specifically the Attack action) to throw any amount of pebbles, limited by the number of attacks you can make in a single action.