Im sure we’ve asked this before, but...

Magic Stone imbues pebbles with special rules, which describe how another creature can make an attack with them using your modifiers. Describing the attack in this way makes the pebble sound more like a one-use magic item consumeable, and less like a weapon. Other items, like Acid (vial) are generally viewed as using [Tooltip Not Found] to attack, but that may be because they have “As an action...” language in their description, which the Magic Stone lacks.

What’s the verdict? When your ally throws your stone, are they doing Attack, or [Tooltip Not Found]? Familiars obviously care about this, and Thiefs as well, but also other classes who potentially lose out on their class features if they can’t throw the stone with Attack like they want.

They are making an attack because the spell says so.

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

Of course they’re making an attack. But with Attack, or [Tooltip Not Found]?

Quote from Chicken_Champ >>

Of course they’re making an attack. But with Attack, or Use an Object?

 It's an attack because it says to make a ranged spell attack. If the rocks did something other than attack I would say it was use and object.

Well it's not really that simple though.

Acid (Vial) says:
"As an action, you can splash the contents of this vial onto a creature within 5 feet of you or throw the vial up to 20 feet, shattering it on impact. In either case, make a ranged attack against a creature or object, treating the acid as an improvised weapon."

Magic Stone says:
"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."

So the first says "ranged attack" and the other says "ranged spell attack" and the first says "as an action" (which has been clarified to be an [Tooltip Not Found] action) while the other is silent.

So how does Magic Stone attack? By [Tooltip Not Found], by Attack action or by [Tooltip Not Found] action? I would say by an Attack action but it isn't crystal clear.

The rules can be vague sometimes by what is left out, just go by what was included. Since it doesn't say action I would just go with attack. And it does say at the end of magic stone, 'to the attack roll.' which means according to the rules in the PHB. it is an attack.

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.

The acid vial includes the attack as part of the action to use the item.  The spell does not include the attack in the casting of the spell, nor does it call for a separate action to use, thus you use the default attack action.

I'm trying to think of situations where making an attack is done via the use an object action. I can't think of any.

It can get confusing because if you take the action to cast a spell, you have taken the cast a spell action not the attack action and then if your spell requires an attack roll such as a melee spell attack or ranged spell attack does it count as both taking the cast a spell action and the attack action for purposes of triggering things?

For the Use an Object action, can you use that to activate a magic item, perhaps one that allows you to cast a spell, a spell that requires an attack roll... would that count? 

If the attack is included as part of another action, such as Use an Object with the acid (vial) or Cast a Spell with Booming Blade, that is the action taken, not the attack action.  If the spell grants you the ability to use something for attacks, such as with Magic Stone or Shadow Blade, you use the attack action to make those attacks.

Looking at magic stone and the attack action, I'd say it takes an attack action. That means you can throw multiple with extra attack.

For those that just keep saying “it says make an attack, so it’s an Attack”, you’re entirely missing the point, and not particularly adding to the conversation, because there are a wealth of actions that include making an attack outside the Attack action, and nobody is debating whether or not you use the stone to make an attack. Its like you’re answering “is a tomato a vegetable or fruit?” by saying “it’s a vegetable, because it’s a plant.” Im not interested in conclusions, but rather reasonings, dig deeper. 

But that said, I think I DO agree that the stone is different from Acid. [Tooltip Not Found] says it’s required “When an object requires your action for its use...” Thats precisely the language difference between Magic Stone and attack items like Acid, there’s no “as an action” language.

Id be inclined to say MS is not an item that is used, but instead a temporary magical weapon that is used to make spell attacks using Attack, like Shadow Blade. But, that means, a fighter can throw two or more stones as part of their attack, a Bloodhunter with cruelty can throw one as a bonus action, an echo Knight throws an extra illusory one from the echo’s square, etc etc etc...

If it’s thrown with Attack like a normal weapon, then it’s also open to other types of actions and feature interactions that thrown weapons are used with. If it’s thrown with [Tooltip Not Found], then it’s more insulated.

It is an Attack. It is the only “spell attack” I know of that uses the Attack Action. What’s more, since spell attacks only have one range increment. So when the attack is made with a sling, the 120’ range is used without disadvantage, and counts as “an attack made using a ranged weapon” so that it is also eligible for use with Sneak Attack, and Sharpshooter.

Quote from Chicken_Champ >>

For those that just keep saying “it says make an attack, so it’s an Attack”, you’re entirely missing the point, and not particularly adding to the conversation, because there are a wealth of actions that include making an attack outside the Attack action, and nobody is debating whether or not you use the stone to make an attack.

 You're right, I didn't explain (or read anyone else's answers) and answers like that aren't helpful.

So here is my line of thinking:

Magic stone doesn't grant/require a specific type of action, it simply says "[You] can make a ranged spell attack with one of the pebbles." So if an action allows you to make such an attack, that attack can be with the pebble.

The attack action lets "you make one melee or ranged attack." So you can make a ranged spell attack with a pebble using this action.

I think the answer that requires the fewest assumptions is that it’s neither the Attack action nor the Use an Object action: it’s a special action granted by the spell. Nowhere is the magic stone described as a weapon, so I don’t think there’s any hard indication that you’re able to take the Attack action with it as anything other than an improvised weapon.

That said, I will always rule that it’s the Attack action, because that produces the best gameplay results.

[EDIT] I actually misread the spell and inserted an “as an action” that doesn’t exist, and with that revelation, I find DxJxC’s post above the most convincing. Though I also now think it’s a poorly worded spell.

Quote from IamSposta >>

It is an Attack. It is the only “spell attack” I know of that uses the Attack Action. What’s more, since spell attacks only have one range increment. So when the attack is made with a sling, the 120’ range is used without disadvantage, and counts as “an attack made using a ranged weapon” so that it is also eligible for use with Sneak Attack, and Sharpshooter.

 There is no such rule (if there is please direct me to it). A sling has a normal range and long range, attacking with it (even a spell attack) uses the rules associated with those ranges:

Some ranged attacks, such as those made with a longbow or a shortbow, have two ranges. The smaller number is the normal range, and the larger number is the long range. Your attack roll has disadvantage when your target is beyond normal range, and you can't attack a target beyond the long range.

Notice that is never says "weapon attacks" or "spell attacks."

And since sharpshooter only removes disadvantage with ranged weapon attacks, magic stone would still have disadvantage at long range (can't ignore cover either, but can get the damage bonus).

Quote from SagaTympana >>

I think the answer that requires the fewest assumptions is that it’s neither the Attack action nor the Use an Object action: it’s a special action granted by the spell. Nowhere is the magic stone described as a weapon, so I don’t think there’s any hard indication that you’re able to take the Attack action with it as anything other than an improvised weapon.

That said, I will always rule that it’s the Attack action, because that produces the best gameplay results.

 That requires assuming it grants an action like other spells/magic items. The attack action doesn't require it to be a weapon attack.

Magic stone says you can attack with it, the attack action says you can make an attack. So naturally, you can use the attack action to attack with a magic stone. No assumptions.

That said, I don't think there is another "spell attack" not tied to a specific action to be made with an attack action available to players (there are a few monsters with such actions, so a sword bard or bladesinger with shapechange is the only thing I can think of that could use them twice with extra attack.)

Monster actions are proper actions, unless used as natural weapons as part of the attack action, which is a whole separate convo...

... but in the context of Magic Stone, I agree with DxJxC thus far. So, no pebbles for Familiars, and no bonus action pebbles for Thieves.

Quote from DxJxC >>

Quote from IamSposta >>

It is an Attack. It is the only “spell attack” I know of that uses the Attack Action. What’s more, since spell attacks only have one range increment. So when the attack is made with a sling, the 120’ range is used without disadvantage, and counts as “an attack made using a ranged weapon” so that it is also eligible for use with Sneak Attack, and Sharpshooter.

 There is no such rule (if there is please direct me to it). A sling has a normal range and long range, attacking with it (even a spell attack) uses the rules associated with those ranges:

Some ranged attacks, such as those made with a longbow or a shortbow, have two ranges. The smaller number is the normal range, and the larger number is the long range. Your attack roll has disadvantage when your target is beyond normal range, and you can't attack a target beyond the long range.

Notice that is never says "weapon attacks" or "spell attacks."

And since sharpshooter only removes disadvantage with ranged weapon attacks, magic stone would still have disadvantage at long range (can't ignore cover either, but can get the damage bonus).

 No, it wouldn’t get the benefit of the other bullet points from Sharpshooter, only the additional damage.

And I know I read something about it somewhere regarding the attack range, but I’ll have to dig to find it. Maybe it was a Sage Advice...? I’ll have to dig. (But if that isn’t the case then the standard attack range would drop when using a sling, which makes no sense.)

Quote from Chicken_Champ >>

Monster actions are proper actions, unless used as natural weapons as part of the attack action, which is a whole separate convo...

 That is indeed a separate convo, but maybe one we should have since if monster actions are only actions and not attacks, then the only opportunity attack they can make is unarmed strikes.