Can I use the Booming Blade/Green-Flame Blade cantrips from hiding and still have advantage (for Sneak Attacks)?

Question

As stated here, you can use cantrips like booming blade and green-flame blade to trigger Sneak Attack under given conditions, naming one is you got advantage on your attack.

Now, one of the factors that can give advantage is hiding - being unseen and unheard - and then attacking.

Can I use those cantrips while hiding for sneak attacks, given that they have verbal components?

I would argue yes, because the attack is part of the spell and is performed sort of simultaneously with the verbal components, so it doesn't break the hiding advantage.

I'm not asking, if I can hide in combat to the get sneak attack, just if I can hide and use cantrips to trigger it without losing advantage from being hidden.

Answer 1

If you are hidden from the target at the time of casting then you get advantage:

Unseen Attackers and Targets (PHB p.194)

When a creature can’t see you, you have advantage on attack rolls against it.

The green flame blade spell, as an example, has this in the description:

Green-Flame Blade (Sword Coast Adventurer's Guide p.143)

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, otherwise the spell fails. On a hit, the target suffers the attack's normal effects.

The action taken to cast the spell is also the action that "contains" the attack. If the target is unaware of the caster at the time of casting they are equally unaware of the caster at the time the attack is made.

Therefore the attacker has advantage on the attack roll and may use sneak attack.

Answer 2

Yes, you can attack while hidden with a cantrip

Your specific request is of attacking while successfully Hidden by using GFB or BB cantrips.

The use of the cantrip (or of an attack) is not the trigger for losing your hidden status. The hidden status is removed when your attack hits or misses.

Jeremy Crawford via Sage Advice confirms this:

If you attack while hidden, you have advantage on the roll, and you reveal your position on the hit/miss (see PH, 194–5).

Just like you can make an attack while hidden to get advantage, you can cast the spell while hidden to get advantage. It is the after effects of those actions that result in the loss of your Hidden status.

Answer 3

I'd say when you cast a spell when hiding you still surprise them. When using the spell(green flame blade) you make an attack. So yeah, why wouldn't it?

Green flame blade:

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, otherwise the spell fails

Unseen attackers:

"Unseen Attackers and Targets", PHB p.194

Combatants often try to escape their foes’ notice by hiding, casting the invisibility spell, or lurking in darkness.

When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target’s location correctly. When a creature can’t see you, you have advantage on attack rolls against it.

Answer 4

Yes, you can use cantrips with verbal components to sneak attack, or else the Magical Ambush feature gained at 9th level from the arcane trickster class wouldn't make sense. PHB page 98 says:

Starting at 9th level, if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell on this turn

If casting a spell with verbal components from hiding would make the caster lose the benefit before the spell takes effect, then this feature wouldn't be that useful, as there are few spells without verbal components.

Answer 5

Booming Blade and Green Flame Blade cantrips require the caster to make an attack, but they don't affect the attack's conditions. If you have (dis)advantage on the attack roll, you still have it.

If you are invisible, you get sneak attack

If you are under the Invisibility spell effect, or fighting in darkness, the enemy cannot see you, so you get the advantage:

When a creature can’t see you, you have advantage on attack rolls against it.

You have advantage, so you can apply the sneak attack damage as well.

If you just attack from hiding, you don't get sneak attack

As being discussed in this answer, to get the advantage, you have to be unseen when you attack. If you attack in melee, the enemy can see you:

In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you.

(PHB, p. 177)

The enemy can be Surprised though, so you still get benefit from the ambush, but you cannot apply the sneak attack damage.