For example, an Arcana cleric takes Firebolt with Arcane Initiate, then multiclasses with Artillerist artificer, and also takes Firebolt as one of the Artificer cantrips. Assuming this character has 5 levels as an Artillerist and 8 levels in Arcana Domain, would casting Firebolt through an Arcane Firearm count as both an artificer spell for the Arcane Firearm feature, and a cleric cantrip for the Potent Spellcasting feature?
[RPG] Does learning the same spell from different sources allow it to benefit from bonuses from all sources
dnd-5emulti-classingspellcasting
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Best Answer
Abilities trigger when they are related to the class you cast the spell "as", but some abilities don't refer to class
From the Multiclass Spellcasting rules:
Even if you know a spell through two classes, you probably haven't used two different preparation slots to prepare it for two classes. But if you have prepared it in both classes (a bit of a waste of prepared spells, but whatever), then it becomes associated with a certain class when you cast it:
Thus, there are successive points in time when the class of a spell could be fixed - when you know it (if you know just one), when you prepare it (if it can be prepared and you prepare just one), or when you cast it (which forces you to choose the class at that point if you have not done so already).
Once a spell is associated with a certain class, it then triggers class-specific interactions as that class and not others.
Arcane firearm says "when you cast an Artificer spell through the firearm", so you would need to have known/prepared/cast the spell as an Artificer to use this ability.
Potent Spellcasting says "Starting at 8th level, you add your Wisdom modifier to the damage you deal with any cleric cantrip," so you would need to have known/cast the spell through the cleric class to use this ability.
Potent Cantrip (evocation wizard) says "Starting at 6th level, your damaging cantrips affect even creatures that avoid the brunt of the effect. When a creature succeeds on a saving throw against your cantrip..." In this case, it does not matter that a cantrip is known to you, or cast by you, through a class other than wizard, it is still your cantrip and this ability would trigger.