[RPG] How does a changeling’s Divergent Persona affect bard spells cast using musical instruments

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My character is a level 1 Changeling Bard. In my character's story, his Bard personality is what I'm using for his Divergent Persona racial feature. So it says under the description for divergent persona that I can select "one tool of your choice" for which "the related proficiency bonus is doubled" which I chose as his instrument he uses for his spells.

Does this mean he gets the double proficiency bonus when he casts spells using that instrument? Or is it more in regards to performance checks and such?

It does finish off saying "the related proficiency bonus is doubled for any ability check you make that uses that proficiency". However I think the proficiency bonus I gain from spells doesn't come from my instrument and is instead just my natural proficiency bonus. But theoretically I am still using the instrument to make that spell ability check, so how does this work?

Either way I'm stumped, if anyone is able to logically state an answer for me, I'd be ever grateful!

Best Answer

Divergent Persona doesn't affect spellcasting

The changeling's Divergent Persona trait says (WGtE, p. 61; emphasis mine):

You gain proficiency with one tool of your choice. Define a unique identity associated with that proficiency; establish the name, race, gender, age, and other details. While you are in the form of this persona, the related proficiency bonus is doubled for any ability check you make that uses that proficiency.

Note the term "ability check" at the end? Spellcasting does not generally involve ability checks. If you are casting an attack spell like Fire Bolt, it is an attack roll, not an ability check. The save DC for your spells is also unrelated to any kind of ability check.

My reading of the ability is that it would not work even with spells that can use ability checks such as dispel magic or counterspell. While both of these do allow for an ability check, if you are trying to counter or dispel a spell cast from a higher spell slot, neither allow you to add your proficiency bonus to the roll. You can add half your proficiency from the Jack of All Trades feature from the bard class, but that is not related to the instrument selected in the Divergent Persona trait.