The following things are true:

  1. Bards are allowed to use their musical instruments as spell focuses.
  2. Proficiency with a musical instrument allows you to add your proficiency bonus to ability checks made with that instrument.
  3. Using an musical instrument in place of a material component means you are using the instrument to cast the spell.

If a spell has you make an ability check as part of casting the spell and your instrumental focus can be used, does your proficiency bonus from instrument apply to the check?

This is a purely theoretical / fringe theorycrafting situation as I have not found a spell that requires a material component that has the caster make an ability check.

The three spells that I know of that call for ability checks (dispel magic, counterspell, and telekinesis), do not require material components, so as far as I understand it, the instrument focus cannot be used. The spell fabricate specifically targets proficiency with artisan's tools as being used (which is a different question if Jack of All Trades applies, and thus anything can be created...)

But considering the Magical Secrets feature that every bard gets and the College of Lore's Additional Magical Secrets feature, there might be a spell out there that that fits this description.

Could the bard use their instrument focus to apply a proficiency bonus to the ability check?

Alternatively, if a spell that calls for an ability check is cast from a magical instrument (such as an Instrument of the Bards), would the proficiency bonus for being proficient in that instrument apply to the spell's ability check?

up vote 11 down vote accepted

No, this wouldn't work

The Tools section of the basic rules tells you what proficiency with a musical instrument does:

Musical Instrument. Several of the most common types of musical instruments are shown on the table as examples. If you have proficiency with a given musical instrument, you can add your proficiency bonus to any ability checks you make to play music with the instrument. A bard can use a musical instrument as a spellcasting focus. Each type of musical instrument requires a separate proficiency.

Musical instrument proficiencies are only relevant to whether you can add your proficiency bonus to checks made to play the instrument.

As discussed in the linked questions here and here, bards don't technically have to play an instrument to use it as a spellcasting focus - but more importantly, an ability check that a spell requires you to make is not an ability check made to play the instrument itself. As such, your proficiency bonus would not be applied to the check unless the spell description states that it is.

This usually wouldn't work

An ability check made from a spell is not "using" the instrument. The instrument is used to cast the spell, but the ability check is a separate part of the casting (or part of the spell effect). Only if the hypothetical spell specifically calls out an ability check with the material component or focus would you be able to add your proficiency bonus.

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