During my research into some homebrew I may have found an issue. I will use official feats and spells in my examples below.

Example 1

Cleric 18 with the Magic Initiate (Cleric) feat. Add Toll the Dead as a regular Cleric cantrip and as a MI Cleric cantrip:

The MI Cleric version does not add the additional damage of the WIS ability modifier to the damage whereas the regular version does.

Example 2

Warlock 9 with Agonizing Blast invocation and the Magic Initiate (Warlock) feat. Add Eldritch Blast as a regular Warlock cantrip and a MI Warlock cantrip:

The MI Warlock version does add the additional damage of the CHA ability modifier to the damage just like the regular version.

These are just official examples of this issue. The root cause is me trying to make a homebrew cleric spell work with a feat to add an extra spell to the Cleric's prepared spell list. Adding the homebrew spell normally adds the modifier correctly, but adding it using a feat does not.

Quote from Mairondil >>

During my research into some homebrew I may have found an issue. I will use official feats and spells in my examples below.

Example 1

Cleric 18 with the Magic Initiate (Cleric) feat. Add Toll the Dead as a regular Cleric cantrip and as a MI Cleric cantrip:

The MI Cleric version does not add the additional damage of the WIS ability modifier to the damage whereas the regular version does.

Example 2

Warlock 9 with Agonizing Blast invocation and the Magic Initiate (Warlock) feat. Add Eldritch Blast as a regular Warlock cantrip and a MI Warlock cantrip:

The MI Warlock version does add the additional damage of the CHA ability modifier to the damage just like the regular version.

These are just official examples of this issue. The root cause is me trying to make a homebrew cleric spell work with a feat to add an extra spell to the Cleric's prepared spell list. Adding the homebrew spell normally adds the modifier correctly, but adding it using a feat does not.

 you could manually add the modifier on each sheet using it but strange

Quote from Sillvva >>

So after a bit of discussion and reading on the feat, it may not actually count for Potent Spellcasting by RAW.

Potent Spellcasting only gives a bonus to Cleric cantrips. The feat does not actually say it counts as a [insert class] spell. It only says "you learn two cantrips of your choice from the [insert class] spell list".

Feel free to discuss. Perhaps somebody can find a sage advice somewhere.

My example may very well be a rules discussion, but the issue I'm having is certainly technical.

I'm giving a player an extra damage dealing cantrip. It's a homebrew cantrip and gives a level scalable Xd12 damage. I use a feat to give them this extra spell because it does not count against the cantrips known. For my high level Cleric with Potent Spellcasting, ddb does not give the appropriate damage die calculation in the dice roller because of this issue. For the purposes of this discussion, the cantrip is a Cleric cantrip.

Quote from Sillvva >>

Gotcha. Yes the homebrew system does not currently support adding spells as class spells via a feat due to the lack of official precedent. One option would be to homebrew the user's subclass to add the spell to their class's spell list.

 At this point, does “lack of official precedent” still matter all that much? Just because WotC hasn’t done it yet, the homebrewers want functionality like this. Why not let some slack in those reigns for us?

Quote from Sillvva >>

Gotcha. Yes the homebrew system does not currently support adding spells as class spells via a feat due to the lack of official precedent. One option would be to homebrew the user's subclass to add the spell to their class's spell list.

 I disagree. From the Sage Advice Compendium:

If you have spell slots, can you use them to cast the 1stlevel spell you learn with the Magic Initiate feat? Yes, but only if the class you pick for the feat is one of your classes. For example, if you pick sorcerer and you are a sorcerer, the Spellcasting feature for that class tells you that you can use your spell slots to cast the sorcerer spells you know, so you can use your spell slots to cast the 1st-level sorcerer spell you learn from Magic Initiate. Similarly, if you are a wizard and pick that class for the feat, you learn a 1st-level wizard spell, which you could add to your spellbook and subsequently prepare.

In short, you must follow your character’s normal spellcasting rules, which determine whether you can expend spell slots on the 1st-level spell you learn from Magic Initiate.

 Using this logic a Cleric should be able to apply Potent Spellcasting to Magic Initiate (Cleric) spells.

Quote from IamSposta >>

Quote from Sillvva >>

Gotcha. Yes the homebrew system does not currently support adding spells as class spells via a feat due to the lack of official precedent. One option would be to homebrew the user's subclass to add the spell to their class's spell list.

 At this point, does “lack of official precedent” still matter all that much? Just because WotC hasn’t done it yet, the homebrewers want functionality like this. Why not let some slack in those reigns for us?

Especially if I'm going to depend on ddb's convenient dice roller. If I can't homebrew a solution, why would I use this as my primary tool?

I'm not even suggesting this is going to be an easy fix. But to dismiss this as a rules question or make an entire subclass to get around the issue is kind of insulting. If it's not a bug, it's a feature request and I would hope the moderator team would deal with it as such.

Spells added by feats do not work with a Cleric's Potent Spellcasting and from what I saw on discord this morning, neither does attuning to the Rod of the Pact Keeper as a Warlock. Related issues? Possibly

I actually ran in to a very similar issue and was breaking my head over why it was not working as expected until I found this thread.

In my case, spells added by my feat do not add the regular spell casting modifier to the spell attack roll, whereas if I add the same (homebrew) spell the normal way, the spell casting modifier is added to the spell attack roll.

I will actually be using a Magic Item as they do not suffer from the same problem (and in retrospect, it also ties in better with game play: we received little creatures that have a few spells from our DM), but I thought I'd add my 2 cents to this thread in case it helps anyone else who runs in to this.

Quote from rwmailing >>

I actually ran in to a very similar issue and was breaking my head over why it was not working as expected until I found this thread.

In my case, spells added by my feat do not add the regular spell casting modifier to the spell attack roll, whereas if I add the same (homebrew) spell the normal way, the spell casting modifier is added to the spell attack roll.

I will actually be using a Magic Item as they do not suffer from the same problem (and in retrospect, it also ties in better with game play: we received little creatures that have a few spells from our DM), but I thought I'd add my 2 cents to this thread in case it helps anyone else who runs in to this.

 Feats will work, but you need to specify what ability score modifier is used for the spellcasting (a subclass already "knows" which spellcasting modifier to use.) This is why DDB has multiple versions of feats like Fey Touched, one for each mental ability score.

Thank you for this very helpful hint!

Turns out this was largely my mistake: I assumed the Ability Score drop down was used in the Number of Uses calculation, but there is actually a separate field for that :-/
It would be helpful, however, to have a tool tip for the field that indicates that this will be used to calculate the Spell Casting Modifier when the spell is cast through the feat.

For anyone else looking how to do this: when adding your Spell to your Feat, in the Edit the Spell for <feat> screen, set the Ability Score drop down to whatever you need.