There are some problems. The problems aren't exactly that the ability is too good. The problem is that you're trying to build something to improve in-combat healing, but people who read this are going to see it as a way to do pre-combat buff spells without requiring concentration.

The first problem is that your requirement "whenever you use magic to restore one or more hit points to a creature" is easily gameable. Thomas Markov's answer highlights this issue when he starts with: "If I punch my friend in the arm for 1 damage..." This needs to be replaced with a better requirement that won't get gamed.

The second problem is that you haven't placed a time limit on the temporary hit points. As written, the cleric could overheal each of their allies at the start of the day, and then they just walk around with extra hit points until they need them. (The rules say that, by default, temporary hit points last until you finish a long rest.)

The third problem is that, as you note, certain high-level spells give out very large amounts of healing, and this wasn't seen as a design problem because the people designing those spells expected them just to heal the target to full hit points. You're going to need to add rules to handle this, or else exclude those spells from this ability entirely.

The fourth problem is that the ability is pretty weak, because most spellcasters only use healing on characters that are very damaged, so the risk of hitting someone's hp cap is normally quite low. One fix might be to make this into a "half feat" that also gives +1 stat.

I think the ability you want to design looks like this:

Whenever you use magic to restore one or more hit points to a creature, if the creature was damaged by an enemy in the last minute, then any healing that would exceed the target's max hp is instead gained as temporary hit points. These temporary hit points fade after one minute.

This ability does not function on a spell of sixth level or greater.

When you take this feat, increase your Wisdom or Charisma score by 1, to a maximum of 20.

With this rules text, the ability is no longer viable as a pre-combat buff spell. The second paragraph excludes heal and mass heal from the effect.

There are some problems. The problems aren't exactly that the ability is too good. The problem is that you're trying to build something to improve in-combat healing, but people who read this are going to see it as a way to do pre-combat buff spells without requiring concentration.

The first problem is that your requirement "whenever you use magic to restore one or more hit points to a creature" is easily gameable. Thomas Markov's answer highlights this issue when he starts with: "If I punch my friend in the arm for 1 damage..." This needs to be replaced with a better requirement that won't get gamed.

The second problem is that you haven't placed a time limit on the temporary hit points. As written, the cleric could overheal each of their allies at the start of the day, and then they just walk around with extra hit points until they need them. (The rules say that, by default, temporary hit points last until you finish a long rest.)

The third problem is that, as you note, certain high-level spells give out very large amounts of healing, and this wasn't seen as a design problem because the people designing those spells expected them just to heal the target to full hit points. You're going to need to add rules to handle this, or else exclude those spells from this ability entirely.

The fourth problem is that the ability is pretty weak, because most spellcasters only use healing on characters that are very damaged, so the risk of hitting someone's hp cap is normally quite low. One fix might be to make this into a "half feat" that also gives +1 stat.

I think the ability you want to design looks like this:

You gain the following benefits:

• When you take this feat, increase your Wisdom or Charisma score by 1, to a maximum of 20.

• Whenever you use a spell of 5th level or lower to restore hit points to a creature, if the creature was damaged by an enemy within the last minute, then any healing that would exceed the target's hit point maximum is instead gained as temporary hit points. The target loses any remaining temporary hit points from this feat after 1 minute.

With this rules text, the ability is no longer viable as a pre-combat buff spell. The spell level restriction excludes heal and mass heal from the effect.

There are some problems. The problems aren't exactly that the ability is too good. The problem is that you're trying to build something to improve in-combat healing, but people who read this are going to see it as a way to do pre-combat buff spells without requiring concentration.

The first problem is that your requirement "whenever you use magic to restore one or more hit points to a creature" is easily gameable. Thomas Markov's answer highlights this issue when he starts with: "If I punch my friend in the arm for 1 damage..." This needs to be replaced with a better requirement that won't get gamed.

The second problem is that you haven't placed a time limit on the temporary hit points. As written, the cleric could overheal each of their allies at the start of the day, and then they just walk around with extra hit points until they need them. (The rules say that, by default, temporary hit points last until you finish a long rest.)

The third problem is that, as you note, certain high-level spells give out very large amounts of healing, and this wasn't seen as a design problem because the people designing those spells expected them just to heal the target to full hit points. You're going to need to add rules to handle this.

I think the ability you want to design looks like this:

Whenever you use magic to restore one or more hit points to a creature, any healing that would exceed the target's max hp is instead gained as temporary hit points. The maximum number of temporary hit points you can grant is equal to the amount of damage the target had before healing; only damage that was inflicted by enemies in the last minute counts for this purpose. These temporary hit points fade after one minute.

With this rules text, the ability is no longer viable as a pre-combat buff spell. The rule about "the maximum number of temporary hit points you can grant" will prevent unexpected results from mass heal.

I still suspect that this feat is a bit unbalanced: it's not good enough before heal is available (because you won't be healing people that are close to full hit points), and after heal is available it's too good.

Maybe a better solution would be to also say: "this does not work on spells of sixth level or greater", and the feat would give you +1 wisdom or +1 charisma as well.

There are some problems. The problems aren't exactly that the ability is too good. The problem is that you're trying to build something to improve in-combat healing, but people who read this are going to see it as a way to do pre-combat buff spells without requiring concentration.

The first problem is that your requirement "whenever you use magic to restore one or more hit points to a creature" is easily gameable. Thomas Markov's answer highlights this issue when he starts with: "If I punch my friend in the arm for 1 damage..." This needs to be replaced with a better requirement that won't get gamed.

The second problem is that you haven't placed a time limit on the temporary hit points. As written, the cleric could overheal each of their allies at the start of the day, and then they just walk around with extra hit points until they need them. (The rules say that, by default, temporary hit points last until you finish a long rest.)

The third problem is that, as you note, certain high-level spells give out very large amounts of healing, and this wasn't seen as a design problem because the people designing those spells expected them just to heal the target to full hit points. You're going to need to add rules to handle this, or else exclude those spells from this ability entirely.

The fourth problem is that the ability is pretty weak, because most spellcasters only use healing on characters that are very damaged, so the risk of hitting someone's hp cap is normally quite low. One fix might be to make this into a "half feat" that also gives +1 stat.

I think the ability you want to design looks like this:

Whenever you use magic to restore one or more hit points to a creature, if the creature was damaged by an enemy in the last minute, then any healing that would exceed the target's max hp is instead gained as temporary hit points. These temporary hit points fade after one minute.

This ability does not function on a spell of sixth level or greater.

When you take this feat, increase your Wisdom or Charisma score by 1, to a maximum of 20.

With this rules text, the ability is no longer viable as a pre-combat buff spell. The second paragraph excludes heal and mass heal from the effect.