Is there a mechanism for reducing and controlling the damage of a spell?

Question

We're in between campaigns right now and I asked a player (my wife) what she was up to after the arduous adventuring of the last few months. We were in the kitchen together and she was making an omelette.

She says: Making an omelette.

Me: I mean Sapphire.

W: She's making omelette's too. Except she's using her new Disintegrate spell to crack the egg.

M: I think that might more than crack an egg. That would reduce half her kitchen to dust.

W: No, she just shot a mini disintegration ray at the dragon egg. And the shell disappeared in a puff.

M: You have a dragon egg?

W: Not any more. But it was tasty!

So apparently Sapphire just took a spell with the potential destructive force of 100hp (Disintegrate) and used it to crack a dragon egg. Or as my wife put it, "Disintegrated the shell off the dragon egg. Cracking eggs is too messy for Sapphire."

After further conversation I think this usage will definitely come up in our next campaign. Sapphire is really enjoying the Disintegrate spell, and in her downtime has apparently used it to:

• Burn the britches off an unsuspecting and cantankerous nobleman
• Scour some grimy iron pans
• Remove the hair from her own legs
• Remove the hair from previously mentioned nobleman's head
• Vandalize the castle wall on numerous occasions

Sapphire is a Level 11 forest gnome sorcerer, and she's a bit mischievous.

This is with disintegrate, a spell that specifically states (PHB 233):

This spell automatically disintegrates a Large or smaller nonmagical object or a creation of magical force.

I know melee attacks can be pulled to knock out an adversary rather than kill them (PHB 197, Knocking a Creature Out) , but what about spells?

Sorcerers do have the Careful Spell metamagic option available at Levels 3, 10 or 17 which is the closest feature I could find to what I'm looking for (PHB 102):

Careful Spell

When you cast a spell that forces other creatures to make a saving throw, you can protect some of those creatures from the spell’s full force. To do so, you spend 1 sorcery point and choose a number of those creatures up to your Charisma modifier (minimum of one creature). A chosen creature automatically succeeds on its saving throw against the spell.

However this does not fully address the ways in which Sapphire has used the Disintegrate spell - for example, on parts of an object/person in close proximity to the rest of that object/person. Like the dragon egg shell (but not the egg itself), and the nobleman's hair (but not his head or body).

Is there a RAW mechanism for reducing and controlling the damage of the Disintegrate spell?

And if not, would allowing casters to reduce the damage of their spells be balanced?

Answer 1

No, there is no RAW mechanism to reduce damage of spells.

You cannot even use cantrips at their original power when you go up a level, which creates some problems for Twinned Spells, for example.

At best you can improve target saving throw or temporary reduce your spellcasting ability to lower save DC, give target resistance to spell element, give target temporary hp or use Abjuration specialist Arcane Ward.
All of these are impossible or very hard to apply to objects.

Allowing casters to reduce the damage of their spells should not upset balance too much.

If you require that spells be casted using original spell slot. While this might open some unexpected interactions (like wizard using 1d6 fireballs to keep horde of barbarians raging) it should not break the game.

If you allow to use spell slot of lower level for reduced effect, you will need to have a good understanding of effects relative power and expected damage per spell level. You will have to essentually invent new spell and assign it level each time it is cast. Which is not easy.

But there is yet another way: invent a new spell.

Many of effects described in your post should be possible using Prestidigitation cantrip. Make Sapphire take this cantrip and reflavor it to be a small disintegration beam when she uses it, if applicable. For something you feel too powerful for cantrip, create a first-level spell Lesser Disintegrate, which can, for example, shave beards, deal 1d6+4 damage, or destroy up to 1 cubic foot of nonliving matter (shaped in a way caster desires).