What is the correct damage calculation when casting spells at higher levels?

Question

I cast Spray of Cards (UA: Wonders of the Multiverse, p. 11) at 5th level, choosing the Cutting Cards effect:

Cutting Cards. Each creature in a 15-foot cone must make a Dexterity saving throw. A creature takes 2d10 force damage on a failed save or half as much damage on a successful one.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage of cutting cards increases by 1d10 for every two slot levels above 2nd.

I interpreted this as 2d10 * 1 or 1/2 depending on whether the targets succeeded on the saving throw + (x)d10 for every 2 level slots over 2.

The DM interprets this as (2d10 + (x)d10) * 1 or 1/2 based on the same determinations.

In my mind, the writing is clear and the damage bonus from higher levels is separate from the base damage calculations, since they are separate clauses. However, the intention could be either.

What is the correct calculation?

Answer 1

Your DM is correct

The correct damage calculation for Spray of Cards cast at higher levels is

• (2 + 1 per each 2 levels above 2nd)d10 * 1 or 1/2

depending on whether the target makes their save. At 5th level, this would be 3d10 before the save.

This is common to many other spells that feature similar "At Higher Levels" text. For example Fireball normally deals 8d6 damage on a failed save, or half as much damage on a successful one. It has the text:

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.

Casting Fireball with a 5th level spell slot deals 10d6, or half at much on a successful save.

Damage Rolls

From the Basic Rules for damage rolls we have (emphasis mine):

Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target. Magic weapons, special abilities, and other factors can grant a bonus to damage.

(...) A spell tells you which dice to roll for damage and whether to add any modifiers.

If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them. For example, when a wizard casts fireball or a cleric casts flame strike, the spell's damage is rolled once for all creatures caught in the blast.

If you are only intended to roll the damage of the spell once, we can infer that the spell's damage cannot be dependent on whether or not the target makes its save. Otherwise the damage calculation would not be correct for mixed results. Therefore "damage" refers to only the 2d10 in the spell's description. When casting at a higher level, this 2d10 is replaced with the new number and the rest of the spell is left unchanged.

Roll your total damage once. Then targets that save take half of that number.

Answer 2

Upcasting the spell only increases the damage that you roll

Regardless of whether or not a target succeeds on their saving throw, and regardless of any other damage modifiers such as resistance and vulnerability, you always roll the same damage for the spell when cast at base level: 2d10. Casting this spell at a higher level increases this damage roll. Effectively, you are replacing the "2d10" in the spell description with (in your example) "3d10" while keeping the rest of the spell text unchanged. You aren't adding a new damage roll, you are increasing the existing damage roll. Once you roll the damage, the rest of the spell's effects (and any other relevant damage-changing effects) still occur as normal, which means that on a successful save, the target takes half of 3d10 damage.