Does the "higher level spell slot" clause in the Globe of Invulnerability spell do anything?

Question

The description of the Globe of Invulnerability spell says, in part (PHB, p. 245):

Any spell of 5th level or lower cast from outside the barrier can't affect creatures or objects within it, even if the spell is cast using a higher level spell slot.

Interpretation 1: For example, Fireball is a level 3 spell, so it doesn't affect anyone inside a Globe of Invulnerability, even if it's cast as a level 6 spell.

Then I read the actual rules of the game on casting a spell at a higher level (PHB, p. 201):

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level.

Interpretation 2: Back to the example, Fireball is whatever level slot it's cast with. Fireball in a level 6 slot is a level 6 spell. Level 6 spells affect someone in a Globe of Invulnerability. Therefore, Fireball cast as a level 6 spell affects someone in a Globe of Invulnerability.

Okay, but then why the heck does the spell say, "even if the spell is cast using a higher-level spell slot"? There's no such thing as a spell of 5th level or lower that's cast with a spell slot of 6th level or higher!

I suspect the PHB editors just screwed up with their wording. I suspect that, if I asked a PHB editor what level Fireball was, they'd say "level 3", not "whatever level slot is used to cast it." Heck, that's what I would say. It's only because of spells like Counterspell, Dispel Magic, and Globe of Invulnerability that I'd pause and say, "Actually, no, 3 is the minimum level slot that can be used to cast Fireball. Per the rule on p. 201, a spell's level the level of the spell slot used to cast it."

Which interpretation is correct? What does the "higher level spell slot" clause of the Globe of Invulnerability spell actually do?

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Answer 1

Globe of invulnerability is an exception to the general rule

"General vs. Specific" is one of the foundations of the D&D 5e rules. It can be found on page 7 of the Player's Handbook and is reiterated on page 5 of Xanathar's Guide to Everything. The short form:

If a specific rule contradicts a general rule, the specific rule wins.

Rule here includes general rules and "racial traits, class features, spells, magic items, monster abilities, and other game elements" (both quotes from the PHB version of the rule).

You have correctly identified the general rule in this case: that spells are of the level of the slots they were cast from (or their base level if not cast from a slot).

The globe of invulnerability spell creates an explicit exception to this rule. This exception makes globe of invulnerability look at the spell's base level at all times. That's what "even if the spell is cast using a higher-level spell slot" effectively means.

The consequence of this is fairly simple. Upcasting globe of invulnerability enables it to block spells whose base (or printed) level is higher (equal or lower than the globe's cast level). For instance, a 6th-level casting of globe of invulnerability doesn't protect against sunbeam (a 6th-level spell), but a 7th-level casting of globe of invulnerability does.