Globe of Invulnerability reads, in part (emphasis mine):
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.
Okay, so a level 6+ spell penetrates the barrier. Normally, “a level 6+ spell” would include ANY spell cast using a level 6+ slot. Globe of Invulnerability has that extra “even if…” clause, though, so that we still check Fireball’s base level even when it’s cast with a level 6 slot. Great. This was all covered in this related question.
The question I’m writing now, though, comes down to whether that “even if…” clause can be circumvented by upcasting the spell with something other than a slot. Enter The Staff of the Magi:
While holding the staff, you can use an action to expend some of its
charges to cast one of the following spells from it, using your spell
save DC and spellcasting ability: conjure elemental (7 charges),
dispel magic (3 charges), fireball (7th-level version, 7 charges)
Interpretation 1: That fireball IS a level 7 fireball. That’s what the staff’s description says.
That fireball is NOT a “spell of 5th level or lower”. Moreover, that level 7 fireball was NOT cast using a higher level slot, so it doesn’t run afoul of the Globe’s “even if…” clause. The fireball penetrates
Interpretation 2: The fireball is normally a level 7 spell, but that’s per the rule about casting spells with higher level slot. Let’s look at the actual rule:
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 (PHB p.201).
One might argue that the Globe’s “even if…” clause means that it’s an exception to the above rule. The fireball fails to penetrate.
Interpretation 3: When The Staff of the Magi says, “7th-level version”, it means that, with respect to all other rules or exceptions-to-rules or whatever, pretend like the spell was cast from a level 7 spell slot (just don’t cross of a level 7 slot if you have one). The fireball fails to penetrate.
Staff of Power
On a lighter note, what got me thinking about this was encountering a Staff of Power in-game, which can (among other things) cast Globe of Invulnerablity. It’s only a Very Rare staff, whereas the Staff of the Magi is legendary. I find it interesting to see the little advantages the Staff of Power has over the Staff of the Magi.