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Straight to the point: A spell given by an Invocation that does NOT expend a Warlock spell slot, that does NOT set a level limit, is that spell automatically upcast? If not, could it be?
At base, all spells from a warlock are automatically at max strength; Cantrips are stronger, spells are upcast, and I’ve never seen or heard of players having a choice that they’re not.
Most invocation spells, like Arcane Eye, Detect Magic, and Jump, don’t have added effects when cast with a higher spell slot, so the text makes no mention of it.
Fiendish Vigor specifically states it must be cast at 1st level regardless of the caster’s level. I think that sets a precedent.
Animate Dead via Undying Servitude and Invisibility via Shroud of Shadows do have benefits at higher levels. Their invocations make no mention about what level they are cast at.
Shroud of Shadows grants a 3rd level slot while Chains of Carci grants a 5th level but has the same warlock level requirement. Possibly coincidence, but I feel it’s still relevant to the post.
As an added detail I think should be considered, casting Invisibility in combat is a good reason for a PC or NPC to cast Counterspell. Knowing if Invisibility is 3rd or 5th level makes the difference between instantly losing the spell or they make a DC15 check with their spellcasting ability. Even more important if the one casting counterspell has disadvantage due to Hex, Bestow Curse, or similar effects.
So would casting Animate Dead or Invisibility using their respective invocations upcast these spells to 5th level?
I searched for over an hour. Anything I could find was either specifically asking about Fiendish Vigor or the invocation(s) in question used a warlock spell slot when cast.