D&D 5E question on lower level spells in higher level slots

[Mort]

My reading of the rules is that a lower level spell (say 1st level magic missile) upcast to a higher level slot (say 7th level) becomes that level of spell - the magic missile is treated as a 7th level spell in all respects - Including affecting a monster that's not affected by 6th and lower level spells.

Support (from the PHB):
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. Effectively, the spell expands to fill the slot it is put into.

I ask because I caught a live stream of a high level game, where the DM ruled the exact opposite (a magic missile upcast to 7th level had no effect on a monster immune to 6th and lower level spells). Pretty sure that was incorrect, but am I missing something?

 

[jmartkdr2]

That's how I've always read the rule, but I can see how someone might read it differently.

 

[Arial Black]

You're correct, but check out the spell globe of invulnerability, the monster might have a similar defence.

 

[Mort]

You're correct, but check out the spell globe of invulnerability, the monster might have a similar defence.

Interesting - so that could block up to 8th level spells (as a 9th level spell).

But in this case - it was a specific monster ability and the DM even called out the fact that an upcast spell retains it's original level.

edited for accuracy.

 

[TaranTheWanderer]

I rule it as a higher level spell so if you want to counterspell a 7th level magic missile, you need to use a 7th level slot or succeed on you caster check.

 

[TaranTheWanderer]

I rule it as a higher level spell so if you want to counterspell a 7th level magic missile, you need to use a 7th level slot or succeed on you caster check.

And furthermore, doing it this way allows you to use it as a strategy to make it harder for enemy caster to dispel or counterspell your spells

 

[Mort]

And furthermore, doing it this way allows you to use it as a strategy to make it harder for enemy caster to dispel or counterspell your spells

That's generally the way I do it too -

Though pretty sure by RAW you get to either identify the spell being cast OR do a counterspell - no time for both (I don't know of anyone personally who plays it that way though).

 

[TaranTheWanderer]

That's generally the way I do it too -

Though pretty sure by RAW you get to either identify the spell being cast OR do a counterspell - no time for both (I don't know of anyone personally who plays it that way though).

It creates an interesting conundrum though. What if I use a 9th level slot to cast arcane lock. Knock will still bypass the spell temporarily but using a dispel magic to remove it permanently would be very difficult. How would someone know the level at which it’s cast?

 

[jmartkdr2]

It creates an interesting conundrum though. What if I use a 9th level slot to cast arcane lock. Knock will still bypass the spell temporarily but using a dispel magic to remove it permanently would be very difficult. How would someone know the level at which it’s cast?

In this particular case: without time constraints, I imagine the pc's could do a little Investigation to learn the spell is upcast, and with a really good plan and/or roll what level it was cast at.

But in combat, you just gotta guess (or roll well).

 

[Mort]

It creates an interesting conundrum though. What if I use a 9th level slot to cast arcane lock. Knock will still bypass the spell temporarily but using a dispel magic to remove it permanently would be very difficult. How would someone know the level at which it’s cast?

The DM assigns a level (many modules say Arcane Lock (15th level) or some such, at least they used to haven't looked recently). Nothing wrong with having a higher level arcane lock be better.