Would casting the Glibness spell on yourself count as "casting a spell that affects the enemy creature", for the purpose of determining whether a Sanctuary spell on you ends? Or is it simply a personal buff?
I feel like the latter is the case, but I'm not sure.
The glibness spell has the following characteristic:
Range: Self
Also, it does not target or affect any other creature on its own. Thus it would not break sanctuary.
Szega's conclusion is correct, glibness will not end sanctuary.
However, their process for explaining why this is so involves faulty reasoning and, as an accepted answer, may cause more confusion than it resolves.
Their answer first starts by emphasizing that glibness has a range of "self", but this is completely irrelevant to the question at hand.
Sanctuary ends if:
...the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature
For a spell that ends sanctuary, the only requirement is that it be a spell "that affects an enemy" - there is nothing in there about range. Szega might have an intuitive sense that a spell with a range of Self can't affect other creatures, but this would be an erroneous conclusion, as there are plenty of examples of such spells that do. Or, they might even have misread the spell range rules, which say:
Most spells have ranges expressed in feet. Some spells can target only a creature (including you) that you touch. Other spells, such as the shield spell, affect only you. These spells have a range of self.
This rules statement can be taken to mean:
Spells that affect only you have a range of self.
However, the logical converse of this is not true:
Spells that have a range of self affect only you.
Again, this second statement is not what the spellcasting rules are saying, and it is false. This can easily be shown by pointing to examples of spells with a range of Self that do indeed affect other creatures (and here Thomas Markov has compiled a useful list of 42 such spells).
Thus, there is no reason to look at the range of a spell at all when considering whether it would drop sanctuary. Nor do we need to concern ourselves with whether a given spell targets another creature, as Szega also claims.
Instead, we must look only at whether or not the spell affects enemy creatures. For glibness, the only mechanical effect the spell produces on creatures is for yourself:
Until the spell ends, when you make a Charisma check, you can replace the number you roll with a 15.
Contrast this with, for example, the friends spell (Range: Self), which says:
you have advantage on all Charisma checks directed at one creature of your choice that isn't hostile toward you. When the spell ends, the creature realizes that you used magic to influence its mood and becomes hostile toward you.
The first effect, modifying your Charisma checks, is directly analogous to glibness. However, friends also makes a non-hostile creature become hostile. It is directly affecting the opposing creature in a way that glibness does not, and in precisely the way that would drop sanctuary.
