What's the proficiency bonus of a monster with class levels?

[OgataiKhan]

We know that a character's proficiency bonus depends on their level.

We also know that a monster's proficiency bonus depends on their CR, as per page 8 of the MM.

But what happens if a dragon gains a Wizard level? Would it keep its proficiency bonus for everything, including its Wizard spells? Would its proficiency bonus reset to 2? Or would it have two separate proficiency bonuses, one for its Wizard spells and one for its dragon attacks and abilities?

[Lupine]

this is DM territory. DM chooses. I would advise that you choose the higher one, unless you want your dragon to feel gimped.

[prabe]

A monster with class levels is still a monster. I think the intent is that you choose the proficiency bonus that best fits the monster's CR with those class levels. With a dragon--especially an older one--it probably takes a lot of class levels before that matters.

[Xapi]

Personally I'd do the proficiency bonus for a Monster of a CR = the monsters CR + the levels it has, or something like that.

[HappyDaze]

It's based entirely off of the CR. Adding class levels might (probably will) increase the CR in some cases, and if so, that might be enough to raise the proficiency bonus. Adding a single level of Wizard to most dragons isn't going to do anything to its CR, but adding a level of Barbarian to most orcs almost certainly will have an effect on its CR.

[BurgerBeast]

It's based entirely off of the CR. Adding class levels might (probably will) increase the CR in some cases, and if so, that might be enough to raise the proficiency bonus. Adding a single level of Wizard to most dragons isn't going to do anything to its CR, but adding a level of Barbarian to most orcs almost certainly will have an effect on its CR.

This. Add the class levels. Then re-calculate CR.

I would, instead of adding class levels, go through the class abilities and (1) only take those class abilities relevant to your monster and (2) convert them to more monster-y abilities. For example, if you add Barbarian levels, you could just give it rage stats and flavour it as fighting in a rage, because generally it will only be having one combat anyway. For Extra attack, just make it multiattack. If it fights with two weapons, just incorporate that into multiattack. If it’s a wizard, does it need every subclass benefit? Or the short rest spell slot recovery? Probably not.

[DevilMcam]

An archmage is more or less a level 17th wizard but is only CR 12 and use the proiciency for that CR, so I'd go for CR everytime. As mentionned before Addingclass levels/features will most likely increase the CR.

[JellyPooga]

This. Add the class levels. Then re-calculate CR.

I would, instead of adding class levels, go through the class abilities and (1) only take those class abilities relevant to your monster and (2) convert them to more monster-y abilities. For example, if you add Barbarian levels, you could just give it rage stats and flavour it as fighting in a rage, because generally it will only be having one combat anyway. For Extra attack, just make it multiattack. If it fights with two weapons, just incorporate that into multiattack. If it’s a wizard, does it need every subclass benefit? Or the short rest spell slot recovery? Probably not.

This.

Creating an NPC, whether it be a recurring ally, BBEG Villain or Generic Mook, is about pitching encounters at the PCs, not about building characters. Adding Class Levels to a monster is an easy patch to give a creature a familiar "package" of abilities, but at the end of the day, many of the features a Class offers simply aren't appropriate or applicable to the way a monster is going to fight or act in a given encounter. For example; the distinction between an ability that recharges on a Short Rest compared to one that recharges on a Long Rest is entirely irrelevant if the PCs only encounter an NPC with that feature once. It's why you tend to see more "Recharge" features on monsters than "Short Rest" ones. Cherry picking Class Features is often much easier than trying to calculate the CR of a creature with all the Class Levels required to get them as a PC. For Example; if I want to add Shadow Jump to a creature, it's a lot easier to account for simply tacking that ability onto a creature than trying to account for all the Monk features I'd need to add to get it from Class Levels.