Balfore
Explorer
Such a great explaination! Thank you so much!Okay, here's the official rule from the introductory section of the Monster Manual (or the DM's Basic Rules):
"SPELLCASTING
A monster with the Spellcasting class feature has a spellcaster level and spell slots, which it uses to cast its spells of 1st level and higher (as explained in the player's D&D basic rules and the Player's Handbook)..."
So you cannot use the Spellcasting trait from any statblock, because all of the shapechanging options in the game (as far as I'm aware of) specifically prohibit you from using class features.
You can use the Innate Spellcasting feature unless something else tells you you can't, however.
Interestingly, the Spellcasting trait is the only one called out as a class feature in monster statblocks. One might assume that something like Sneak Attack on the assassin would count as a class feature too...but that becomes problematic. For example, that would mean that a creature could polymorph into a CR 6 Hobgoblin Warlord and get 4d6 Martial Advantage, but not into a CR 5 Master Thief and get 4d6 Sneak Attack.
There are other examples. The major problem is that the NPC statblocks in the appendices are not substantially different than the NPC statblocks included as parts of monster entries (such as Hobgoblins and Drow). So if you take "has the same name as a class ability" as the standard, you end up with really arbitrary and inconsistent restrictions that drive you to transform into hobgoblins and drow instead of half-orcs and high elves, because the former have monster entries and special abilities that you can use and the latter don't.
I stick with the most conservative restriction to avoid that problem. Anything in a monster statblock (including the NPC statblocks in the appendices) is by definition a monster ability, not a class ability, unless something says otherwise. The only one that fits that bill is Spellcasting, which has been explicitly defined as a class feature.
As an alternative, you might note that the Spellcasting class feature lists a class and level in its description. You could use the same rules for other features: if the statblock says that it uses the feature as a character of a certain class and level, then it you could count it as a class feature. Off the top of my head, I can't recall any features that do that other than Spellcasting, but I may be missing something in the Volo's Appendix.
Honestly, I think that's probably how the designers intended it. They didn't care if a dragon turned into a hobgoblin or an assassin, but they didn't want monsters picking up spellcasting with the Change Shape feature. They phrased it as "class ability" and defined Spellcasting as being such, in order to be clear that it applied to Spellcasting, while leaving space for adding other class abilities to monsters in the future by calling them out as such--future compatibility with Change Shape.
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