I have seen many questions on the web related to Jack of All Trades, revealing facts new to me such as that it adds to initiative, on account of being an ability check which Jack of All Trades indeed affects.
However, one question I haven't been able to find an answer to is whether Jack of All Trades affects AC. If we compare it to initiative, it seems like AC should be an ability check as well:
- It uses an ability to arrive at the final value (dex)
- It's possible to add proficiency bonus to the check (defensive duelist)
- It's a check that is checked for each turn — it's not a static value since different effects can affect it
Does Jack of all trades affect AC? Is there an official ruling on this?
Best Answer
No. Armor Class is not an ability check.
The PHB does not describe armor class(AC) as an ability check. Armor class is its own in-game thing.
While the Dexterity ability adds or subtracts to AC in some cases via the Dexterity modifier, it is not universal:
If you wear heavy armor, your Dexterity isn't added to your AC.
If you wear medium armor, there is a limit (2, or 3 with the Medium Armor Mastery feat) to how much dexterity modifies your AC.
If you wear light armor or no armor, your full Dexterity modifier is applied to determine your armor class. (Armor Table: PHB, p. 145).
The game term used is Dexterity modifier, not a Dexterity based ability check. Further that point, your AC is generally a static value (with specific exceptions depending on feats, cover, magic items, using your shield or not, etc. as you noted). You don't roll a d20 to determine your AC, whereas for an ability check you roll a d20 against a DC to determine success or failure.
Ability Checks (PHB p. 174)
@T.J.L. made the following point in a comment that may aid in understanding:
If you are trying to view AC vis a vis ability checks, AC more closely resembles a DC(Difficulty Class) that an ability check tries to beat.