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Arcane Lock says that it increases the DC of checks to break the object or pick any of its locks by 10. However, there is no listed increase in the object's AC or HP.
Does this mean that a character making an attack against an Arcane Locked object bypasses the spell's protection, because the object's AC is unchanged?
Does this mean that a character making an attack against an Arcane Locked object bypasses the spell's protection, because the object's AC is unchanged? (Emphasis mine.)
RAW, yes, because making an attack specifically opposes AC, not DC.
The target number for an ability check or a saving throw is called a Difficulty Class (DC). The target number for an attack roll is called an Armor Class (AC). (PHB p.7, emphasis in original)
Arcane lock specifically changes "the DC to break it or pick it," not the object's Armor Class. (AC would be implied by an attempt to damage the object, rather than to break it).
That said, in the DMG's section on Objects (pp. 246-7) an object's Armor Class is described as "how difficult it is to deal damage to the object when striking it." So there's definitely leeway for a GM to read an object's AC as a sort of DC. But that's not RAW.
The spell prevents it from being opened, not from taking damage.
"You and the creatures you designate when you cast this spell can open the object normally.
The clarification arrives in this part of the spell description:
You can also set a password that, when spoken within 5 feet of the object, suppresses this spell for 1 minute. Otherwise, it is impassable until it is broken1 or the spell is dispelled or suppressed.
It, means the object that the spell was cast upon. The meaning of "broken" can mean an object taking damage until it is broken1.
There is more than one way to skin a cat. Will you rely on your strength? You can, and check your DC against this enhanced barrier.
Or, you can work smarter, not harder, and use a tool. Consider what the simplest tool, the lever, does for increasing the effective force you apply to something.
Example: On p. 247 of the DMG, a small chest which is "resilient" has 3d6 hit points. If it is made of wood, you'd expect the AC to be 15, if made of Adamantine, and AC of 23. If it is protected with an arcane lock, someone can still hit it (with a weapon) enough times to smash it and render any lock, magical or mundane, moot. What that does to the contents of the chest is another matter.
Doors and other structural items will have hit points as the DM assigns, and AC per the table on page 246.
The DM may choose to waive the "autohit with a 20" on objects (which I would as DM) so that particularly strong objects take a lot more work to break via brute force.
1 Breaking something and attacking it to deal damage are separate things in D&D 5e. One is a Strength check to force it open / snap it in two / cave it in / whatever. It succeeds or fails in one roll. The other takes normal hitting with a weapon and dealing HP damage until it runs out of HP, at which point it becomes broken. Credit to @SevenSidedDie and @Theo Brinkman
The "Doors" section of the DMG (page 103) specifically describes methods for getting through locked doors. (This guidance could be applied to chests or other locked objects. I would suggest it should be, as most objects with locks have some sort of door.)
Arcane Lock specifically protects from the first two methods:
Arcane Lock increases the DC for either of these attempts by 10.
Hulk Smash
The third method is (emphasis added):
This would be handled by attacks vs AC (and factoring the door's damage threshold (DMG, Objects, p. 247) so it is not protected by Arcane Lock.
Break Locks, Smash Doors
By RAW, attacking the lock itself is not included as way to get through the door, it's the door that is the object you attack.
Attempts to "break the lock" (note, that is a quote from the spell description) would be considered as interacting with an object and handled as strength checks (PH, p. 185) subject to the higher DC.
Let Play Balance Guide You
If an Arcane Lock on a solid door can be defeated by a couple of whacks at the keyhole, you are doing it wrong. On the other hand, an Arcane Lock on a dilapidated barn door would not make the door impregnable. (It's Arcane Lock not Arcane Door.)
