When does Armor of Agathys take effect?

Question

When does Armor of Agathys take effect given that Damage Reduction happens first?

Consider a character with Heavy Armor Master which says:

• While you are wearing heavy armor, bludgeoning, piercing, and slashing damage that you take from non magical weapons is reduced by 3.

and Armor of Agathys which says:

You gain 5 temporary hit points for the duration. If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage.

If the character is struck for 3 or fewer points—all damage would be negated by Heavy Armor Master—does the attacker still take the 5 points of damage from the Armor of Agathys? or because the Armor of Agathys did not absorb any damage is the attacker unaffected?

Answer 1

I would say the Armor of Agathys would still take effect - it's description says if a creature hits you with a melee weapon, not damages you. There are a variety of effects that could mitigate the damage that wouldn't prevent it from being a hit in the first place.

This also makes in-game-world sense; the spell is putting off cold that hurts people from you and your gear; the heavy armor master ability is letting you shrug off attacks that hit your armor, so there's no good simulationist reason the cold shouldn't affect the attacker either.