Simple RAW 3

[Flashy]

Q159 - Armor of Agathys. Does the cold damage dealt to an attacker depend on the amount of damage prevented by the temporary hit points, or is the damage always the static 5 per spell slot level?

e.g. if I make a melee attack against a warlock with 5th level Armor of Agathys, and I deal 8 damage, does the Armor of Agathys retaliate me with 8 cold damage or 25 cold damage?

A159: The cold damage is dependent on spell level, not on the amount of damage done to the temporary hit points. The spell is worded as "If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage" with the upcasting text specifying that both the number of hit points and the damage per hit scale with level.

In your specific example the Armor of Agathys retaliates for 25 cold damage.