Question about Armor of agathys and getting temp hp

[The-Sorcerer12]

If I have AoA up then use a ability to give me more temp hp then AoA had. I get the higher temp hp but do I still keep AoA up or is it gone When I loss the temp hp it came with.

[RogueJK]

You can only have one set of Temp HP at a time, from one source. If you gain additional Temp HP from another source, you choose whether to keep your existing Temp HP total or go with the new Temp HP total.

Therefore, if you have AoA running, and you choose to gain Temp HP from another source, it overrides your AoA Temp HP and AoA ends, because AoA's description specifically 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."


The official Sage Advice Compendium actually addresses this very scenario. From https://media.wizards.com/2020/dnd/downloads/SA-Compendium.pdf :


Can you extend the duration of armor of Agathys by gaining more temporary hit points?

The spell is meant to work only as long as you have the temporary hit points that the spell grants. When those temporary hit points are gone, the spell is done. Keep in mind that temporary hit points aren’t cumulative (see PH, 198). If you have temporary hit points and receive more of them, you don’t add them together, unless a game feature says you can. You decide which temporary hit points to keep. As an example, let’s say you’re a warlock with the Dark One’s Blessing feature, which gives you temporary hit points when you reduce a creature to 0 hit points. You currently have 2 temporary hit points remaining from armor of Agathys, you just slew a monster, and your Dark One’s Blessing can now give you 4 temporary hit points. If you take those temporary hit points, they replace the ones from armor of Agathys and end that spell, so you might not want to take them and keep the spell going instead, so as to continue benefiting from the cold damage it delivers.

[Dualight]

With the question already answered, I'd like to point out that the "Simple RAW" threads exist for questions such as this one. The active one is permanently at the top of the 5e subforum.

[Chronos]

And there are multiple factors that might go into your decision of which source of temp HP you want to keep. Some sources of temp HP come with riders, like Armor of Agathys. Some sources grant more of them. Some sources last longer. Depending on the situation and the exact numbers, you might choose any of them.