Fiendish Vigor & Armor of Agathys

If a level 2 warlock takes Fiendish Vigor, can they keep Armor of Agathys going for the full duration just by recasting False Life constantly? It seems like AoA doesn't care where a character's temporary hit points come from, just that they never hit 0.

If you're using the 5.5e version of Armor of Agathys, yes. This was a deliberate change made in 5.5e; the 5e version of the spell only stayed up as long as its own temp HP remained.

At higher levels this will be less effective, as eventually you'll reach the point where enemies are doing enough damage to erase the 12 temp HP from Fiendish Vigor in a single hit, but it's great for a while. 

Quote from wagnarokkr >>

At higher levels this will be less effective, as eventually you'll reach the point where enemies are doing enough damage to erase the 12 temp HP from Fiendish Vigor in a single hit, but it's great for a while. 

 There's no real opportunity cost for doing this, though, right? It seems like the warlock can just swap out the invocation once it loses its teeth.

Quote from Nebuul >>

Quote from wagnarokkr >>

At higher levels this will be less effective, as eventually you'll reach the point where enemies are doing enough damage to erase the 12 temp HP from Fiendish Vigor in a single hit, but it's great for a while. 

 There's no real opportunity cost for doing this, though, right? It seems like the warlock can just swap out the invocation once it loses its teeth.

Yeah, that's true, you can always swap it out when you gain a level.

Quote from Nebuul >>

If a level 2 warlock takes Fiendish Vigor, can they keep Armor of Agathys going for the full duration just by recasting False Life constantly?

 In theory yes but typically no, because the spell ends immediately if your temp hp drop to 0, temp HP do not stack, and there's a good chance enemies will do more than 2d4+4 damage between your turns.

Quote from Pantagruel666 >>

Quote from Nebuul >>

If a level 2 warlock takes Fiendish Vigor, can they keep Armor of Agathys going for the full duration just by recasting False Life constantly?

 In theory yes but typically no, because the spell ends immediately if your temp hp drop to 0, temp HP do not stack, and there's a good chance enemies will do more than 2d4+4 damage between your turns.

Note that the 2024 version doesn't make you roll, it just gives you a straight 12 every time. Makes it more viable for tier 1 and early 2, I'd say.

Quote from wagnarokkr >>

If you're using the 5.5e version of Armor of Agathys, yes. This was a deliberate change made in 5.5e; the 5e version of the spell only stayed up as long as its own temp HP remained. [...]

 I agree. In fact, the 5e and 5.5e SAC have different answers to the same question:

(5e) 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.

(5.5e) Can you extend the duration of Armor of Agathys by gaining more Temporary Hit Points?

The spell requires you to have Temporary Hit Points, and they don’t need to be from the spell.

Quote from The_Ace_of_Rogues >>

Note that the 2024 version doesn't make you roll, it just gives you a straight 12 every time. Makes it more viable for tier 1 and early 2, I'd say.

Wonder why they even have it be casting false life, instead of just gaining temp hp as a magic action (which would allow them to use a different hp formula, probably with level scaling). 

Quote from Pantagruel666 >>

Quote from The_Ace_of_Rogues >>

Note that the 2024 version doesn't make you roll, it just gives you a straight 12 every time. Makes it more viable for tier 1 and early 2, I'd say.

Wonder why they even have it be casting false life, instead of just gaining temp hp as a magic action (which would allow them to use a different hp formula, probably with level scaling). 

 Because it's better to use extant mechanics when possible, rather than making new ones.

By having it be the spell, most every question one could ask about how it interacts with any other ability is already answered. No question of whether you can, say, use it in a beholder's eye beam.

Quote from jl8e >>

 Because it's better to use extant mechanics when possible, rather than making new ones.

Temporary hit points are an extant mechanic. Which is simpler:

• As a magic action, gain warlock level + charisma modifier temporary hit points.
• You can cast False Life on yourself without expending a spell slot. When you cast the spell with this feature, you don’t roll the die for the Temporary Hit Points; you automatically get the highest number on the die.