Does damage that "can't be reduced" bypass temporary hit points?

Question

As per title. The temporary hit points description says:

Some spells and special abilities confer temporary hit points to a creature. Temporary hit points aren't actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.

And there are options in game that deal damage, which cannot be prevented in any way, like Life transference spell:

[...] You take 4d8 necrotic damage, which can't be reduced in any way, [..]

Or the Vital Sacrifice feat from Tal'Dorei campaign setting reborn, which sparked the debate at my table:

As a bonus action, you can choose to take 1d6 necrotic damage to gain a blood boon. [...] The damage you take to gain a blood boon can't be reduced in any way.

So, would temporary hit points block the damage to the real hit points from these options? My player goes on to provide as an example a RPG-bot ruling otherwise in their feat rating for the book, and Jeremy Crawford's tweet on similar matter with concentration:

When temporary hit points absorb damage for you, you're still taking damage, just not to your real hit points.

arguing that you do take damage, and temp hit points are not a reduction, so they work against the options presented above. For me it's... complicated, as I think it's counter intuitive, easily abusable and the temp hit points straight up say they are a buffer against damage, so that damage would go past them, and on the other hand is my player, and the rest of the group which stands by "it just works, it makes sense", which, of course they do, it's just beneficial to the player/party.

Answer 1

Temporary HP do not reduce damage

Therefore, they work in this case. The feature description says:

The damage you take <...> can't be reduced in any way.

Temporary hit points increase your HP pool. But they do not reduce damage. You still take damage normally. This damage will be subtracted from your hit points (temporary first):

When you have temporary hit points and take damage, the temporary hit points are lost first, and any leftover damage carries over to your normal hit points.

Basically, temporary hit points are the kind of hit points which can't be restored by healing, but can go above your HP maximum.

Answer 2

Maybe

You're concerned it's "easily abusable". That means you're concerned that in the long run, it's less fun. That tops everything. If you think it makes the game less fun, you should disallow it.

If you think it's easily abusable, but you're not sure, that seems like a worthwhile question in itself.

The language isn't definitive

Encryptor's answer provides a rationale for saying this is RAW.

I think there are counter-arguments.

These features say, you can't reduce the damage "in any way". Isn't that what the temp HP is doing?

There are other arguments. The Vital Sacrifice feat says:

You’ve learned secrets of hemocraft that grant you esoteric power at the price of your own life force.

Temporary hit points aren't "your own life force".

The text of the rules just isn't definitive, and the DM has to decide.

Rules as intended?

Rules as intended? We don't know, but the language "can't be reduced in any way" seems pretty clear.

It implies an intent to come up with an umbrella term to prevent finding ways around the reduction, which is what using temporary hit points does.

Making a ruling

Obviously, if it seems more fun to allow it, then allow it. If it feels like an exploitable rules loophole that's going to make the game less fun, don't allow it.

If you're inclined to disallow it, one pretty reasonable approach is to disallow on a case-by-case basis. For Vital Sacrifice, the "price of your own lifeforce" seems like a powerful argument.

Depending on your table, you could allow it provisionally. Some tables, that works, some tables, once the players have a shiny, it's difficult to pry it from their temporary-hit-point-empowered fingers.

Answer 3

The damage bypasses the temporary HP

Temporary hit points do reduce damage, which means these features get around them and affect your real hit points directly.

What does it mean to "reduce damage"?

Damage reduction is not a defined term in the rules, so we have to use a natural-language interpretation. I would say that, whenever something makes the amount of damage done to your character smaller than it would be otherwise, that thing is reducing the damage.

Do temporary hit points reduce damage?

If you interpret temporary hit points as part of your character, then no, they don't reduce damage. But if you interpret them as something around or on top of your character—like a kind of magical shield—then yes, they do reduce damage. This answer to another question judges that "can't be reduced" damage would bypass an Arcane Ward, which similarly absorbs damage until its own hit points are depleted.

So, are temporary hit points part of the underlying character, or are they separate?

Based on their description, I would say "separate".

Temporary hit points aren't actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.

As described, this "buffer" or "pool" is its own entity, which is distinct from the character ("you") that is being protected.

This is reinforced by the fact that characters can die even if they still have temp HP:

If you have 0 hit points, receiving temporary hit points doesn't restore you to consciousness or stabilize you. They can still absorb damage directed at you while you're in that state, but only true healing can save you.

If "absorb[ing] damage directed at you" doesn't qualify as reducing damage, I'm not sure what does.