Is a character aware of how effective one casting is over another

dnd-5eeldritch-invocationsspellswarlock

My warlock takes the Fiendish Vigor (PHB p.111) invocation, allowing them to cast False Life (PHB p.239) as a first level spell at will.

False Life
1st-level necromancy
Casting Time: 1 action
Range: Self
Components: V, S, M (a small amount of alcohol or distilled spirits)
Duration: 1 hour
Bolstering yourself with a necromantic facsimile of life, you gain 1d4 + 4 temporary hit points for the duration.

My intent was to recast this ability several times between battles until I hit the maximum effect, 8 temporary hit points. However, my DM ruled that my character is unaware of how effective the spell cast was, and can't determine whether a given cast was for 5, 6, 7, or 8 temporary hit points. I went along with it at the time, but I'm wondering if there is any precedent of a character recasting spells to maximize the effect.

Best Answer

"You" get to choose which temporary hit points to keep

The rules for temporary hit points state:

If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22.

But who is "you"? The player or the character? The Combat chapter (which contains the temp HP rules) switches between the two meanings:

When attacking with a weapon, you add your ability modifier--the same modifier used for the attack roll--to the damage. A spell tells you which dice to roll for damage and whether to add any modifiers.

The "you" that attacks with a weapon is arguably both the character and the player. The "you" that rolls damage dice is clearly the player. False life works the same way. The player chooses to cast the spell, the character casts it, the player rolls dice for the spell, and then the player chooses which temporary hit points to keep.

There is precedent for the DM making some rolls for you

The DMG's Running the Game chapter first establishes a baseline:

Establish expectations about rolling dice. Rolling in full view of everyone is a good starting point.

Later on, it provides one scenario where the DM may choose to roll for the player:

You might choose to make a roll for a player because you don't want the player to know how good the check total is. For example, if a player suspects a baroness might be charmed and wants to make a Wisdom (Insight) check, you could make the roll in secret for the player.

In my experience, certain skill checks are the only time I've ever seen a DM roll dice for a player. I have never seen a DM roll damage (or healing) for a player's spell - and particularly not secretly.

Combining magical effects

I believe that the rules for temporary hit points are more specific than the general rules for combining magical effects. This would mean that you only choose which temp HP to keep each time you cast - you don't simultaneously have 4+ different possible temp HP values. If your DM keeps the old and new temp HP values secret, you won't know which one to choose.

If the temporary hit points from the "most potent" casting of false life simply suppressed the other castings, then what would happen in the following situation:

  1. You cast false life. The DM secretly rolls 6 temp HP.
  2. Thirty minutes pass without you taking damage.
  3. You cast false life again. The DM secretly rolls 5 temp HP.
  4. You take 3 damage.
  5. Thirty minutes pass, meaning that the first casting ends.

How many temporary hit points do you now have? Do you suddenly jump up to 5, since the "more potent" casting has ended? Or do the suppressed castings of false life somehow also take the damage? I believe the most straightforward answer is that each casting forces you to make a one-time choice between the old and new value at the time of casting, per the rules for temporary hit points.

Establish expectations

In the end, it's up to your table. However, here are some things to consider when discussing how to proceed:

  • This is at most a difference of 3 temporary hit points. Does your DM feel that the temp HP is unbalanced, or are they just concerned about (potential) metagaming?
  • Does your DM also roll weapon attacks or damage for magic missile? After all, knowing that I crit for 25 damage instead of hitting for 4 could change my tactics.
  • Is your DM willing to track your temporary hit points for you? Your DM will need to remember that 2 (out-of-game) hours ago, you got 6 temp HP from false life, so they can subtract that from future attacks. If not, you'll have to know the actual value.
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