Does dealing 0 damage to a concentrating spellcaster require a saving throw?

Question

Wizard is currently concentrating on a spell. Wimpling has a -1 Strength modifier, and makes a melee attack on Wizard. The attack hits, but Wimpling rolls a 1 on the damage roll, meaning that after applying his Strength modifier, he deals 0 damage.

Per the SRD, Damage Rolls (and in more recent Player's Handbook printings):

With a penalty, it is possible to deal 0 damage, but never negative damage.

From the Spellcasting chapter, Concentration:

The following factors can break concentration:

…

Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon’s breath, you make a separate saving throw for each source of damage.

But I'm not sure if "dealing 0 damage" means that the target "takes damage". Does Wizard need to make a Constitution saving throw to continue concentrating on the spell?

I'd prefer official sources or references if available. If there aren't any, then I would accept any semi-official or well-informed well-reasoned arguments, preferably backed up by whatever evidence is available.

Answer 1

No saving throw is triggered

I found two relevant Q&A twitter posts from Jeremy Crawford (the Lead Designer and Managing Editor for DnD 5e) that shed some light on this. However, Jeremy's tweets are no longer considered official, but they can provide some guidance into his thought process at the time he wrote the tweet.

Question:

if a raging barbarian makes no attack, but takes one damage that is reduced to zero, does rage drop?

Answer:

Taking 0 damage is the same as taking no damage.

If you took no damage, you didn't take any damage.

Taking 0 damage does not count as taking damage, therefore it shouldn't trigger any effect conditioned upon taking damage.

I also found a specific example of a different effect that triggers on damage (in this case, drow poison from the DMG):

Question:

If the Battlemaster maneuver parry, reduce the weapon Drow damage to zero, the poison damage still work ?

Answer:

Drow poison in the DMG is delivered by piercing/slashing damage (0 dmg = 0 poison).

To me, this lays out a clear line of logic. Dealing damage applies Drow poison. If you deal 0 damage, it does not apply the poison according to Jeremy Crawford. Therefore, dealing 0 damage does not count as dealing damage for effects that trigger on dealing/taking damage. This would logically include concentration checks.

Answer 2

No concentration saving throw is required.

Fifth Edition strives to use natural language whenever possible. This was a stated design philosophy from the writers of the game. From the D&D Podcast, Jan. 2017 at approximately the 11:39 mark, Lead Designer Jeremy Crawford says:

"This is a general principal in our rules. If the rules do not specifically add meaning to an English word, or take meaning away, or completely change the meaning it simply means what it means in idiomatic English"

Web searches on "5e" "design philosophy" and "natural language" will turn up many other hits, and Crawford and Mearls will often use similar language when answering rules questions.

In this case, "0 damage" is the functional equivalent of "no damage". The saving throw is not contingent on being hit, but upon taking damage. Since "no damage" was taken, no saving throw is required.

Keep in mind that the rules allow a DM to impose saving throws for other circumstances. From the same section:

The DM might also decide that certain environmental phenomena, such as a wave crashing over you while you’re on a storm—tossed ship. require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell

It is unlikely that Wimpling is sufficiently distracting, but a DM could require a saving throw whenever he feels it appropriate to the circumstance. In the case of straight out damage though, Wizard is safe.

Answer 3

Taking 0 damage is taking damage: the DC 10 saving throw is triggered.

The Sage Advice Compendium explicitly clears up that you can take 0 damage. 0 is the damage minimum.

Sage Advice Compendium V.2.3 (11):

Can damage be reduced to 0 by resistance or another form of damage reduction? There is no damage minimum in the rules, so it is possible to deal 0 damage with an attack, a spell, or another effect.

The rules on concentration state, in part (PHB 203-204):

Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. [...]

Taking 0 damage still counts as taking damage and thus interrupts your concentration. The DC is 10.

Dealing 0 damage might sound unintuitive because the outcome is that you are at the same Hit Points (PHB 196) that you have had before:

Whenever a creature takes damage, that damage is subtracted from its hit points.

But this is merely from a mathematical mindset, from a plain English standpoint you still take damage (and subtract it). That your hit point total doesn't have to change is emphasised by looking at receiving damage to temporary hit points Sage Advice Compendium V.2.3 (13):

If I have 10 temporary hit points and I take 30 damage from an attack while concentrating on a spell, what is the DC of the Constitution save to maintain my concentration? The DC is 15 in that case. When temporary hit points absorb damage for you, you’re still taking damage, just not to your real hit points. In contrast, a feature like the wizard’s Arcane Ward can take damage for you, potentially eliminating the need to make a Constitution saving throw or, at least, lowering the DC of that save.

So unless a game effect specifically calls out that it absorbs damage like the Arcane Ward does (PHB 115) or changes the recipient like the Aura of the Guardian (XGtE 39) et cetera eliminating the need to make a saving throw, then you still need to make a saving throw even when the damage does not change your actual hit points.

Answer 4

A saving throw is triggered

Damage Rolls [..] You roll the damage die or dice, add any modifiers, and apply the damage to your target.

Hit Points [..] Whenever a creature takes damage, that damage is subtracted from its hit points. (PHB 196)

The case of damageValue=0 is not called out as an exception anywhere and produces no contradiction. Thus RAW you can take 0 damage, and it should trigger any effect that depends on taking damage.

In the scenario you mention, it could be described as the weakling waving his arms in the wizard's face quite annoyingly, thus distracting him.