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A few attacks, such as the Shocking Grasp cantrip, can prevent reactions on a target when they hit it.

A few "defensive reactions", such as the Shield spell (+5 to AC including against attack roll), can prevent an attack from succeeding.

I have recently faced the following scenario : I had an AC of 18, and an opponent attacked me with an equivalent of Shocking Grasp, touching with, I believe, a 21. At that moment, I was not sure if I could use my reaction to cast a Shield to jump my AC to 23 (therefore blocking the attack), or if the attack already prevented me from using Shield.

As far as I know, an attack has three phases (make the roll, determine if it hits, then apply the damage and effects). I would believe that the Shield reaction occurs between phase 2 and phase 3, therefore potentially protecting from an attack like Shocking Grasp, but I could be wrong.

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Yes it does protect you from Shocking Grasp.

Shield from the SRD.

An Invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering Attack, and you take no damage from Magic Missile.

It doesn't say that it can't block cantrip damage. In addition it increases the AC for the attack that you use the reaction on.

Shocking Grasp from the SRD.

Lightning springs from your hand to deliver a shock to a creature you try to touch. Make a melee spell Attack against the target. You have advantage on the Attack roll if the target is wearing armor made of metal. On a hit, the target takes 1d8 lightning damage, and it can't take reactions until the start of its next turn.

It doesn't say that it can't be blocked by the spell shield. It is a melee spell Attack which according to shield it just needs to be an 'Attack' for the reaction trigger. In addition you take damage and can't take reactions together so if you can prevent the first you have to prevent the other since it is a chain linked by 'and' in this situation.

After you cast shield the resolution happens because shield allows you to increase your AC before the Attack so you can potentially prevent the attack from hitting you.

From these two spells this is how the flow seems to turn out using page 194 of the PHB 'Making an Attack' step three.

You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

This wording the shield effect would take place on the comma in this interpretation of "On a hit, you roll damage..." So it breaks the flow into these actions steps:

  1. Determine hit is successful. Player doesn't know total so they can't metagame in this situation.
  2. Shield reactions are triggered allowing you to increase your AC, including the triggering attack.
  3. Re-determine if the hit is successful. There is still a chance that it is if the player can't metagame this knowledge. e.g. the player's AC is 15, but the attacker rolled a 20 total for hitting the player.

Since the reaction prevention only comes with the damage that is dealt and shield could prevent the damage from being dealt in the first place it seems to reason that shield can be used since you would be able to prevent the damage.

Now if the attack roll and damage roll happened at the same time then no you wouldn't be able to prevent this, but there is an ordered set of steps in the 'Make an Attack' step 3 outline where hit must come before damage.

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  • 1
    \$\begingroup\$ You might highlight that the shield spell specifies that it works against the triggering attack. \$\endgroup\$ – Chemus Aug 16 '17 at 22:25
  • \$\begingroup\$ That seems to cover it. Thanks for the improvement! \$\endgroup\$ – SevenSidedDie Aug 17 '17 at 3:51
  • \$\begingroup\$ Your answer is good, therefore I will accept it. Note, however, that the metagame information of the total enemy attack roll is something that a person with the Shield spell can possibly request to know, according to this : thesageadvice.wordpress.com/2014/10/11/shield-before \$\endgroup\$ – Gael L Aug 17 '17 at 15:05
  • \$\begingroup\$ "It doesn't say that it can't be blocked by the spell shield" — is there any feature or spell, that explicitly says "it can't be blocked by the spell shield"? If there is none, this is not relevant, isn't it. \$\endgroup\$ – enkryptor Aug 13 at 19:35
  • \$\begingroup\$ I disagree with " it is a chain linked by 'and'". The word "and" is a conjunction used to make a list. It doesn't declare a precondition. A list of things happen on hit. The items list are separated by the conjunction. Both happen on hit. \$\endgroup\$ – GcL Aug 14 at 15:18
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Yes, you are protected

To me, there are two issues to resolve:

  1. Can shield turn the triggering hit into a miss?
  2. Does shocking grasp prevent shield from triggering?

Hit into miss

To answer the first, I believe that yes, shield can turn the triggering attack into a miss. From the DMG:

For example, the opportunity attack and the shield spell are clear about the fact tat they can interrupt their triggers. If a reaction has no timing specified... the reaction occurs after its trigger finishes (DMG 252).

From this, I gather that shield is treated differently than other reactions. The only reason for this behavior is if shield can prevent the hit that triggered it from hitting. Otherwise, there would be no need to call it out as an exception to the general rule. Furthermore, since shield does not heal damage, the trigger has to go before the attack actually lands.

Prevention of triggering

Based on shield being able to turn a hit into a miss, it follows that shocking grasp cannot prevent shield from being cast. From the description of shocking grasp:

On a hit, the target takes 1d8 lightning damage, and it can't take reactions until the start of its next turn (PHB 275).

Emphasis mine. In other words, the damage and the no-reactions clause come together. Since shield (when cast) prevents the lightning damage, it also prevents the no-reactions clause from occurring.

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Shield

An Invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering Attack, and you take no damage from Magic Missile.

Since Shield can be used against the triggering attack, it can be used before the triggering attack hits in order to make it miss.

Shocking Grasp

Lightning springs from your hand to deliver a shock to a creature you try to touch. Make a melee spell Attack against the target. You have advantage on the Attack roll if the target is wearing armor made of metal. On a hit, the target takes 1d8 lightning damage, and it can't take reactions until the start of its next turn.

The target isn't denied actions until after they are hit.

Thus, you can use Shield.

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