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Me and the DM are fairly new to D&D so we're still trying to figure out the rules. DM says that spells with a duration of "1 reaction" (ie. Shield) work as described below.

I totally acknowledge that the DM could make this a house rule, but that's not where we are (he said he thought it was in the rules somewhere). I'm not able to find anything confirming his interpretation in the PHB, so I wanted to get the internet's interpretation before asking our DM about it again.

  1. On my turn, "pre-cast" Shield as my action.
  2. Before my next turn, I'm hit by an attack.
  3. After learning the attack hits, I use the reaction to get +5 AC.
  4. DM compares to his attack roll and determines whether the attack hits retroactively.
  5. +5 AC lasts until the start of my next turn.

DM claims step 1 is necessary and consumes a spell slot. I claim that step 1 is not necessary and the spell slot is consumed after I use the reaction. Thoughts?

Spell description:

Shield

1st-level abjuration

Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell
Range: Self
Components: V, S
Duration: 1 round

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.

And...

Reactions

Some spells can be cast as reactions. These spells take a fraction of a second to bring about and are cast in response to some event. If a spell can be cast as a reaction, the spell description tells you exactly when you can do so.

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    \$\begingroup\$ To correct a term, Shield does not have a Duration of 1 reaction, it has a Casting Time of 1 reaction. That's an important distinction (though I don't know what a duration of 1 reaction would actually mean) \$\endgroup\$ – Kamil Drakari Apr 18 '18 at 13:24
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    \$\begingroup\$ In fact, the spell clearly states that Shield has a Duration of 1 round. \$\endgroup\$ – Mooing Duck Apr 18 '18 at 17:22
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You are correct, reaction spells cannot be readied or precast

Reaction spells are cast as a reaction in response to a trigger that is spelled out in the spell description. In the case of shield, that trigger is "when you are hit by an attack or targeted by the magic missile spell". When that triggers occurs, you can cast the spell and spend the spell slot at that time to do it.

You do not need to Ready a reaction spell in order to use it in this way. It is simply the way reaction spells work.

Your DM is likely just confusing the rules a bit since every other spell (with a casting time of 1 action), in order to be Readied and cast as a reaction, actually does need to be precast and then held until the trigger occurs. In fact, according to the rules, reaction spells cannot be readied at all.

To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration.

Since these spells have a casting time of reaction they do not qualify. Regardless, reaction spells do not need this though since they have that functionality built-in.

In this sense, reaction spells are practically always Readied, with the exception that you do not need to precast them as you do with a Readied spell. You only spend the spell slot when the trigger occurs.

If reaction spells did not work this way, then you would have to Ready feather fall in order to prevent a fall. Obviously it would be ridiculous to require a character to Ready feather fall every round (spending a slot each time) in case they fall, and in fact would make the spell essentially useless.

Jeremy Crawford has also said as much:

If an effect, like the shield spell or an opportunity attack, lets you use your reaction, you don't Ready it.

So the order of events would be:

  1. I'm hit by an attack.
  2. After learning the attack hits, I spend a spell slot to cast shield as a reaction to get +5 AC.
  3. DM compares to his attack roll and determines whether the attack hits retroactively.
  4. +5 AC lasts until the start of my next turn.
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    \$\begingroup\$ Shield seems to confuse people, because it relies on the player already being hit. The thing they have to realise is, shield is a time travel spell. \$\endgroup\$ – Doctor Jones Apr 18 '18 at 8:28
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No, you do not need to ready the reaction spell.

The "Casting Time" of spell XYZ presents you with the time required to complete casting after the player states, "I wish to cast the spell XYZ." A spell that requires 1 minute demands one minute of game time, 1 action requires the character to use an action on his or her turn, 1 bonus action requires the use of the character's bonus action on his or her turn, etc. Similarly, a "reaction" spell requires the use of the character's Reaction, one of which is available per Round (i.e., it's refreshed at the start of your turn, and available until the end of the turn preceding yours).

You can use your turn to take a separate action, and possibly a bonus action, and still cast Shield on another character's turn as long as you do so when hit by an attack or targeted by Magic Missile. In terms of your order, remove Step 1, and change Step 3 to say "After learning the attack hits, I use my reaction to cast Shield, and get +5 AC."

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You don't have to precast a reaction spell in order to be able to use it so you are correct. Jeremy Crawford has said so himself. However if you intend to use shield to avoid an attack of oppurtunity you will lose your ability to use a spell on your bonus action and if you did use a spell you will not be able to use shield to avoid an attack of oppurtunity per Jeremy Crawford's words.

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    \$\begingroup\$ If it takes away your ability to cast a spell using a bonus action, it does so only if you use the Reaction spell on your turn, which is atypical but possible (e.g., to use Shield to avoid an opportunity attack). Per PHB 202, under Bonus Action spells, "You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action." Here, of course, "turn" =/= "round". \$\endgroup\$ – pondrthis Apr 18 '18 at 3:20

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