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As this blog post suggests, there seems to be a hole in the D&D Basic rules. Maybe. The rules for readying an action state:

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

This seems to imply that it is impossible to interrupt a spell in 5e. If you set your trigger to be "I attack the Wizard when he casts a spell", then you make the attack after the spell has been cast. Does this mean that spells can no longer be interrupted in combat like they could be in previous editions?

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Well, presumably one could set their trigger as "I attack the wizard when he starts to cast a spell." – Arkhaic Jul 22 '14 at 5:11
up vote 18 down vote accepted

The Basic rules call out two situations where a Concentration check is required to cast a spell:

  • The spell is Readied but not yet cast (p.72)
  • The spell has a cast time longer than one action (p.79)

There is no specific rule that I have seen for interrupting spells which can be cast in a single Action, and this seems to be deliberate.

I would be extremely wary of adding such a rule, unless you're actively looking to nerf combat casting and it's something your players can live with.

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You're talking about the specific instance where one of your bullet-points is met and the caster takes damage, right? I think you should make that clear: there's no general need for a concentration check to cast a readied spell or a long-casting-time spell. – nitsua60 Jun 2 at 21:48

RAW, it says that Concentration may be broken when you take damage. This will interrupt sustained Concentration spells or readied spells. Concentration is also required to cast spells with longer casting time than a single action.

While concentration may not be needed for quicker spells, "mental focus" is.

Casting in Armor
Because of the mental focus and precise gestures required for spellcasting, you must be proficient with the armor you are wearing to cast a spell. You are otherwise too distracted and physically hampered by your armor for spellcasting.

I think it's reasonable to say that getting injured would be more distracting than wearing unfamiliar armor!

While not explicitly stated by RAW, a trigger of "I attack the Wizard when he starts casting a spell" should work.
You attack "right after" the Wizard starts casting.

The example triggers are split-second things that interrupt movement.

Examples include “If the cultist steps on the trapdoor, I’ll pull the lever that opens it,” and “If the goblin steps next to me, I move away.

"If the cultist steps on the trapdoor..." - you're not waiting for him to step there & stand around for the rest of his round, if he tries to walk over the trapdoor, you're pulling that lever the moment he steps on it, interrupting his move action.

Same should apply for spellcasting, unless maybe they have a "reaction" casting time (taking a fraction of a second, too fast to interrupt).

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That solution seems... really cheesy. In that case, the "right after the trigger finishes" text is meaningless, since you could reasonably apply the "when they start to X" trigger to any readied action, which doesn't seem like it's the intended behavior. – DuckTapeAl Jul 22 '14 at 5:17
It isn't cheesy. You just don't take your action before the trigger in some kind of timey-wimey nonsense. The trigger happens, you notice it, the trigger has finished at this point by virtue of it having happened in the past, you take your action. If your trigger is "when someone walks through this door", do you wait until someone's walked through the door, or do you go charge in before they've even done so? (Note they said: "right after the trigger finishes", not "right after the triggering action.") – doppelgreener Jul 22 '14 at 5:20
@Zachiel I think the "after the trigger finishes" is to make it clear that you need to pick an observable trigger and that you can't stop that trigger from happening. For example, you can't say "when someone starts to sneak attack me, I'll dodge." The first clue you have of a sneak attack is the blade in your ribs. You can't say "when the floor starts to give way, I won't take that step" because the floor collapsing happens too quickly. However, seeing the wizard waving his hands is something you can respond to. Stealthy wizards are, presumably, safer. – Gregory Avery-Weir Jul 22 '14 at 14:33
@Zachiel How is that not the intended behaviour? What makes you think the rules don't allow for interruptions—because they don't have a special Interrupt action type? Doesn't it look like this serves that exact purpose, just with simpler mechanics? – SevenSidedDie Jul 22 '14 at 14:35
@Zachiel 5e isn't locked down that way. Anything a PC can react to in "real" life can be stated as a trigger. (Yes, readied actions happen after the trigger is observed by the PC.) – SevenSidedDie Jul 22 '14 at 22:00

Actually, the post does miss something: it is possible to interrupt spell-casting.

Specifically, there is the 3rd level spell Counterspell on page 228 of the PHB. Furthermore, this spell is only included on the Sorcerer, Warlock and Wizard spell lists.

Clearly this option is not intended to be generally available. It also seems that there has been a deliberate effort to reduce spell interruption options.

Nothing in the Concentration or Casting Time sections of the PHB states that concentration is required for spells with casting time of "1 action". On the contrary: concentration is specifically associated with (a) maintaining a spell and (b) casting a spell with a long casting time. So the implication is that it's deliberately not possible to interrupt 1-action spell via damage.

If we go a step further and take a look at the Mage Slayer feat, it states:

  • When a creature within 5 feet of you casts a spell, you can use you reaction to make a melee weapon attack against that creature.
  • When you damage a creature that is concentrating on a spell, ...

Again there's a (seemingly) deliberate distinction between casting and concentrating.

Note: Maintaining concentration after taking damage requires a CON save at minimum DC 10. Since CON saves don't scale with level for most casters, it's not an easy ask. This would probably make the ability "to ready an attack to interrupt a caster starting to cast a spell" far too OP.

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protected by C. Ross Jul 26 '14 at 21:04

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