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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 at 5:11

2 Answers 2

up vote 7 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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RAW, it says that Concentration is (possibly) broken when you take damage. This will interrupt sustained Concentration spells or readied spells.

It is reasonable to assume that concentration is required to cast any spell.

While not specified 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.

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

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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 at 5:17
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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 at 5:20
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@SevenSidedDie because the rules don't allow for interruptions, and the proposed trick is turning a reaction into an interruption by wordplay. –  Zachiel Jul 22 at 12:03
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@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 at 14:33
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@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 at 14:35

protected by C. Ross Jul 26 at 21:04

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