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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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The Basic rules call out two situations where a Concentration check is required to cast a spell: the caster takes damage and...

  • 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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    \$\begingroup\$ Of note: Even the mage slayer feat doesn't let you interrupt a spell about to be cast. You get an opportunity attack, but it's AFTER the cast. I would recommend adding this to the answer as it further demonstrates that even with a feat designed to counter mages, you still can't interrupt the cast. \$\endgroup\$ – Lino Frank Ciaralli Mar 2 '18 at 5:43
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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 task. 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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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).

This tweet from Jeremy Crawford supports my interpretation:

Can silence interrupt a spell caster? I.e. cleric readies silence, trigger is enemy begins spell casting.

As DM, I'd allow a readied silence spell to interrupt the casting of a spell with a verbal component.

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  • \$\begingroup\$ The only problem with that tweet is that a caster trying to do the vocal part of the spell and hearing no sound would immediately discern they're in a silenced area, move out, and finish the spell anyways. It's not so much a counter as it is a temporary inconvenience. \$\endgroup\$ – Lino Frank Ciaralli Mar 2 '18 at 5:47
  • \$\begingroup\$ @LinoFrankCiaralli Isn't a spell that is interrupted a spell that is lost? Meaning, once they start the spell, don't they lose the slots if they don't get to finish it? \$\endgroup\$ – Praxiteles Mar 16 '18 at 5:16
  • \$\begingroup\$ Good question. Is the spell slot burned at the beginning of casting or when casting is completed? \$\endgroup\$ – Lino Frank Ciaralli Mar 16 '18 at 17:39
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    \$\begingroup\$ @LinoFrankCiaralli: For spells with a casting time longer than 1 action, failing a concentration check during casting does not burn the spell slot. open5e.com/spellcasting/… It seems reasonable that only being Counterspelled with the actual spell can waste your spell slot before you finish casting. \$\endgroup\$ – Peter Cordes Nov 9 '18 at 4:00
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    \$\begingroup\$ @LinoFrankCiaralli: In the worst case, a DM might rule that Silence takes effect during the cast time, wasting the opposing caster's Action but not their spell slot. (But Actions are usually atomic, aka uninterruptible, in the RAW). Or if they didn't have enough movement remaining to leave the area before attempting a cast, then Silence as a reaction before their cast-a-spell Action starts would shut them down and maybe let them do something else with their Action that turn. \$\endgroup\$ – Peter Cordes Nov 10 '18 at 1:25
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You can get an attack off before the spell is cast, but it might not affect the spell.

Although I allow a person with a readied action to state "I wait until he starts to cast a spell" at my table (Many believe this to be within the rules, though some may rule otherwise), the spell still goes off after the readied action unless something specifically stops it. Examples could be a stunning monk strike, a grapple which prevents somatic gestures, an entangling effect, being knocked unconscious, etc.

The mere act of taking damage should not interrupt a spell—though it can interrupt concentration, which is another matter entirely—unless something in the spell description states it specifically. I am not aware of any spell which states this, however.

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    \$\begingroup\$ Well, some spells sort of state that, in that they have casting times longer than 1 action--those ones require concentration during the multi-round casting and therefore would require a concentration check upon taking damage. (But not many people are casting multi-round spells during combat.) \$\endgroup\$ – nitsua60 Dec 19 '16 at 21:22
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    \$\begingroup\$ True. The question seemed more centered on a combat situation. I only gave an answer because parts of my answer were to be found in the other answers, and I felt they needed to be tied together into one place to form one idea. \$\endgroup\$ – keithcurtis Dec 19 '16 at 23:04

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