[5E] Interrupting a Spellcaster via Ready Action

[TheSword]

The rule to make the Arcana check does use your reaction. Since the readied action allows you to use your reaction in response to the trigger, if you make the Arcana check you have no reaction remaining to act when the readied action is triggered.

That’s ok I used my google-fu and found the details under new used for skills in xanathars guide.

However I think there has been some assumption here. That ruling is about deducing specifically which spell was cast. Not about telling whether someone was casting which requires just clear sight/hearing.

Edit (just seen the earlier post acknowledging this)

 

[DND_Reborn]

How about another example. Let's say the fighter readied this action: "I throw my spear at the Flayer if it starts moving towards us". As a DM, would you really insist the the Flayer finish its movement - perhaps it moves 20' to get next to the wizard, attacks, then moves its last 10' to get next to the fighter - before allowing the fighter to throw the spear? I think this case is very clear: the trigger finishes as soon as the Flayer takes one step closer and the fighter can throw the spear or ignore the trigger, per the rules.

That is a great example. Perhaps a caster readies Lightning Bolt when a target moves. The target then moves up to engage an ally. Does the caster have to wait until the target has finished his move and thus risk hitting his ally with the Lightbolt as well?

It is similar to the example in the PHB, "If the goblin moves towards me, I move away." Would the goblin get to finish its move and be next to you before you move away? If so, then it would also get an OA when you move away. That would pretty much negate the purpose of having a readied action to move away--so the goblin could NOT actually get to attack you.

 

[TheSword]

There is nothing in the ready rule that says the trigger must be a complete action. It seems very reasonable that the trigger may be, ‘starts to cast a spell’ in which case then readied action takes place after the spell is begun but before the spell is completed.

A large portion of the effort for the readied action has already been spent. There is an assumption that the readied fighter has the spear in a throwing position, aimed at the mind flayer with the fighter concentrating on the exact moment. It doesn’t seem unreasonable for this to happen in the intervening 3 seconds it takes to cast a spell.

 

[Torquar]

Aside from specific effects like Counterspell, interrupting a 1 action spell isn't possible, they are too fast. Not even the Mage Slayer feat allows it. Since you can't interrupt attacks either, this seems fair enough.

 

[Li Shenron]

There is no such thing in 5e as "interrupting someone's action".

There is only interrupting someone's move with an OA or interrupting someone's spell with Counterspell.

Ready doesn't interrupt anything.

You either set "casting a spell" as the trigger and take your readied action after the spell, or set "moving hands" as trigger and take your readied action before he casts, but he can restart/resume casting (unless your readied action kills it or gives it a condition during which it cannot cast.

You can of course make it lose concentration (in the first case), but only if applicable i.e. it casts a concentration spell or a spell with casting time longer than 1 action.

 

[Shiroiken]

By RAW, it would be 2B, since 5E doesn't allow spell disruption via damage. A logical argument could be made that unless the fighter is very close (say 10 ft), the words and motion would be complete by the time the attack reaches the target anyway. Some DMs could houserule that spellcasters could be forced to make said Concentration check (mostly fans of 3E and older). It really depends on how the DM wants to run the game, so long as they remain consistent.

 

[TheSword]

Discussion at the table in a session zero is probably the right thing to do, as the rules can be played either way. 5e is deliberately not too specific about what can and can’t be done with actions in order not to limit play.

“What is good for the goose is good for the gander” so beware if you use this as players expect the DM to do it to you and vice versa.

 

[DND_Reborn]

Aside from specific effects like Counterspell, interrupting a 1 action spell isn't possible, they are too fast. Not even the Mage Slayer feat allows it. Since you can't interrupt attacks either, this seems fair enough.

More is the pity. D&D went from having spell take several seconds (measured in segments before) to most spells the somatic component is appearantly snapping your fingers, you don't have to actually have material components in hand, and the verbal components are "go!" or something.

Of course you can interrupt attacks, it is called Armor Class. ;-)

Seriously, though, there are ways to stop attacks from hitting (which is really, in essence, interrupting the attack). The Shield spell is a perfect example, although there are others. As a reaction you add +5 AC, after the hit even, making it miss. What else would you call that?

A lot of timing elements have been removed from D&D for simplicity, but like others I think they went overboard. We allow spells to be disrupted during casting, but it requires a readied action, a successful hit, and a failed concentration check. Fortunately, this works both in the players favor at times and against them at other times.

And why are spells so fast? I can understand the verbal only, maybe, and certain others. But if you play the material components must be in hand, that takes time, if the somatic components are complex in any way, that takes time, if the verbal components are more than a single word, it takes time. You put all that together and IMO spells absolutely should be long enough to stop while casting.

Finally, the "action" they take has nothing to do with the time required. Actions lack duration, as discussed at length in other threads.

 

[Torquar]

Of course you can interrupt attacks, it is called Armor Class. ;-)

Seriously, though, there are ways to stop attacks from hitting (which is really, in essence, interrupting the attack). The Shield spell is a perfect example, although there are others. As a reaction you add +5 AC, after the hit even, making it miss. What else would you call that?

Armour Class works against combat spells, Saving Throws too. The Shield spell would be one of those specific effects that I mentioned in the post you quoted, Readied actions aren't.

 

[5ekyu]

Just a couple of points.

If you allow ready to be specified on the "starting stuff" of an event which is an action- when he starts casting a spell by blah blah - and resolve the ready before the action completes- you have basically made the explicit statement in the tule about "right after the trigger finishes" totally useless. You can ready for "wgphen he starts his sword towards me" instead of "attacks me" and "draws a potion" instead of "drinks a potion." Etc etc etc.

For movement

But notice that two examples of movement triggers specifically are identified that show its triggered by moving onto a space or into a range etc. Movement is not an action, just a thing you do, but they do make it clear you do not have to be talking about finished movement.

For me, I have no identifiable house rules in play for this and so ready vs spell casting - the gestures and chants etc are what makes the casting perceivable not the event. The strike would land after the cast a spell got done, no interruption unless it was longer cast or concentration.

Movemrnt can be ready-set-triggers for specific spots or ranges and go off then before they move farther.

So, basically, ready is react not interrupt.