D&D 5E Invisibility, non-instantaneous spells, and spell effects

[Stormonu]

If the caster of Magic Missile had a reaction they could use, I'd allow it, maybe with some sort of check. In this case I'd partly be assuming the caster is recognizing the words, actions or fading from sight and identifying that the target is disappearing and is fast enough to react before the target's gone. Probably a Dexterity check vs. the spellcaster's DC to get them before they go invisible, or whomp the target before they've moved sufficiently to be missed.

 

[Irlo]

My problem with this is that it effectively nullifies the statement that the readied action comes after the trigger. "When the archer draws their bow" is very obviously an attempt to react to the archer's attack; but the player is trying to split hairs so they get to go before the attack actually happens. You can almost always find a way to split that hair fine enough if you try, and I have no desire to get into that.

Treating the action as an atomic whole allows me as DM to draw a clear line which doesn't invite endless debate: Once an action starts, it can no longer be stopped (unless you have a special reaction like counterspell that specifically interrupts other actions). You have to decide which is more important to you, knowing the enemy's plan or striking first.

I get what you're saying. I don't see the tactic of readying attacks to be especially useful or advantageous enough to worry about players exploiting loopholes, but, yes, if you have players given to debate, then keeping it simple is certainly best. In the exampe of the invisibility spell, the advantage is a chance to break concentration. The disadvantage is that you have to be pretty darn certain that the caster is going to use invisibility, and the circumstances where that is true are rare enough not to worry about.

 

[Irlo]

Just ask your GM about.

Oh, I am the GM. How would you adjudicate this, and why? That's what I'm interested in.

 

[Umbran]

5e isn't big on keywords. When the rules are written in more natural language, you probably shouldn't lean on fine grained details of specific words like "instant".

It just doesn't make a lot of sense that one spellcaster can finish a spell, and before the spell can take effect, another can 1) recognize what is happening, 2) cast their own spell and, 3) have that spell complete, before the first takes effect. "Instant" or not, the invisibility isn't taking its own sweet time lingering over coffee - it is getting about the business of making the spellcaster not visible.

If the caster of Magic Missile had a reaction they could use, I'd allow it, maybe with some sort of check. In this case I'd partly be assuming the caster is recognizing the words, actions or fading from sight and identifying that the target is disappearing and is fast enough to react before the target's gone.

The Inquisitive Rogue's "Eye for Detail" allows the rogue to use a bonus action to spot a hidden creature or uncover clues. Which suggests to me that normally observations are full actions, or this subclass ability isn't meaningful.

Maybe we can allow observing the spell to be a reaction, as the spell happens on the opponent's turn anyway. But then, they don't have another reaction for the readied spell casting.

 

[Irlo]

No way. A reaction occurs after the trigger. At no point does invisibility say that the target takes any time to turn invisible. You cast and then the target is invisible, poof, almost, ahem, almost like magic. It's too late to target them unless you can see invisible creatures.

If you ready an action to magic missile some guy if he casts a spell, you don't interrupt his spellcasting.

Perfectly reasonable. Your principle, then, is that spell effects occur instantaneously upon competion of the casting unless otherwise specified. I heard an argument elsewhere that nothing was instantaneous unless specifically stated to be so, which is part of what prompted this thread.

 

[Irlo]

5e isn't big on keywords. When the rules are written in more natural language, you probably shouldn't lean on fine grained details of specific words like "instant".

It just doesn't make a lot of sense that one spellcaster can finish a spell, and before the spell can take effect, another can 1) recognize what is happening, 2) cast their own spell and, 3) have that spell complete, before the first takes effect. "Instant" or not, the invisibility isn't taking its own sweet time lingering over coffee - it is getting about the business of making the spellcaster not visible.

In this case, the casting of the magic missle was mostly completed on that player's turn and held, per the ready action rules, to be released as a split-second reaction. That's why I even question that a spell effect might be interrupted. So it's:
1) Be watching for the enemy to start to become invisible, regardless of the cause. (The trigger is "start to become invisible," although perhaps "start to disappear" might be better?)
2) Interrupt with the reaction to release the spell.

Is there a split second of fading visibility that's long enough for my split second reaction to take place? Whether one's answer is yes or no, is the answer in the rules or strictly by DM's rulling? (I think it's a ruling, but I'm interested in how others read it.)

 

[Shiroiken]

I would have the player who is readying identify when they will cast magic missile:

1) When an enemy begins to cast a spell? (Any enemy, the first enemy?) - ok.
2) When an enemy goes invisible? For me, that's not a sufficient trigger. What happens if they Misty Step? It looks like invisibility when they vanish, for example.

Pretty much how I look at it. You could even say when a specific character begins to cast a spell, or even when casting/triggering a spell/ability that you can recognize, but if you don't know explicitly they are going invisible, you can't determine the trigger.

 

[TaranTheWanderer]

I always think about baseball when thinking about readying.

The batter readies to swing: "I ready until the pitcher throws the ball." The readied action goes immediately after he throws because if you swung too soon, you'd miss.

On the other hand: "I ready for when the pitcher winds up. When he does, I throw my bat at him."

You're clearly not waiting for the ball to be thrown. You're trying to interrupt mid-pitch. The guy can still throw the ball, obviously, even after he's been hit by the bat.

But I see @EzekielRaiden reasoning that, throwing the bat at the pitcher might not throw off his pitch. (or, in the example in the OP, prevent the spell from triggering.) But, I don't know. It might.

 

[James Gasik]

My problem with this is that it effectively nullifies the statement that the readied action comes after the trigger. "When the archer draws their bow" is very obviously an attempt to react to the archer's attack; but the player is trying to split hairs so they get to go before the attack actually happens. You can almost always find a way to split that hair fine enough if you try, and I have no desire to get into that.

Treating the action as an atomic whole allows me as DM to draw a clear line which doesn't invite endless debate: Once an action starts, it can no longer be stopped (unless you have a special reaction like counterspell that specifically interrupts other actions). You have to decide which is more important to you, knowing the enemy's plan or striking first.

I'm not sure exactly how this nullifies anything. The rules don't say the trigger has to be an action. They say the trigger is a perceivable circumstance. If it said the trigger had to be "an opponent takes the attack action" or "an opponent casts a spell", you'd be correct.

I mean, if you're the DM of course, you are correct, but I'm just pointing out the rule isn't that precise.

It seems to indicate you can set the trigger for things like "when the enemy steps on the trap", "when someone opens the door", or "when someone looks at me funny".

What actual action they are engaged in shouldn't matter once the trigger's circumstance is met. That trigger occurs, you do the thing.

In addition to this, there are actions you could not possibly react to. Dash or Dodge, for example, don't do anything on their own- they simply affect what the person who takes them can do on their turn.

 

[Blue]

Under the Ready action, there are examples that break up what a target is doing, such as "If the cultist steps on the trapdoor, I'll pull the level that opens it". As such it seems that the only limit to the trigger is "perceivable circumstance", there is no explicit or implied limitation that it can not interrupt part of an action. And the trigger occurs after the perceivable experience completes, not the whole action completes. It happen when they step on the trapdoor, not when they have completed a move that takes them off the trapdoor.

We know from the Identifying a Spell section of Xanathar's that a reaction can be used to identify a spell explicitly before it is cast. So there is a period after casting is started before the casting is finished that can accommodate a Reaction during that period.

So the rules are clear that we can have a Ready action that goes off "when the target begins casting". Begining casting occurs -> Ready -> casting (may) complete -> spell goes off.

Now, unless they are casting a multiple round spell (a special case listed in the PHB), there is no need for Concentration spells to cast. So triggering while casting would not require Concentration checks - the spell is not up yet to need it either.