background: was trying to think of a fun (not necessarily effective) way to use 4 elements monk. I noticed one of their few free abilities can be read to make a thin wall of water, fire, mist, or earth big enough to make total cover. Which got me thinking about a kind of support monk, using their held action to throw up a wall in defense of themselves and allies.
Actual question: how would this interact in game? Whether its this monk ability, control water, or wall of stone, If a kobold shoots an arrow at my ally and I throw up total cover over my ally in response, does the attack just fail? Is it canceled? Would you suggest the DM rule the wall only stops some amount of damage?
Since a reaction is in response to a trigger and occur immediately after it, the trick is to select one that take place before the attack if you want to interrupt it somehow. In your example, if you Ready an Action when the kobold pull an arrow from its quiver, you will be able to react immediately after it finishes doing so.
Since a reaction is in response to a trigger and occur immediately after it, the trick is to select one that take place before the attack if you want to interrupt it somehow. In your example, if you Ready an Action when the kobold pull an arrow from its quiver, you will be able to react immediately after it finishes doing so.
I think the easy solution then would be to specify your trigger as "if the enemy *begins to attack* X, then I erect a barrier" or "once the enemy *commits* to attacking X..."
You're not waiting for the hit, you're waiting for the motion.
Since a reaction is in response to a trigger and occur immediately after it, the trick is to select one that take place before the attack if you want to interrupt it somehow. In your example, if you Ready an Action when the kobold pull an arrow from its quiver, you will be able to react immediately after it finishes doing so.
I think the easy solution then would be to specify your trigger as "if the enemy *begins to attack* X, then I erect a barrier" or "once the enemy *commits* to attacking X..."
You're not waiting for the hit, you're waiting for the motion.
This is why the use of held actions is very DM dependent.
Can you use "Begins to cast a spell" as a trigger? Or does the spell get cast before the trigger completes?
If a creature "Begins to make an attack" ... is it possible to know what the target of that attack is before the attack is made? I could see a lot of circumstances where the target may not be known until the instant before the attack is made which would not be sufficient time to react (at least in my opinion).
The rules on reactions indicate that the reaction occurs immediately after the trigger completes if the timing isn't specified (and by this they mean in the rules NOT in the wording of the readied action - Ready actions ALWAYS occur after the trigger completes). In the case of the Ready action, it is explicitly called out in both the DMG and the PHB as happening after the trigger completes.
DMG:" Adjudicating Reaction Timing: Typical combatants rely on the opportunity attack and the Ready action for most of their reactions in a fight. Various spells and features give a creature more reaction options, and sometimes the timing of a reaction can be difficult to adjudicate. Use this rule of thumb: follow whatever timing is specified in the reaction’s description. For example, the opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action."
PHB: "First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. 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."
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger."
The question becomes - Is "Beginning" a perceivable circumstance? When does "Beginning" end ... since the reaction can only be taken after the trigger completes?
Answers to both those questions are very DM dependent. Some will allow "beginning" as a trigger in itself ... even if "beginning" doesn't have a well defined completion point in terms of a trigger. Some will rule that beginning isn't perceivable. Does beginning start when the character thinks of what they will do? Does it start when they lift their arm or leg? Move a hand? Reach for a spell casting focus? Does beginning start when the creature starts to speak? Are they saying regular words or magic words? Or are they pretending to utter magic words that won't do anything? How does another creature know what that creature intends to do BEFORE it actually happens?
Specific reactions like Shield and Counterspell allow the spellcasting to interrupt the trigger. Counterspell uses a being able to see a spellcaster casting a spell within 60'. Counterspell has special wording allowing its effect to occur before the trigger (cast a spell) completes. A comparable Readied action based on a creature casting a spell would have to wait until the trigger completes and the spell is cast before the reaction can be taken.
The bottom line is that RAW, reactions including the Readied action require the trigger to complete. In terms of a trigger based on a creature casting a spell, counterspell gives an example which can interrupt a trigger which is explicitly not allowed for other reactions including the Ready action. "Begins to cast a spell" doesn't appear to be an acceptable trigger in this case since "Beginning" may not be sufficiently perceivable ... but various DMs may choose to rule it differently.
P.S. Also note that there are specific reactions which do not require a perceivable circumstance to be used. The Artificer Flash of Genius ability is called out as being triggered by the d20 roll in the SAC. Whatever causes that d20 roll, in many cases, is unlikely to be perceivable. e.g. Failing a wisdom saving throw - the effects may never be visible. Flash of Genius is an exception since it is a reaction triggered by a game mechanic and not a perceivable circumstance.
"What is the timing of Flash of Genius?
You use Flash of Genius immediately after the triggering d20 is rolled and before any of the effects of the roll are applied. Unless a rule tells you otherwise, a reaction occurs immediately after its trigger."
I don't think this one works. And I'm aware that I have taken the opposing position in a relatively similar discussion in the past about readying an action with a trigger designed to beat the opponent to the reaction. Here's why I don't think it works in this case. As others have said, a reaction follows an action unless the reaction says otherwise, and readying an action does not say this. Now you might think you could ready the casting action on a carefully-worded trigger that would give you enough time, like "...if the kobold raises his bow," or "...if the kobold reaches for his bow" but I think this still doesn't work, because raising your bow (aiming?) is part of the attack action, and it means the kobold is already attacking, so your response will be too late. And even reaching for his bow can be considered an object interaction taken as part of the attack action, so you're still in the same situation. It's also kind of metagamey, but that's beside the point I'm making.
You would have to have a trigger that is either part of a preceding action, or else part of the kobold's movement, but neither of those are surefire ways to beat the kobold to the shot, because the kobold may or may not move before firing and the kobold may or may not take some other kind of action before firing if that is even possible in your situation.
background: was trying to think of a fun (not necessarily effective) way to use 4 elements monk. I noticed one of their few free abilities can be read to make a thin wall of water, fire, mist, or earth big enough to make total cover. Which got me thinking about a kind of support monk, using their held action to throw up a wall in defense of themselves and allies.
Actual question: how would this interact in game? Whether its this monk ability, control water, or wall of stone, If a kobold shoots an arrow at my ally and I throw up total cover over my ally in response, does the attack just fail? Is it canceled? Would you suggest the DM rule the wall only stops some amount of damage?
I think that it wouldn't work, because a readied action, when triggered, takes place after the triggering event finishes. See https://www.dndbeyond.com/sources/phb/combat#Ready
Since a reaction is in response to a trigger and occur immediately after it, the trick is to select one that take place before the attack if you want to interrupt it somehow. In your example, if you Ready an Action when the kobold pull an arrow from its quiver, you will be able to react immediately after it finishes doing so.
I think the easy solution then would be to specify your trigger as "if the enemy *begins to attack* X, then I erect a barrier" or "once the enemy *commits* to attacking X..."
You're not waiting for the hit, you're waiting for the motion.
This is why the use of held actions is very DM dependent.
Can you use "Begins to cast a spell" as a trigger? Or does the spell get cast before the trigger completes?
If a creature "Begins to make an attack" ... is it possible to know what the target of that attack is before the attack is made? I could see a lot of circumstances where the target may not be known until the instant before the attack is made which would not be sufficient time to react (at least in my opinion).
The rules on reactions indicate that the reaction occurs immediately after the trigger completes if the timing isn't specified (and by this they mean in the rules NOT in the wording of the readied action - Ready actions ALWAYS occur after the trigger completes). In the case of the Ready action, it is explicitly called out in both the DMG and the PHB as happening after the trigger completes.
DMG:" Adjudicating Reaction Timing: Typical combatants rely on the opportunity attack and the Ready action for most of their reactions in a fight. Various spells and features give a creature more reaction options, and sometimes the timing of a reaction can be difficult to adjudicate. Use this rule of thumb: follow whatever timing is specified in the reaction’s description. For example, the opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action."
PHB: "First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. 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."
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger."
The question becomes - Is "Beginning" a perceivable circumstance? When does "Beginning" end ... since the reaction can only be taken after the trigger completes?
Answers to both those questions are very DM dependent. Some will allow "beginning" as a trigger in itself ... even if "beginning" doesn't have a well defined completion point in terms of a trigger. Some will rule that beginning isn't perceivable. Does beginning start when the character thinks of what they will do? Does it start when they lift their arm or leg? Move a hand? Reach for a spell casting focus? Does beginning start when the creature starts to speak? Are they saying regular words or magic words? Or are they pretending to utter magic words that won't do anything? How does another creature know what that creature intends to do BEFORE it actually happens?
Specific reactions like Shield and Counterspell allow the spellcasting to interrupt the trigger. Counterspell uses a being able to see a spellcaster casting a spell within 60'. Counterspell has special wording allowing its effect to occur before the trigger (cast a spell) completes. A comparable Readied action based on a creature casting a spell would have to wait until the trigger completes and the spell is cast before the reaction can be taken.
The bottom line is that RAW, reactions including the Readied action require the trigger to complete. In terms of a trigger based on a creature casting a spell, counterspell gives an example which can interrupt a trigger which is explicitly not allowed for other reactions including the Ready action. "Begins to cast a spell" doesn't appear to be an acceptable trigger in this case since "Beginning" may not be sufficiently perceivable ... but various DMs may choose to rule it differently.
P.S.
Also note that there are specific reactions which do not require a perceivable circumstance to be used. The Artificer Flash of Genius ability is called out as being triggered by the d20 roll in the SAC. Whatever causes that d20 roll, in many cases, is unlikely to be perceivable. e.g. Failing a wisdom saving throw - the effects may never be visible. Flash of Genius is an exception since it is a reaction triggered by a game mechanic and not a perceivable circumstance.
"What is the timing of Flash of Genius?
You use Flash of Genius immediately after the triggering d20![]()
is rolled and before any of the effects of the roll are applied. Unless a rule tells you otherwise, a reaction occurs immediately after its trigger."
I don't think this one works. And I'm aware that I have taken the opposing position in a relatively similar discussion in the past about readying an action with a trigger designed to beat the opponent to the reaction. Here's why I don't think it works in this case. As others have said, a reaction follows an action unless the reaction says otherwise, and readying an action does not say this. Now you might think you could ready the casting action on a carefully-worded trigger that would give you enough time, like "...if the kobold raises his bow," or "...if the kobold reaches for his bow" but I think this still doesn't work, because raising your bow (aiming?) is part of the attack action, and it means the kobold is already attacking, so your response will be too late. And even reaching for his bow can be considered an object interaction taken as part of the attack action, so you're still in the same situation. It's also kind of metagamey, but that's beside the point I'm making.
You would have to have a trigger that is either part of a preceding action, or else part of the kobold's movement, but neither of those are surefire ways to beat the kobold to the shot, because the kobold may or may not move before firing and the kobold may or may not take some other kind of action before firing if that is even possible in your situation.
"Not all those who wander are lost"
Thanks comrades. It would have been a janky thing to begin with, but the clarity on triggers makes it pretty clear I should go a different direction.