Related to this question and this question.
In the case of a readied action where the trigger is specific enough to occur before the attack completes, does the attack complete?
The timeline:
Character A readies action to move out of range of Character B if they start to attack (either with a spell attack or weapon attack)
Character B begins the attack
Character A trigger occurs, moves out of range.
Character A is now out of range and no longer a valid target, what happens?
You can't react to the start of an attack to avoid the effect of the attack. You should probably just take the Dodge action.
You ask:
In the case of a readied action where the trigger is specific enough to occur before the attack completes, does the attack complete?
Your question implies that there is an interruptible span of time between an attack's initiation and its completion.
Your assumption is flawed: In 5e an attack is finished as soon as it's begun. To evidence this we need only the basic rules for attacking:
When you make an Attack, your Attack roll determines whether the Attack hits or misses. To make an Attack roll, roll a d20 and add the appropriate modifiers. If the total of the roll plus modifiers equals or exceeds the target’s Armor Class (AC), the Attack hits.
The second sentence adds helpful detail here, but the first is completely sufficient. When an attack is made, an 'Attack roll determines whether the Attack hits or misses'. The attack roll and the attack result (i.e. hit or miss) are both simultaneous, essential qualities of the attack.
If there has been no Attack roll, there has been no Attack. And, generally speaking, if there has been no hit/miss resolution, there has been no attack.
Note that there is at least one exception to this rule: the Goblin Boss' redirect attack is a reaction that's triggered by being targeted by an attack and changes the attack's target. This specific rule interrupts the usual protocol and does not set a precedent for what can count as a 'perceivable circumstance' in the case of readying an action. It's worth noting that several reactions available to players, such as those granted by the Shield spell and Defensive Duelist feat, are triggered by an attack hitting and are intended to retroactively prevent the attack from hitting. In these cases the sequence of rolls and reactions perceived by the player are clearly distinct from the in-universe sequence that a character experiences.
In summary: Determining whether the attack hit or missed is not a step that follows the Attack action, but is rather essential to the Attack action. In the default case there is never a time, however brief, at which an attack exists but is yet to hit or miss. Every attack that has been made has already hit or missed (except in the case of exceptions).
Hopefully it's clear by now that, if you're the target of an attack, you can't move
out of range and [cease to be] a valid target
You can only be the target of an attack that's happening, and if you're being targeted then the attack roll has hit or missed, because that resolution is essential to the process of being targeted (Unless you are a Goblin Boss - see the section above).
Want to avoid enemy attacks and willing to spend an action on making that happen? Take the Dodge action to impose disadvantage on all attacks, and get advantage on Dex saves. It's almost always better; the benefit applies to all attacks before the start of your next turn, and it's much simpler to adjudicate.
Ready Action(PHB 193):
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.
So in your example this would be the timeline:
Character A readies action to move out of range of Character B if they attack (either with a spell attack or weapon attack).
Character B begins the attack.
Character A trigger occurs, have to wait until trigger finishes.
Character B finishes the attack.
Character A moves out of the range, possibly triggering an AoO, or chooses to ignore the trigger.
The Ready action states:
First, you decide what perceivable circumstance will trigger your reaction.
In ordinary language you might be tempted to say that you could perceive someone "starting" to attack. The problem is this makes every attack interruptible. Note that official spells or features that affect an attack don't actually prevent the attack from happening, they merely change the outcome. The protection fighting style imposes disadvantage on the attack, and the shield spell or defensive duelist feat can cause an attack that would hit to miss by changing your armor class, but the attack still happens.
This is not to say that you can't specify "when the creature attacks" as a trigger, but following the rules, the reaction has to occur "after the trigger finishes". Separating "starting the attack" from "the attack" essentially would rewrite the Ready rules so that the readied action occurs before the trigger finishes.
If "starting to attack" were separated from the attack itself, then this chain of events would happen:
Not only is this not RAW, it would be a somewhat silly chain of events, and would make combat very difficult to adjudicate.
During combat, attacks happen very quickly. There are very specific reactions (such as the ones mentioned above) that can affect an attack, and additional ones should be created with clear rules for how they affect the attack, but they should not prevent the attack from happening. Once the creature's takes the Attack action, nothing should be allowed to rob the creature of the action. The action may of course not succeed, but it can't be taken away. Of course, things could happen that prevent them from taking th action in the first place. If the creature moves in to attack, and their opponent has readied the action "When the goblin moves within 30 feet of me, I run away," the goblin might not get within range of you, but could conceivably choose a different action at that point because they haven't yet attacked, such as Dash, or Dodge, or Attack someone who is still within range.
There is one circumstance where I imagine a player might think that they want to respond to "starting to attack", and that is before combat has started. Two parties face off at some distance, hurling insults and making threatening displays. A player thinks "I don't actually want to fight, so if the opponent starts to attack, I will run away."
This is not actually a Ready action. Until combat starts, there is no initiative order. When one side commits to the combat--they "start" to cast a spell, fire a missile weapon, or move toward the other party with ill intent--initiative is rolled. Whoever wins, goes first. The person who "starts" combat does not get to automatically go first. So if the player wins the initiative roll, they can run away before the combat starts, but on their turn, the opponents could move to chase them.
A number of questions on this site deal with the issue of who gets to act first at the start of combat:
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.” (PHB, p. 193)
So basically RAW you can ready to make an action or to move.
The readied action you described only had move.
What would have happened if you just moved? The attack would have still happened (an opportunity attack) but you would have been out of range of any other melee attacks.
Just disengaging still leaves you in range so you can't say "I ready myself to disengage and run when he starts attacking." If you wanted to get away then you would have to use your action on your turn to disengage, then move away, then their turn would start.
The rules specifically mention "If the goblin steps next to me, I move away.", so we know that using your Reaction to move is valid.
However "start to attack" sounds too specific a trigger to me, and implies a level of observation that would normally be in the purview of a Perception check (not to mention that a skilled or canny opponent could easily fake your PC into using his Reaction in this way).
I realise that counterspell does work in this way, however the casting of a spell could easily be seen as a more complex and identifiable process than the feints of a blade.
It also causes complications with Opportunity Attacks: if a Reaction is available, should the attacking creature get to swing at you on the way out of his melee range?
All that being said, a PC is potentially using his entire Action and Reaction just to (for example) move behind a wall when a spell goes off. Why not just take your Action as normal and get out of sight afterward?
From the rule on Contests:
This situation also applies when one of them is trying to prevent the other one from accomplishing a goal (Basic Rules, p. 58)
This kind of situation takes us back to where a DM uses the basic structure of the game to adjudicate potential rules conflicts:
DM narrates the result (Korvin'sKaveat: roll dice only when necessary)
While this approach is "getting a bit fiddly" as compared to the cleaner way D&D 5e was built to be played (fewer die rolls to resolve combat), if the rules don't clearly spell this out then a way to provide both the player and the NPC a chance for their attempt to succeed fits into a contest. Who was quicker?
As a DM, I'd be more likely to rule situational advantage to the player with the Ready Action due to their having sacrificed their other actions for that; reward planning ahead and using one's wits? But that's not necessary, advantage or disadvantage based on the situation will vary with the scenario.
For Melee, use dodge, move or dash
If it's a melee attacker not already in melee range, a Ready Action to Dash/move away would seem to solve the problem. (Provided that the character can move). You move out of range and avoid an attack. Likewise, the choice to dodge to impose disadvantage on the attack would reduce the odds of, but not nullify, a successful attack.
For Ranged attack, the question poses a slight difficulty
Dodge is an easier option to apply to such a situation
While dodge was probably the rule intended to deal with a situation like this, as it would grant the attacker disadvantage on the attack. But, Dash and Move are actions eligible for use in a reaction or Ready Action.
That said, if the player somehow knows how far the attacker can shoot, why not try to get out of the way rather than risk a (reduced) chance to be hit? This situation requires that the PC knows what's coming, doesn't it? The PC has to know what the range is for an attack before applying this "get out of range" attempt. That takes us to ...
A problem I run into with this question is the point made in Xanathar's Guide to Everything: it takes a reaction to cast counterspell, but it takes another action/actor/action economy cost to know the spell someone is casting. While that is related to the discussion on counterspell, it may apply here as well. Beyond that, many ranged attack spells have a duration of "instantaneous" which argues for the amount of time being available to do this as negligible. That points me back to "dodge" as being the clearer way to try and avoid an attack, but what is being asked is how to nullify the ranged attack by exceeding the attack's range.
It may be obvious due to how long the combat has been going on, or the wand in the attacker's hand, but there may be some meta knowledge involved also. You'd need to know what spell it is to know what "out of range" means. The Q&A at the link delves into the details and comes to the conclusion that "the rules are not that clear."
This is a bit of frame challenge.
As I was reviewing the other answers as well as the citation raised by AncientSwordRage, I think one of the disconnects between the querent and the respondents is the interpretation of the trigger for the reaction. I disagree with assertions that this isn't a permissible trigger, but rather that most DMs should probably ask for clarification.
Within the citation, the trigger indicates some sort of perceptable trigger in the form of a character aiming a weapon. To be clear, this is not an attack, this is somebody raising a weapon.
As this question is written, on Step 2 it states that, "Character B begins the attack." How this gets interpreted is critical and hence why I indicate that this is a matter for DM adjudication, because the stated trigger is not, "If Character B attacks me," rather it's if Character B starts to attack.
This phrasing of the trigger is open to a lot of interpretation, but this DM would understand that, "If they start to attack," in a manner that benefits Character A whom is specifically expending their turn to avoid an attack by Character B. Thus, I would interpret this trigger to mean that when Character B begins to cast a spell/raise a bow in a perceptable manner that would satisfy Character A's trigger and thus they could move out of range and deny Character B a target that's in range.
That said, this does not mitigate the fact that at this time, Character B has not completed an attack on their turn, nor expended their Action or anything else. Only that the field has changed in reaction to something that they did. Thus, if Character B wishes to move closer to bring Character A back into range that's an option for them.
This is only one interpretation of these triggers, but I don't see there to be more than two. If the DM interprets things in the sense that the trigger is Character B must make an attack before the trigger conditions are met, then I defer you to Lovell's answer.
The rules for the Ready action read:
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."
Character A readies action to move out of range of Character B if they start to attack (either with a spell attack or weapon attack)
"Start to attack" is more than a bit vague, and as a DM I would try to get them to be more clear about trigger. I'd ask, "What exactly is your character looking for? How will know they are going to attack?" If it remains vague, I'll roll with what I got.
"Move out of range" isn't a guaranteed option, just an intention. Where the character moves the player will get choose when the trigger occurs. We'll talk about this more later in the answer.
Character B begins the attack
A weapon attack should be the more clear case. The player they gave up the action on their turn on the "begin the attack", so I'd let them use their before the attack roll.
If it is a melee attack, the first attack would count as "used", but the attacker can use what moment they have left to attempt to move after the PC and take their second attack of either the Attack action with Extra Attack feature, Multi Attack action, Action Surge or the like. Because the attacker is expressly allowed to move between the attacks in a Attack action.
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. For example, a fighter who can make two attacks with the Extra Attack feature and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet, and then attack again.
Also the fleeing character's (A's) movement would still provoke a potential Opportunity Attack using the attacker's reaction, if available.
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
If it is a ranged attack, the attacker could try to adjust the aim and "follow the target," and the movement might cause the DMs to rule it imparts disadvantage on the first attack for the running around, if the player character doesn't actually get out of range.
The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.
The other attacks the attacker makes, if available, would be made with straight rolls if the fleeing character is still in range. If the character is able to actually get out of range, the ranged attacker will be able to use whatever movement they have left to move to attempt to get the target back in range, given they have multiple attacks from Extra Attack feature, Multiple Attack action, or Action Surge or the like. The question remains, can they move out of range. See "move out range" section later in this answer.
In the case the "attacker" uses a spell, it should require a Intelligence (Arcana) check to see if the character thinks the spell is an attack or not. I do this because it seems appropriate given:
Your Intelligence (Arcana) check measures your ability to recall lore about spells, magic items, eldritch symbols, magical traditions, the planes of existence, and the inhabitants of those planes.
If the character perceives (correctly or incorrectly) that it is an attack, then it should trigger their movement.
Character A trigger occurs, moves out of range.
If the trigger is successfully tripped, then the player must tell the DM where they would like to move, but the DM has no obligation to the player what the range of the attack. So on the readied move, the player choses to move the character up to the character's movement in a direction the player or character thinks would get them out of range.
- The DM describes the environment.
- The players describe what they want to do.
- The DM narrates the results of the adventurers’ actions.
If it isn't out of range the first attack can still happen, and the movement can, as mentioned previously, provoke opportunity attacks from hostiles creature whose melee range they exit, possible including the original attacker.
Readied Actions (emphases mine):
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. Remember that you can take only one reaction per round.
Readied actions occur after their trigger, and what you are asking to do is to move after an attack starts, but before it concludes. To do this, you will need the trigger that indicates the start of the attack to be perceivable (per the quoted rules for readied actions), and also to be distinct from the conclusion of the attack, since your readied action will occur after the trigger finishes. That is, you will need to be able to parse the different parts of an attack at a scale that is apparent to your character, not just from a rules standpoint.
The parts of an attack (emphases mine):
- Choose a target. Pick a target within your attack's range: a creature, an object, or a location.
- Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.
- Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.
Let's assume that Step 2, Determine Modifiers, is completely 'meta' and not something your character could actually perceive or react to in-game. However, Steps 1 and 3 are distinct (and separated by Part 2). Thus, if by "begins the attack" you mean, [the attacker chooses me as a target] and if by "the attack completes" you mean, [the attacker resolves the attack by making an attack roll and damage roll], then potentially you have something to react to. In theory, it might be possible to react to being selected as the target of an attack, and then taking your reaction after the target selection finishes but before the attack roll begins because your character can perceive themselves as being the target of an impending attack.
Most answers to this question say that this is not actually possible1, that you cannot parse an attack so finely, and that 'right after the trigger finishes' means after the entire attack finishes. In the face of this skepticism, then, it is useful to cite a few examples of abilities that do precisely that.
The Goblin Boss has:
Redirect Attack. When a creature the goblin can see targets it with an attack, the goblin chooses another goblin within 5 feet of it. The two goblins swap places, and the chosen goblin becomes the target instead.
The Light Domain Cleric (PHB61) has:
WARDING FLARE Also at 1st level, you can interpose divine light between yourself and an attacking enemy. When you are attacked by a creature within 30 feet of you that you can see, you can use your reaction to impose disadvantage on the attack roll, causing light to flare before the attacker before it hits or misses.
Warding Flare was the only ability I found in the PHB2 that allows you to use your reaction in response to being targeted, but I would not be surprised if a non-PHB (but still official) class or race had such an ability.
There are thus at least two official examples of abilities which use reactions in response to being targeted, and which resolve before a hit roll is made and before the attack concludes. Note that both of the examples are limited to being used only when the user can see the attacker, which supports the idea that what they are responding to is an in-game recognition that a character is the target of an attack before the attack itself is made (rather than a meta-level decision of the player). Thus, there is mechanical support for the idea that your Character A is capable of reacting to being targeted.
While these two abilities allow characters to introduce reactions after one step of the attack sequence that are resolved before another step, many other abilities allow characters to parse the attack sequence even more finely. For example, consider the School of Enchantment Wizard's Instinctive Charm:
When a creature you can see within 30' of you makes an attack roll against you, you can use your reaction to divert the attack, provided that another creature is within the attack's range. The attacker must make a Wisdom saving throw against your wizard spell save DC of 17. On a failed save, the attacker must target the creature that is closest to it, not including you or itself. If multiple creatures are closest, the attacker chooses which one to target. On a successful save, you can't use this feature on the attacker again until you finish a long rest.
You must choose to use this feature before knowing whether the attack hits or misses. Creatures that can't be charmed are immune to this effect.
In the case of Instinctive Charm, we see that the attack has proceeded through Step 1 (Choose a target), Step 2 (Determine modifiers), and has begun Step 3 (Resolve the attack) to the point of making the attack roll. It is only at that point, when an attack roll against the wizard is made (but before the resultant number is known), that the PC uses their reaction to force a save, and on a failed save makes the attacker return to Step 1 (Choose a target). Thus, it is actually possible for a character to react at an even finer scale trigger than simply being the target of an attack; Enchanters can react to having an attack roll made against them and can take their reaction before that attack roll is determined to have hit or not.
Similarly, consider the College of Lore Bard's feature Cutting Words:
When a creature that you can see within 60 feet of you makes an Attack roll, an ability check, or a damage roll, you can use your Reaction to expend one of your uses of Bardic Inspiration, rolling a Bardic Inspiration die and subtracting the number rolled from the creature’s roll. You can choose to use this feature after the creature makes its roll, but before the GM determines whether the Attack roll or ability check succeeds or fails, or before the creature deals its damage. The creature is immune if it can’t hear you or if it’s immune to being Charmed.
Like Instinctive Charm, the use of this ability reacts to an attack roll being made, and thus interrupts the attack sequence within Step 3 (and not between steps). However, the interruption here is slightly later in the sequence, after the roll has 'registered' and its value is known but before it is declared a success or failure.
An even later interruption is produced by a Mark of the Sentinel, where:
Vigilant Guardian. When a creature you can see within 5 feet of you is hit by an attack roll, you can use your reaction to swap places with that creature, and you are hit by the attack instead. Once you use this trait, you can’t do so again until you finish a long rest.
Here the ability interrupts step 3 after the attack has been declared a hit but before damage is done.
All five of the abilities mentioned are powered by reactions, and each one requires the user to be able to see the attack, suggesting that it is based on what the in-game character perceives.
It is abundantly clear that the rules permit such fine parsing of triggers as you wish to do. Many features grant this ability. That is not to say that what you want to do, moving as a readied action in response to an attack, is permitted. We know that in the listed features the mechanical requirements for a readied action have been met - a perceivable circumstance and a level of parsing that allows you to act after an attack is declared but before it resolves. But this does not assure us that a generic readied action may be used to this end - it could be that doing this requires a feature (like the five mentioned) that specifically grants you the ability to interrupt an attack. Unfortunately, both examples given for generic readied actions rely on responding to movement ('the cultist steps', 'the goblin steps'), and not responding to attacks, spells, etc. Since these are examples, not a complete list of possibilities, we simply do not know whether a generic readied action may be used to do what you want - move in response to being targeted by a spell attack.
Since the rules are silent on this, it is up to the DM to decide whether to allow a readied action to move away from an attack before the attack is completed. But at least we can say that allowing this does not contradict RAW, because there are multiple examples of this being permitted under RAW in certain circumstances.
For your second question, you ask what happens when the target is out of range by the time the attack action completes. The ranged attack rules (emphases mine) say:
When you make a ranged attack, you fire a bow or a crossbow, hurl a handaxe, or otherwise send projectiles to strike a foe at a distance. A monster might shoot spines from its tail. Many spells also involve making a ranged attack.
Range. You can make ranged attacks only against targets within a specified range.
If a ranged attack, such as one made with a spell, has a single range, you can't attack a target beyond this range.
Some ranged attacks, such as those made with a longbow or a shortbow, have two ranges. The smaller number is the normal range, and the larger number is the long range. Your attack roll has disadvantage when your target is beyond normal range, and you can't attack a target beyond the long range.
In this case, the archer or caster selects you as the target of the spell - but has not yet made the attack roll. If you are permitted to react to targeting, following the considerations as above, you can then move out of range before the attack is made. In this case the attack simply fails; it cannot be made, whether it is a weapon or spell, because you have made yourself an invalid target.
In my experience as a DM, readied actions are a risky gamble. As often as not they end up being wasted when their trigger doesn't materialize. What if the attacker in question ends up attacking another member of your party? What if they attack you and you move away, but their attack has enough range to still reach you? Either of these outcomes would mean your spent both your action and reaction and got no benefit, which is a poor return on investment in the action economy. If allowing the readied action you suggest helps a player feel more agency but results in little overall benefit to their character, that's an easy yes at my table.
1 Cf.: "Attacks resolve instantaneously, so there is no such thing as 'before the attack completes'." "The rules describe attacks as simultaneously enacted and resolved"
2 The Mounted Combatant feat also allows you to change the target of an attack after Step 1 (targeting) but before Step 3 (attack roll). "You can force an attack targeted at your mount to target you instead." However, unlike every other example above, this neither requires your reaction nor requires you to see the attack. To me, that feels more like a meta ability the player is choosing to use, rather than an in-game decision the character is making.
Yes, you can use a Readied action to avoid an attack by moving out of its range, by readying a Disengage action with a trigger like "when he starts to swing his sword at me". This is something you can perceive, so it's a valid trigger for a Ready action. It'd trigger after they target you but before they roll.
Unlike many of the other answerers, I do believe that the game intends to allow you to use reactions to interrupt an attack, and for this, I will point to the shield spell, which allows you to turn an attack that hit into an attack that missed by increasing your AC as a reaction.
Of course, this will cost you your action on your turn, and prevent you from doing anything actually useful like attacking or casting damaging spells.
