I don't know whether this post belongs here or in homebrew, but I was hoping there would be a by-the-books way of doing things like this. Most DMs would do a sleight-of-hand roll or a DEX roll for how fast a character reacts to something, but there's also generally a mental component to how fast someone's reaction time is. It's how fast someone can process that there is danger, process how to react to the danger, and send the signals to various parts of the body to react to the danger. It's not all just about how fast you are. So how would you do a check/save for this in your D&D game? All suggestions appreciated.
Generally anything that relies on a quick physical response is a DEX save. The mental ones apply to enduring mental assault, not processing and reacting to a physical threat
As The_Ace_of_Rogues said, a Dexterity saving throw represents "can you physically move out of the way of some specific danger." Reacting to a situation, like the beginning of combat, is rolling initiative.
In 4e, both initiative and Reflex saving throws were based on the higher of your Dexterity or Intelligence modifiers, so you're instinct that there could be a mental component isn't far off. But that's not something that 5e does.
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.
If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends. A member of a group can be surprised even if the other members aren’t. ----
Then Sage Advice adds: "A surprised creature stops being surprised at the end of its first turn in combat" --- The first step of any combat is this: the DM determines whether anyone in the combat is surprised (reread “Combat Step by Step” in the Player’s Handbook). This determination happens only once during a fight and only at the beginning. In other words, once a fight starts, you can’t be surprised again, although a hidden foe can still gain the normal benefits from being unseen (see “Unseen Attackers and Targets” in the Player’s Handbook).
To be surprised, you must be caught off guard, usually because you failed to notice foes being stealthy or you were startled by an enemy with a special ability, such as the gelatinous cube’s Transparent trait, that makes it exceptionally surprising. You can be surprised even if your companions aren’t, and you aren’t surprised if even one of your foes fails to catch you unawares.
If anyone is surprised, no actions are taken yet. First, initiative is rolled as normal. Then, the first round of combat starts, and the unsurprised combatants act in initiative order. A surprised creature can’t move or take an action or a reaction until its first turn ends (remember that being unable to take an action also means you can’t take a bonus action). In effect, a surprised creature skips its first turn in a fight. Once that turn ends, the creature is no longer surprised.
In short, activity in a combat is always ordered by initiative, whether or not someone is surprised, and after the first round of combat has passed, surprise is no longer a factor. You can still try to hide from your foes and gain the benefits conferred by being hidden, but you don’t deprive your foes of their turns when you do so. --- Surprise rules work for two opposing sides. What happens with surprise when a third group of combatants sneaks up, hidden from the melee, and ambushes?
The surprise rule is relevant only when a combat is starting. Any ambushes during the fight use the rules for Dexterity (Stealth) checks. --- Being surprised has no effect on saves
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RAW thats probably all there is to guide you. Saving throws, such as the Dex one that is probably the best interpretation of reacting in a physical way to danger, is the best there is and even "surprise" doesn't stop players or NPCs from taking those dexterity saves to try and avoid some danger as part of their instinct.
Seeing the threat coming before it hits (i.e. spotting hidden attackers) would be covered by the passive perception. This represents general awareness of their surroundings, but they can also make active perception rolls each turn to be actively looking for danger and potentially roll higher than their passive perception to see it. If a player has the "Alert" feat they are completely immune to being surprised.
Another rule that can cover this is for hidden attackers who get advantage on their attack against a player they are hidden against. This can actually pair with "surprise" if the attackers remain hidden when they attack, giving them both the first round without actions from opponents who are surprised and letting them attack those opponents with advantage as opposed to a surprise round that might see the attackers jumping out from cover and charging to attack. -----
If you're looking for some sort of custom rule to a situation you have in mind, the idea of using a dexterity saving throw or ability check is your best bet. Then you can apply disadvantage to their roll if you believe the character was caught completely off guard or didn't have a chance to see it happening.
There's also the "Dodge" action players can take each turn to give disadvantage on attack rolls against them and advantage specifically to dexterity saving throws. You could easily house rule this to say that it would give advantage on some sort of other dexterity ability check that would be made in response to an event.
Example: Players are guarding a gem on a pedestal. An enemy caster uses "Catapult" to launch the gem 90 feet towards them. You decide you want the players to have an opportunity to catch the gem out of the air as it flies by and have them roll a Dex saving throw/ability check against the caster's casting DC. 1. Characters who did not notice the caster or weren't watching get no roll, even if they had the dodge action (which doesn't impact attacks from hidden creatures). 2. Characters with no free hands roll with disadvantage. 3. Characters with a free hand roll normally 4. Characters who took the dodge action roll with advantage
You might also decide that this will use the character's reaction, so they must decide if they want to use their reaction for a chance to catch it.
The short answer is to roll Initiative as that is the mechanic used to determine who goes first, but you a re looking for another check. A saving throw is generally used to avoid an attack so I would advise against that. I guess I would suggest a skill check that you deem appropriate in the moment. Perception or Insight would be common options. Animal Handling if there is an animal involved. Survival if it’s a natural hazard. An Intelligence-based knowledge check if one of those seem appropriate. We could come up with situations to justify about anything. I’d suggest mixing it up so that players who invest in less common skills feel like their investment has value. And maybe a good check grants advantage on the initiative check. I hope you play with this and try different paths.
Generally for things that are "instinctive" I use the passive. I only use an active check when the player tells me that he is doing something.
For example, if the player suspects that an NPC is telling a lie, I have them roll for insight. If the player doesn't tell me that he distrusts the NPC, what I do is secretly roll against his passive insight (which I calculate as passive perception).
Another example would be with arcana or nature or whatever. For example, if there is an arcane item in the room and the difficulty of "discovering" it is lower than its passive, I don't ask for a roll. He recognizes the object. Otherwise I don't say anything. And it's up to him whether he wants to study it carefully or not (in which case, I ask for an arcana check).
And for me the passive is always the baseline. Even if its result is lower, the minimum is always its passive. In fact, I don't ask him for a check if his passive already exceeds the difficulty (whether opposed or not).
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I don't know whether this post belongs here or in homebrew, but I was hoping there would be a by-the-books way of doing things like this. Most DMs would do a sleight-of-hand roll or a DEX roll for how fast a character reacts to something, but there's also generally a mental component to how fast someone's reaction time is. It's how fast someone can process that there is danger, process how to react to the danger, and send the signals to various parts of the body to react to the danger. It's not all just about how fast you are. So how would you do a check/save for this in your D&D game? All suggestions appreciated.
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Generally anything that relies on a quick physical response is a DEX save. The mental ones apply to enduring mental assault, not processing and reacting to a physical threat
As The_Ace_of_Rogues said, a Dexterity saving throw represents "can you physically move out of the way of some specific danger." Reacting to a situation, like the beginning of combat, is rolling initiative.
In 4e, both initiative and Reflex saving throws were based on the higher of your Dexterity or Intelligence modifiers, so you're instinct that there could be a mental component isn't far off. But that's not something that 5e does.
The rules that could relate are:
surprised
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.
If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends. A member of a group can be surprised even if the other members aren’t.
----
Then Sage Advice adds:
"A surprised creature stops being surprised at the end of its first turn in combat"
---
The first step of any combat is this: the DM determines whether anyone in the combat is surprised (reread “Combat Step by Step” in the Player’s Handbook). This determination happens only once during a fight and only at the beginning. In other words, once a fight starts, you can’t be surprised again, although a hidden foe can still gain the normal benefits from being unseen (see “Unseen Attackers and Targets” in the Player’s Handbook).
To be surprised, you must be caught off guard, usually because you failed to notice foes being stealthy or you were startled by an enemy with a special ability, such as the gelatinous cube’s Transparent trait, that makes it exceptionally surprising. You can be surprised even if your companions aren’t, and you aren’t surprised if even one of your foes fails to catch you unawares.
If anyone is surprised, no actions are taken yet. First, initiative is rolled as normal. Then, the first round of combat starts, and the unsurprised combatants act in initiative order. A surprised creature can’t move or take an action or a reaction until its first turn ends (remember that being unable to take an action also means you can’t take a bonus action). In effect, a surprised creature skips its first turn in a fight. Once that turn ends, the creature is no longer surprised.
In short, activity in a combat is always ordered by initiative, whether or not someone is surprised, and after the first round of combat has passed, surprise is no longer a factor. You can still try to hide from your foes and gain the benefits conferred by being hidden, but you don’t deprive your foes of their turns when you do so.
---
Surprise rules work for two opposing sides. What happens with surprise when a third group of combatants sneaks up, hidden from the melee, and ambushes?
The surprise rule is relevant only when a combat is starting. Any ambushes during the fight use the rules for Dexterity (Stealth) checks.
---
Being surprised has no effect on saves
-------------
RAW thats probably all there is to guide you. Saving throws, such as the Dex one that is probably the best interpretation of reacting in a physical way to danger, is the best there is and even "surprise" doesn't stop players or NPCs from taking those dexterity saves to try and avoid some danger as part of their instinct.
Seeing the threat coming before it hits (i.e. spotting hidden attackers) would be covered by the passive perception. This represents general awareness of their surroundings, but they can also make active perception rolls each turn to be actively looking for danger and potentially roll higher than their passive perception to see it. If a player has the "Alert" feat they are completely immune to being surprised.
Another rule that can cover this is for hidden attackers who get advantage on their attack against a player they are hidden against. This can actually pair with "surprise" if the attackers remain hidden when they attack, giving them both the first round without actions from opponents who are surprised and letting them attack those opponents with advantage as opposed to a surprise round that might see the attackers jumping out from cover and charging to attack.
-----
If you're looking for some sort of custom rule to a situation you have in mind, the idea of using a dexterity saving throw or ability check is your best bet. Then you can apply disadvantage to their roll if you believe the character was caught completely off guard or didn't have a chance to see it happening.
There's also the "Dodge" action players can take each turn to give disadvantage on attack rolls against them and advantage specifically to dexterity saving throws. You could easily house rule this to say that it would give advantage on some sort of other dexterity ability check that would be made in response to an event.
Example: Players are guarding a gem on a pedestal. An enemy caster uses "Catapult" to launch the gem 90 feet towards them. You decide you want the players to have an opportunity to catch the gem out of the air as it flies by and have them roll a Dex saving throw/ability check against the caster's casting DC.
1. Characters who did not notice the caster or weren't watching get no roll, even if they had the dodge action (which doesn't impact attacks from hidden creatures).
2. Characters with no free hands roll with disadvantage.
3. Characters with a free hand roll normally
4. Characters who took the dodge action roll with advantage
You might also decide that this will use the character's reaction, so they must decide if they want to use their reaction for a chance to catch it.
The short answer is to roll Initiative as that is the mechanic used to determine who goes first, but you a re looking for another check. A saving throw is generally used to avoid an attack so I would advise against that. I guess I would suggest a skill check that you deem appropriate in the moment. Perception or Insight would be common options. Animal Handling if there is an animal involved. Survival if it’s a natural hazard. An Intelligence-based knowledge check if one of those seem appropriate. We could come up with situations to justify about anything. I’d suggest mixing it up so that players who invest in less common skills feel like their investment has value. And maybe a good check grants advantage on the initiative check. I hope you play with this and try different paths.
I would struggle with the question, "Why can't you react if you have an unspent reaction..."
Generally for things that are "instinctive" I use the passive. I only use an active check when the player tells me that he is doing something.
For example, if the player suspects that an NPC is telling a lie, I have them roll for insight. If the player doesn't tell me that he distrusts the NPC, what I do is secretly roll against his passive insight (which I calculate as passive perception).
Another example would be with arcana or nature or whatever. For example, if there is an arcane item in the room and the difficulty of "discovering" it is lower than its passive, I don't ask for a roll. He recognizes the object. Otherwise I don't say anything. And it's up to him whether he wants to study it carefully or not (in which case, I ask for an arcana check).
And for me the passive is always the baseline. Even if its result is lower, the minimum is always its passive. In fact, I don't ask him for a check if his passive already exceeds the difficulty (whether opposed or not).