So I had a player recently ask a question that I can't find any reasoning or logic to counter, and my initial response is that it's "just not in the rules", but I need to be sure.

So the player wants to ready an action to attack an enemy spellcaster at the moment they start to cast, to basically either cause the spell to fail, or at least require a concentration check. For reference, a spell like fireball that is instantaneous cast. I responded with you can ready an action, but it wouldn't interrupt the cast.

He then mentioned that you can't cast spells in armor that you aren't proficient with because it hampers spellcasting, so why couldn't say, a punch to the face?

I mean, he has a point there, but is there something in the rules that states that this isn't a thing, or how do I go about explaining it to the player in a way thats satisfactory?

Or am I wrong, and you CAN interrupt a spell in this way, and I just missed something?

A readied action occurs after the trigger. If the spell takes an action to cast or a bonus action, then your readied action occurs after the casting. If the spell takes longer to cast, such as 1 minute, then your readied action can occur after they have started and are still casting - the normal concentration rules apply (if they take damage they make a Con Save with a DC equal to half the damage received or 10 whichever is greater).

If you want to try and interrupt a spellcaster casting a bonus action or action spell then you need the Mage Slayer feat.

In addition to above, the implications of this would be that you could interrupt any action with a readied action (what makes spells special? armor without proficiency give weapon attacks disadvantage). So the entire game will devolve into both sides readying attacks to interrupt the other team, since any action PCs can do NPCs can also do.

Whether Mage Slayer allows your to force a caster to make a concentration check on a spell that normally doesn't require concentration is debatable. I know this has been brought up in a discussion regarding Mage Slayer and Misty Step here: https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/33651-mage-slayer-feat-vs-misty-step

Also I don't see why Mage Slayer would work but a Readied action wouldn't. The reaction granted by Mage Slayer seems like a reasonably specific trigger a player could declare when taking the Ready Action.

If you are in the Mage Slayer interrupts spellcasting camp I feel like granting the same for Ready Action is reasonable. Giving up your Extra Attack is a costly thing to do for a martial character. Mage Slayer lets you avoid that cost as well as the other benefits of that feat.

Quote from Fangeye >>

Whether Mage Slayer allows your to force a caster to make a concentration check on a spell that normally doesn't require concentration is debatable. I know this has been brought up in a discussion regarding Mage Slayer and Misty Step here: https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/33651-mage-slayer-feat-vs-misty-step

Also I don't see why Mage Slayer would work but a Readied action wouldn't. The reaction granted by Mage Slayer seems like a reasonably specific trigger a player could declare when taking the Ready Action.

If you are in the Mage Slayer interrupts spellcasting camp I feel like granting the same for Ready Action is reasonable. Giving up your Extra Attack is a costly thing to do for a martial character. Mage Slayer lets you avoid that cost as well as the other benefits of that feat.

 That is another good point. It doesn't work for mage slayer which requires a feat and is limited to melee, so it definitely should not work for any readied attack.

As for the "giving up extra attack is costly," that doesn't mean you should be able to do it in exchange for a target's entire action and potentially a limited resource. That is 110% unfair for the caster. What if a caster could choose to do half damage with a cantrip for 1 turn to cause a martial character to lose their action and break their weapon?

Quote from Aradona >>

I mean, he has a point there, but is there something in the rules that states that this isn't a thing, or how do I go about explaining it to the player in a way thats satisfactory?

Or am I wrong, and you CAN interrupt a spell in this way, and I just missed something?

 No, he doesn't have a point really.
Don't get me wrong, I totally get why a martial character would want to be able to do that but the game isn't built like that and you'd quickly get into balance trouble.

Counterspell is the in-game mechanic to interrupt spellcasting.

Quote from DxJxC >>

 That is another good point. It doesn't work for mage slayer which requires a feat and is limited to melee, so it definitely should not work for any readied attack.

As for the "giving up extra attack is costly," that doesn't mean you should be able to do it in exchange for a target's entire action and potentially a limited resource. That is 110% unfair for the caster. What if a caster could choose to do half damage with a cantrip for 1 turn to cause a martial character to lose their action and break their weapon?

 Oh for sure. The point I was trying to make was limited to tables that do allow Mage Slayer to interrupt spellcasting. I was trying to explain why allowing the same for readied actions doesn't make Mage Slayer worthless.

As for the broader point of if interrupting spellcasting should be allowed or not I totally agree. The major advantage of martial classes is that they aren't as resource limited as casters. Counterspell is a third level spell for a reason and allowing it for free would be a nightmare fit casters.

Quote from Aradona >>

So I had a player recently ask a question that I can't find any reasoning or logic to counter, and my initial response is that it's "just not in the rules", but I need to be sure.

So the player wants to ready an action to attack an enemy spellcaster at the moment they start to cast, to basically either cause the spell to fail, or at least require a concentration check. For reference, a spell like fireball that is instantaneous cast. I responded with you can ready an action, but it wouldn't interrupt the cast.

He then mentioned that you can't cast spells in armor that you aren't proficient with because it hampers spellcasting, so why couldn't say, a punch to the face?

I mean, he has a point there, but is there something in the rules that states that this isn't a thing, or how do I go about explaining it to the player in a way thats satisfactory?

Or am I wrong, and you CAN interrupt a spell in this way, and I just missed something?

You made the right call. The only actual factual reason is that the rules don't allow that, but I think it makes sense to say that spellcasters in combat are expecting to be attacked and that a simple attack isn't going to phase them enough to prevent them from doing what they're good at. Remind your player that spellcasters are as practiced at magic as a warriors are practiced with their weapons. Also, the point that even a trained Mage Slayer can't prevent a spell like that.

An alternative you might bring up: If you're already next to the mage and you have an Action available, why not try to do something like grab their hands, cover their mouths, or take away their spell foci? These actions would also fall under DM fiat since they're not specifically covered by the rules, but it seems like acting proactively instead of reactively would make more sense and you have a chance to disable a good portion of their spells, depending on which component you pick.

I appreciate the responses, and those...make a lot of sense. I figured the big picture reason was game balancing, so I thought I'd ask...for all I knew there was a mechanic that allowed this and I missed it. I appreciate the feedback!

Quote from Ophidimancer >>

Quote from Aradona >>

So I had a player recently ask a question that I can't find any reasoning or logic to counter, and my initial response is that it's "just not in the rules", but I need to be sure.

So the player wants to ready an action to attack an enemy spellcaster at the moment they start to cast, to basically either cause the spell to fail, or at least require a concentration check. For reference, a spell like fireball that is instantaneous cast. I responded with you can ready an action, but it wouldn't interrupt the cast.

He then mentioned that you can't cast spells in armor that you aren't proficient with because it hampers spellcasting, so why couldn't say, a punch to the face?

I mean, he has a point there, but is there something in the rules that states that this isn't a thing, or how do I go about explaining it to the player in a way thats satisfactory?

Or am I wrong, and you CAN interrupt a spell in this way, and I just missed something?

You made the right call. The only actual factual reason is that the rules don't allow that, but I think it makes sense to say that spellcasters in combat are expecting to be attacked and that a simple attack isn't going to phase them enough to prevent them from doing what they're good at. Remind your player that spellcasters are as practiced at magic as a warriors are practiced with their weapons. Also, the point that even a trained Mage Slayer can't prevent a spell like that.

An alternative you might bring up: If you're already next to the mage and you have an Action available, why not try to do something like grab their hands, cover their mouths, or take away their spell foci? These actions would also fall under DM fiat since they're not specifically covered by the rules, but it seems like acting proactively instead of reactively would make more sense and you have a chance to disable a good portion of their spells, depending on which component you pick.

I'd also point out that the grappled and restrained conditions do not prevent spellcasting so I would be leery of allowing a character to grab the hands of a caster and say they can't cast spells as a result. It might make "sense" depending on the circumstances but it would likely also be a balance issue. The same goes for the spellcasting focus? There are optional rules for disarming but the basic rules don't allow you to take someone else's weapon so again I'd be leery of letting a character take a spell casting focus from another character in the middle of combat. The other player is alert, aware and likely trained to prevent others from interfering with their spell casting so none of these options would be likely to work at my table without some specific prior set up that might give it a chance to work as a special one off case.

Quote from David42>>

I'd also point out that the grappled and restrained conditions do not prevent spellcasting so I would be leery of allowing a character to grab the hands of a caster and say they can't cast spells as a result. It might make "sense" depending on the circumstances but it would likely also be a balance issue.

I do know that grappling doesn't stop spellcasting, which is why I pointed out that it is not covered by the rules and is DM fiat. I wasn't suggesting allowing it to prevent casting in total, just prevent Somatic components. Spell components are their balancing factors and I think fair game for interference, but the fact that there are three separate types means it's hard to lock down a caster completely.

The same goes for the spellcasting focus? There are optional rules for disarming but the basic rules don't allow you to take someone else's weapon so again I'd be leery of letting a character take a spell casting focus from another character in the middle of combat. The other player is alert, aware and likely trained to prevent others from interfering with their spell casting so none of these options would be likely to work at my table without some specific prior set up that might give it a chance to work as a special one off case.

There are also ways of disarming without resorting to optional rules, though it does require a Battle Master Maneuver. Not everyone is going to have it, but it is a possibility within the rules.

I'd also point out that the grappled and restrained conditions do not prevent spellcasting so I would be leery of allowing a character to grab the hands of a caster and say they can't cast spells as a result. It might make "sense" depending on the circumstances but it would likely also be a balance issue.

Yes. This kind of thing almost invariably devolves into every combat being a series of called shots where people are trying to blind or disarm or otherwise achieve an effect that is normally locked behind limited resources. It becomes a group of 10-year-olds playing Space Battle in the backyard shouting out that they have lasers, but the other one has laser shields, but the first one shot the laser shield generator, but the other one has a backup, ad nauseum. I'm all for allowing creativity in combat, but you need to draw a line. My line is that you cannot prevent other characters from using their primary features - i.e. spellcasting - unless something explicitly says that you can or there are special circumstances that make it clear this is not something you can usually do.

OP, as someone who has had great experience with house rules I would suggest you set up some test situations and try them out. Not that does not mean your idea will work in your game or that your house rule may cause an issue you do not see. But in general I have found (if you have the time) that testing out some examples is beneficial to your game in the long run.  Also as a general rule it can be beneficial to have a general rule for every game you play that says something like this "All rule questions that fall outside of the RAW or that make me to make a complex decision on the spot may be ruled one way during the game and then after some reflection I might change how I will treat that situation in future games."

Stepping outside of the 5e frame work and pulling on my experience from other games, often spell casting is visual in that casters have to perform specific actions or the process creates/generates specific effects that others can react to, for example dancing, chanting, generating mystical lights and sounds, etc. In games that I have played in that allow a person to interrupt a spell by physically damaging them the person has to declare an action to do so, notice the action and then react to that action. Often the declaration of "wait to damage the person when they cast a spell" is limiting in nature in that if the spell caster does not cast a spell they may not get to attack or may only attack at a huge penalty and always requires the person to have a higher init then the caster. Note this system works best when you roll init every round so when you act is variable and it places more emphasis on who goes first and the things that make you go first in combat. 

Another balance issue can be is there a spell/ability that always interrupts melee attacks and you see the need for some balancing factors in the melee persons corner. If so then you may want to test out your house rule and see how it plays out.

In testing I often find testing at extreme levels is easier so high level PC's with high numbers in the areas you are looking to test. If you use test examples with smaller numbers it can be harder to see if it is unbalancing. 

"All rule questions that fall outside of the RAW or that make me to make a complex decision on the spot may be ruled one way during the game and then after some reflection I might change how I will treat that situation in future games." 

  I do that at my tables, and that has worked real well for my players and I. it keeps things fluid, and if I am wrong, I am wrong, we accept it and move on. My players have a lot of fun (as they should) and I'm...not tearing my hair out lol (well, what's left of it anyway). This was brought up to me between sessions and at first I was like yeah man, can't do it, although you can do a readied attack. Then we got to really talking about it, and thought that logistically it makes sense, So I appreciate that. Also, didn't know that grappling and restrained doesn't prevent spellcasting. Good to know.

Often players forget that GM's are human and forget things, make mistakes and just have a bad day once in a while. So it is nice to have some thoughts on paper or said before campaigns to help everyone out.

Quote from Aradona >>

"All rule questions that fall outside of the RAW or that make me to make a complex decision on the spot may be ruled one way during the game and then after some reflection I might change how I will treat that situation in future games." 

  I do that at my tables, and that has worked real well for my players and I. it keeps things fluid, and if I am wrong, I am wrong, we accept it and move on. My players have a lot of fun (as they should) and I'm...not tearing my hair out lol (well, what's left of it anyway). This was brought up to me between sessions and at first I was like yeah man, can't do it, although you can do a readied attack. Then we got to really talking about it, and thought that logistically it makes sense, So I appreciate that. Also, didn't know that grappling and restrained doesn't prevent spellcasting. Good to know.

Ditto.  Pretty much why stupid questions pop up from time to time on the forums because the "I need to research that." condition is applied to me the DM.