What ways are there to disrupt spellcasting in 5e?

[Coffee_Dragon]

If you are reffering to this, then it doesn't say what you think it does. My trigger is "when X starts to cast a spell", as soon as he starts the trigger is finished, the action is not.

As I've said, it would be a bizarre way to design if they meant, "There are some reactions that interrupt, and then there's the rest, including Ready. Ready also interrupts!" I don't think we're getting any further here.

PC: "I will pull the lever when he steps on square X"
Enemy: "I move from Square A to Square B"

[PC][B][X][A]

Clearly interrupting enemy movement.

Movement is incremental in a way that actions (typically) are not. I have already admitted the case can be complicated to the point where you are interrupting something, but in the basic case as described, it isn't movement.

[Rukelnikov]

I don't think we're getting any further here.

Yup, agreed

[MaXenzie]

In regards to a readied action, I believe the specific wording is important.

"I ready an action to cast hold person when the mage casts a spell" will allow the enemy mage to cast a spell before the hold person activates.

"I ready an action to cast hold person once the mage begins to cast a spell" will let the mage waste a spell slot and then get held, as the trigger was them beginning to cast a spell, but specifically not them casting it.

Wording is important, folks.

[Tanarii]

The ready action description gives us two examples of its use, one of them is literally interrupting another creatures movement:

"lf lhe cultist steps on the trapdoor, I'll pull the lever that opens it"

That's not an interruption. It's an after the trigger is completed reaction.

Keep in mind movement is not an action in this edition. It is not indivisible by default.

[TheUser]

In regards to a readied action, I believe the specific wording is important.

"I ready an action to cast hold person when the mage casts a spell" will allow the enemy mage to cast a spell before the hold person activates.

"I ready an action to cast hold person once the mage begins to cast a spell" will let the mage waste a spell slot and then get held, as the trigger was them beginning to cast a spell, but specifically not them casting it.

Wording is important, folks.

The slot wouldn't be wasted; because they failed to provide the components for the spell it never gets cast and hence the slot is not used; whereas counter spell says the spell just fails (as opposed to fails to cast).

[MaXenzie]

The slot wouldn't be wasted; because they failed to provide the components for the spell it never gets cast and hence the slot is not used; whereas counter spell says the spell just fails (as opposed to fails to cast).

That depends on your definition of "beginning" to cast a spell.

Since they have finished beginning to cast a spell, they are in the process of casting it. Meaning the components have been used and arcane energy is swirling and magic is forming, etc, etc.

[JackPhoenix]

That depends on your definition of "beginning" to cast a spell.

Since they have finished beginning to cast a spell, they are in the process of casting it. Meaning the components have been used and arcane energy is swirling and magic is forming, etc, etc.

And once that happens, the spell is cast. There's no distinction between "beginning to cast a spell" and "cast a spell" in the rules, unless the spell has a casting time longer than one action.

They can't finish an act that doesn't exist.

[tieren]

PC: "I will pull the lever when he steps on square X"
Enemy: "I move from Square A to Square B"

[PC][B][X][A]

Clearly interrupting enemy movement.

I think this is a key misunderstanding in the interrupt debate.

On the one hand yes the enemy's attempt to move all the way to B is interrupted, but his movement on the square x was not.

a closer analogy to interrupting spell casting would be:

PC: "I will repelling blast him back when he starts to enter square X"

The question then being, does the enemy move onto X and get blasted back (presumably) or does he raise his foot like he is about to step into X and then gets blasted back without ever stepping on X. That is the question of whether his movement was interrupted mid step rather than interrupted in the sense of not reaching his final intended destination.

I am ok with readying an action to key off a perceivable phenomenon (waving hands, speaking, etc...) and having the readied action take place before the spell is cast.

Now in the case of a readied silence spell. If the caster hadn't used their movement yet I would let them walk out of the area of effect before finishing their spell (space allowing); same with most line of sight obstructions.

Heres one:

Caster A readies shocking grasp for when Caster B starts to cast a spell; Caster B starts to cast fireball, Caster A starts to cast shocking grasp, Caster B then actually casts counterspell on the shocking grasp.

or how about readying an attack to strike when the Caster "begins" to cast the shield spell.

Thug A readies attack, Thug B swings at Caster, Caster "begins" to cast shield, triggering Thug A's attack.

If Thug A's attack happens before the spell is done being cast it seems to counter the point of the shield spell being faster than attacks and preventing them.

[Telok]

Additional issue with equating movement and spellcasting: Not all tables use a mat & minis.

[Rukelnikov]

That's not an interruption. It's an after the trigger is completed reaction.

Keep in mind movement is not an action in this edition. It is not indivisible by default.

Ok, I can get that.

I think this is a key misunderstanding in the interrupt debate.

On the one hand yes the enemy's attempt to move all the way to B is interrupted, but his movement on the square x was not.

a closer analogy to interrupting spell casting would be:

PC: "I will repelling blast him back when he starts to enter square X"

The question then being, does the enemy move onto X and get blasted back (presumably) or does he raise his foot like he is about to step into X and then gets blasted back without ever stepping on X. That is the question of whether his movement was interrupted mid step rather than interrupted in the sense of not reaching his final intended destination.

Guess it depends on the trigger, "When he's about to step on square X" would trigger before he actually stepped in it, "When he steps on Square X" would be once he has set a foot on it. I don't really know what "starting to enter a square" would mean, or if it is even a perceivable circumstance, which is the only constraint for choosing triggers.

I am ok with readying an action to key off a perceivable phenomenon (waving hands, speaking, etc...) and having the readied action take place before the spell is cast.

Now in the case of a readied silence spell. If the caster hadn't used their movement yet I would let them walk out of the area of effect before finishing their spell (space allowing); same with most line of sight obstructions.

Heres one:

Caster A readies shocking grasp for when Caster B starts to cast a spell; Caster B starts to cast fireball, Caster A starts to cast shocking grasp, Caster B then actually casts counterspell on the shocking grasp.

or how about readying an attack to strike when the Caster "begins" to cast the shield spell.

Thug A readies attack, Thug B swings at Caster, Caster "begins" to cast shield, triggering Thug A's attack.

If Thug A's attack happens before the spell is done being cast it seems to counter the point of the shield spell being faster than attacks and preventing them.

Its an interesting case for sure, my stance would have to interpret it as Thug A's attack would target an AC without Shield, Thug B's attacks would have shield applied against them.

Consider this case now:

5th lvl wizard readies action to launch a crossbow bolt at an enemy spellcaster when its starts to cast a spell.

Enemy spellcaster declares casting of Wall of Fire.

5th Wiz realizes enemy spellcaster is doing the components for some spell, he can cast Counterspell to disrupt it, but he cannot pull the trigger?

[TheUser]

And once that happens, the spell is cast. There's no distinction between "beginning to cast a spell" and "cast a spell" in the rules, unless the spell has a casting time longer than one action.

They can't finish an act that doesn't exist.

I mean...by virtue of the fact that counterspell occurs DURING A CAST would seem to indicate that if you went immediately from casting to being finished casting there is no window of opportunity to counter it....and yet counter spell describes inside the ruleset, a window of opportunity between starting and finishing a spell cast.

If you stop a character from providing all the components of a spell -even during the process of casting- the spell does not go off and the spell slot is not used. Counterspell dictates specifically that the spell finishes but instead fails.

[CorporateSlave]

Five pages to read through! Hope I haven't missed something and am just repeating!

First of all, I agree and disagree with everyone.

Clearly there are Reactions that interrupt - even a sword attack - by design, but I don't know that there is any RAW to support being able to disrupt a spell being cast with a Readied physical Attack. In fact, didn't this used to be a feature of earlier editions of D&D that is no longer present in 5e? (could be wrong about this, admittedly)

How to interrupt a sword attack before it is resolved - Protection fighting style (or a few other things that impose Disadvantage). These Reactions trigger when an Attack is made, but before the dice rolling is resolved...as it changes how many dice are rolled (you have to declare its use before the dice roll is known, in any case).

However, this is a specific dedicated Reaction ability, not using a Reaction to pop off a Readied Action, and by the RAW, we know specific trumps general.

There are clearly certain specific abilities that "interrupt" spells, but are they really specific exceptions (I think probably yes)? Counterspell for example, is also a dedicated Reaction, not a Readied Action.

Most of the RAW I have seen does not make any specific examples of how Readied Actions can interrupt Actions, only Movement. But everything can interrupt Movement. You can interrupt your own Movement to Move, Cast A Spell, Move. In a lot of ways, "Movement" is more a group of discrete 5' increments and less one unified activity such like an Action. Besides, per the PHB RAW Movement and Actions are explicitly different things. The Ready Action itself differentiates between Actions and Movement. (it may be of some significance to notice that it is the Ready Action, not "Readying an Action." You can use the Ready Action to trigger either an Action or Movement as your Reaction)

Few people would argue that you could not specify a trigger of "If that Orc takes one step closer to us, I'm going to Attack it with my longbow" and when the Orc closes from 30' away to 25' away you get your shot. But that's Movement interrupted (maybe interrupted, for all anyone knows the orc may only have been intending to move 5' closer), it's not their Action interrupted, and per 5e, RAW, those are two very different things. To imply that the ability to interrupt full potential Movement also means you can interrupt something else entirely, an Action, is just that...a subjective implication that is not supported in the rules. There is no RAW stating an Action and Movement are the same thing. They are entirely different sections in the PHB, in fact.

If I were DM'ing, I would probably not allow the cheese of a PC stating "I Ready an Attack for the moment I see the caster start to cast a spell, then shoot him to interrupt it and force a Concentration check (or otherwise cause it to fail). There are things that per RAW can interrupt or otherwise cancel out a spell. Older editions included attacks/taking damage in this, 5e does not. The fact that there are specific examples/abilities that interrupt or cancel spells yet no description of how to use a Readied Attack Action to do the same really makes me thing that those are Specific examples that beat the General rule that a Readied Attack Action will only occur once the entirety of the trigger is completed, and specifying a trigger of starting to do something, particularly an Action, does not suffice.

The particular wording can currently be rules lawyered perhaps, but maybe the simplest way to explain my view:

There is no general RAW for interrupting spells being cast with a casting time of 1 Action or less. This would have been terribly easy to include in the rules if it was intended by the game designers. It was not included. "Common sense" arguments can be made, but the only RAW general rule relating to Casting a Spell that discusses maintaining any type of Concentration to Cast a Spell concerns spells with a casting time of greater than 1 Action...or using the Ready Action to Cast and Hold the spell.

There are many specific RAW ways to interrupt spells being cast. These specific examples (spells/abilities) beat the general rule that a Readied Action must wait until after the trigger resolves to take place. Simply gaming the system by using tricky wording that attempts to break down an Action into discrete steps that allow for interruption is some fine power gaming, but not something I feel a reasonable DM should allow, and not something RAW specifically states you can do.

[DMThac0]

I mean...by virtue of the fact that counterspell occurs DURING A CAST would seem to indicate that if you went immediately from casting to being finished casting there is no window of opportunity to counter it....and yet counter spell describes inside the ruleset, a window of opportunity between starting and finishing a spell cast.

If you stop a character from providing all the components of a spell -even during the process of casting- the spell does not go off and the spell slot is not used. Counterspell dictates specifically that the spell finishes but instead fails.

This is a case of specific beats general. Counterspell specifically states that it interrupts the spell, where as none of the other actions suggested (aside from the one Ranger subclass) have that specification. That makes the Counterspell unique in this ability not the rule.

[Man_Over_Game]

Five pages to read through! Hope I haven't missed something and am just repeating!

First of all, I agree and disagree with everyone.

Clearly there are Reactions that interrupt - even a sword attack - by design, but I don't know that there is any RAW to support being able to disrupt a spell being cast with a Readied physical Attack. In fact, didn't this used to be a feature of earlier editions of D&D that is no longer present in 5e? (could be wrong about this, admittedly)

How to interrupt a sword attack before it is resolved - Protection fighting style (or a few other things that impose Disadvantage). These Reactions trigger when an Attack is made, but before the dice rolling is resolved...as it changes how many dice are rolled (you have to declare its use before the dice roll is known, in any case).

However, this is a specific dedicated Reaction ability, not using a Reaction to pop off a Readied Action, and by the RAW, we know specific trumps general.

There are clearly certain specific abilities that "interrupt" spells, but are they really specific exceptions (I think probably yes)? Counterspell for example, is also a dedicated Reaction, not a Readied Action.

Most of the RAW I have seen does not make any specific examples of how Readied Actions can interrupt Actions, only Movement. But everything can interrupt Movement. You can interrupt your own Movement to Move, Cast A Spell, Move. In a lot of ways, "Movement" is more a group of discrete 5' increments and less one unified activity such like an Action. Besides, per the PHB RAW Movement and Actions are explicitly different things. The Ready Action itself differentiates between Actions and Movement. (it may be of some significance to notice that it is the Ready Action, not "Readying an Action." You can use the Ready Action to trigger either an Action or Movement as your Reaction)

Few people would argue that you could not specify a trigger of "If that Orc takes one step closer to us, I'm going to Attack it with my longbow" and when the Orc closes from 30' away to 25' away you get your shot. But that's Movement interrupted (maybe interrupted, for all anyone knows the orc may only have been intending to move 5' closer), it's not their Action interrupted, and per 5e, RAW, those are two very different things. To imply that the ability to interrupt full potential Movement also means you can interrupt something else entirely, an Action, is just that...a subjective implication that is not supported in the rules. There is no RAW stating an Action and Movement are the same thing. They are entirely different sections in the PHB, in fact.

If I were DM'ing, I would probably not allow the cheese of a PC stating "I Ready an Attack for the moment I see the caster start to cast a spell, then shoot him to interrupt it and force a Concentration check (or otherwise cause it to fail). There are things that per RAW can interrupt or otherwise cancel out a spell. Older editions included attacks/taking damage in this, 5e does not. The fact that there are specific examples/abilities that interrupt or cancel spells yet no description of how to use a Readied Attack Action to do the same really makes me thing that those are Specific examples that beat the General rule that a Readied Attack Action will only occur once the entirety of the trigger is completed, and specifying a trigger of starting to do something, particularly an Action, does not suffice.

The particular wording can currently be rules lawyered perhaps, but maybe the simplest way to explain my view:

There is no general RAW for interrupting spells being cast with a casting time of 1 Action or less. This would have been terribly easy to include in the rules if it was intended by the game designers. It was not included. "Common sense" arguments can be made, but the only RAW general rule relating to Casting a Spell that discusses maintaining any type of Concentration to Cast a Spell concerns spells with a casting time of greater than 1 Action...or using the Ready Action to Cast and Hold the spell.

There are many specific RAW ways to interrupt spells being cast. These specific examples (spells/abilities) beat the general rule that a Readied Action must wait until after the trigger resolves to take place. Simply gaming the system by using tricky wording that attempts to break down an Action into discrete steps that allow for interruption is some fine power gaming, but not something I feel a reasonable DM should allow, and not something RAW specifically states you can do.

I agree with everything you said.

5e was intended to fix the problems of the past, and there were two big problems that people had with 3.5:

Magic was far too strong, and a mage was useless when he didn't have magic to make him OP.

So they made magic much weaker, but then ramped up the reliability of mages to compensate. Now a Fighter can be on the same level as a Wizard (at least, in comparison to 3.5), but they had to remove all the finicky parts of magery to make sure that mages still felt valid. It's a balancing act.

Now, I don't have a major problem if someone takes some kind of risk/resource to prevent a mage from casting a spell, but it shouldn't be reliable.

[tieren]

Consider this case now:

5th lvl wizard readies action to launch a crossbow bolt at an enemy spellcaster when its starts to cast a spell.

Enemy spellcaster declares casting of Wall of Fire.

5th Wiz realizes enemy spellcaster is doing the components for some spell, he can cast Counterspell to disrupt it, but he cannot pull the trigger?

Personally I don't have a problem with the readied attack occurring before the spell completes (although at my table a more specific trigger would be required), I don't agree with the idea that the attack somehow disrupts the casting, after the crossbow attack is resolved the enemy casts his firewall (assuming they survived the attack)

[stoutstien]

Personally I don't have a problem with the readied attack occurring before the spell completes (although at my table a more specific trigger would be required), I don't agree with the idea that the attack somehow disrupts the casting, after the crossbow attack is resolved the enemy casts his firewall (assuming they survived the attack)

Second this.
now if the Castor was casting a long-term spell that required concentration and the reaction was an attempt to disrupt that I see no problem. Using the firewall example it may not be possible due to lose of line of sight.Trading action/reaction for a chance to end a spell early seems balanced.
Which remind me how bad the mage Slayer feat is as written.

[Contrast]

Consider this case now:

5th lvl wizard readies action to launch a crossbow bolt at an enemy spellcaster when its starts to cast a spell.

Enemy spellcaster declares casting of Wall of Fire.

5th Wiz realizes enemy spellcaster is doing the components for some spell, he can cast Counterspell to disrupt it, but he cannot pull the trigger?

Its also worth just reiterating that this is a game.

Why is a character capable to perfectly jumping the exact same distance every time? Why can only level 20 dex based character whose focused on training with a bow attack 8 times in a round (a fighter) while another can only attack once (a rogue)? Why does a rogue always strike for substantial damage only when he's hidden or an enemy is standing next to his opponent, can't he get lucky? Why do PCs sit there and watch enemies stab their unconscious friend, only leaping into action when its their 'turn'?

The answer to all these things is nothing to do with logic and everything to do with the type of game 5E wants to be. So the reason you can counter a spell with Counterspell and not with a crossbow is nothing to do with the timing and mechanics of firing a crossbow bolt at a man in a funny hat in the real world and everything to do with the fact that we're looking up the rules for Readying an Action in a Rulebook^TM.

A Counterspell can interrupt because the rules says it can. A Readied action triggers when the triggering action finishes because the rules say it does. Obv you don't have to play it that way in your living room (I usually wouldn't allow it to interrupt but would probably allow exceptions depending on circumstance), I ain't the boss of you

[JackPhoenix]

Why can only level 20 dex based character whose focused on training with a bow attack 8 times in a round (a fighter) while another can only attack once (a rogue)?

Because one focuses his training on speed, while the other focuses more on accuracy.

Why does a rogue always strike for substantial damage only when he's hidden or an enemy is standing next to his opponent, can't he get lucky?

He can. It's called critical hit. Also, he could have any other source of advantage than being hidden, or be a swashbuckler.

Why do PCs sit there and watch enemies stab their unconscious friend, only leaping into action when its their 'turn'?

They don't. Turns and rounds only exist for the player's convenience, in fiction, the actions are simultaneous.

[Rukelnikov]

Personally I don't have a problem with the readied attack occurring before the spell completes (although at my table a more specific trigger would be required), I don't agree with the idea that the attack somehow disrupts the casting, after the crossbow attack is resolved the enemy casts his firewall (assuming they survived the attack)

But, I'm in favor of that, damage doesn't prevent/disrupt the spell in any way (unless it drops the caster)

[Rukelnikov]

Why is a character capable to perfectly jumping the exact same distance every time? Why can only level 20 dex based character whose focused on training with a bow attack 8 times in a round (a fighter) while another can only attack once (a rogue)? Why does a rogue always strike for substantial damage only when he's hidden or an enemy is standing next to his opponent, can't he get lucky? Why do PCs sit there and watch enemies stab their unconscious friend, only leaping into action when its their 'turn'?

There's an in world logic for all of those, fortunately JP covered most of them already.

The answer to all these things is nothing to do with logic and everything to do with the type of game 5E wants to be. So the reason you can counter a spell with Counterspell and not with a crossbow is nothing to do with the timing and mechanics of firing a crossbow bolt at a man in a funny hat in the real world and everything to do with the fact that we're looking up the rules for Readying an Action in a Rulebook^TM.

Exactly, the rules say "you decide what perceivable circumstance will trigger your reaction.". If I can perceive the casting of a spell well enough to counter it, then it is a perceivable circumstance, and thus a valid trigger for Ready Action.

A Counterspell can interrupt because the rules says it can. A Readied action triggers when the triggering action finishes because the rules say it does. Obv you don't have to play it that way in your living room (I usually wouldn't allow it to interrupt but would probably allow exceptions depending on circumstance), I ain't the boss of you

Nope, rules don't say that. You don't need to specify an Action as the trigger, you can define any perceivable circumstance as the trigger, and your ready action comes after the trigger has finished.

[ProsecutorGodot]

Nope, rules don't say that. You don't need to specify an Action as the trigger, you can define any perceivable circumstance as the trigger, and your ready action comes after the trigger has finished.

If we're nitpicking "triggering action" to mean "literally an action" for no reason then the discussion has gone far enough off the rails that we might as well just agree to disagree. Does it really matter to specify that it doesn't have to be an action if the point was that the reaction occurs after the trigger, whether it was an action or not?

There's no way for your character to be acutely aware in world of what "beginning to cast a spell" looks like in a form that could be pre-empted by an arrow compared to "casting a spell" in a way that has become inevitable unless interrupted by appropriate magic like counterspell.

I'll advocate for case by case basis, such as a spellcasters duel where creative applications of magic could be used to interrupt a caster or using Silence specifically in response to a spellcasting. These seem like reasonable exceptions to make. What doesn't seem reasonable is creating a phase of spellcasting that doesn't exist to allow weapons to serve the same purpose.

If you're trying to stop a spellcaster from doing something, use your action. What exactly is even gained by trying to lawyer your readied attack when you could have reasonably (in the cases that a spellcaster is trying to cast a spell in your range) attacked them instead?

EDIT: Just as an example.
DM: "While your ally dispatches yet another one of the bandits, the robed one in the back begins erratically waving his arms around and speaking in a way that you don't understand"
P: "Does this trigger my readied action to shoot him as beginning to cast a spell?
DM: "This does look similar to what party Wizard does when he casts a spell, you can choose to respond if you would like."
P: "Okay, I shoot him"
DM: "As the words "wait don't shoot, I surrend-" finally break through his frantic babbling, your arrow hits him straight in the neck and he falls over dead"
P: "But I thought you said he was casting a spell, I wouldn't want to kill a man who was surrendering."
DM : "You thought he was casting a spell, but since you used your reaction to fire an arrow and not to attempt to identify that it was actually a spell you've instead shot a man trying desperately to dissuade you from killing all of his friends and him. You know he wasn't exactly a law-abiding citizen so it's not as if you've killed an innocent in this. Decide for yourself if this weighs heavily on you."

[CorporateSlave]

What doesn't seem reasonable is creating a phase of spellcasting that doesn't exist to allow weapons to serve the same purpose.

This, I suppose.

If the game designers had wanted a spell to be cast in "parts" that lent it to easy disruption (which is what we are talking about here lets face it...allowing any Ready Action trigger to disrupt a spell makes it insanely easy to counter anything but a Subtle Spell), they could easily have added those rules. Where are they? Well, in the part about spells with a casting time greater than one Action, sort of. That casting is broken down into parts...so to speak. But only that extended duration of casting.

As a DM I would not for one second buy the argument...which has zero RAW support...that a viable trigger for a Ready Action's Reaction use is "starting to cast a spell," outside of saying, ok, so your Reaction goes off as soon as the spell is cast, which is pretty clearly the RAI...but the spell is still cast, not interrupted.

Just because something doesn't have any particular RAW contradiction/forbiddance doesn't make it "according to the rules."

Spells with a casting time of 1 Action, 1 Bonus Action, or 1 Reaction simply are not divided up by RAW into any meaningful "phases" such as "beginning to cast, casting, finished casting." However, there are specific things that are able to interrupt. This tells me the RAI is that only these specific things can interrupt the casting. Everything else waits until the spell is cast.

[Kane0]

If you're trying to stop a spellcaster from doing something, use your action. What exactly is even gained by trying to lawyer your readied attack when you could have reasonably (in the cases that a spellcaster is trying to cast a spell in your range) attacked them instead?

EDIT: Just as an example.
DM: "While your ally dispatches yet another one of the bandits, the robed one in the back begins erratically waving his arms around and speaking in a way that you don't understand"
P: "Does this trigger my readied action to shoot him as beginning to cast a spell?
DM: "This does look similar to what party Wizard does when he casts a spell, you can choose to respond if you would like."
P: "Okay, I shoot him"
DM: "As the words "wait don't shoot, I surrend-" finally break through his frantic babbling, your arrow hits him straight in the neck and he falls over dead"
P: "But I thought you said he was casting a spell, I wouldn't want to kill a man who was surrendering."
DM : "You thought he was casting a spell, but since you used your reaction to fire an arrow and not to attempt to identify that it was actually a spell you've instead shot a man trying desperately to dissuade you from killing all of his friends and him. You know he wasn't exactly a law-abiding citizen so it's not as if you've killed an innocent in this. Decide for yourself if this weighs heavily on you."

Because you (the PC) doesn't want to be the first to act in a hostile manner but won initiative? And to me that sounds more like the DM screwing with you really. I would literally be like 'dude, is he casting a spell or not?'

[JoeJ]

Because you (the PC) doesn't want to be the first to act in a hostile manner but won initiative? And to me that sounds more like the DM screwing with you really. I would literally be like 'dude, is he casting a spell or not?'

Initiative is rolled at the beginning of combat. Why would you not want to act in a hostile manner in combat?

[ProsecutorGodot]

Because you (the PC) doesn't want to be the first to act in a hostile manner but won initiative? And to me that sounds more like the DM screwing with you really. I would literally be like 'dude, is he casting a spell or not?'

And I would ask you to make a check to see if you can recognize a spell or not, your PC doesn't have the knowledge of you as a player, he sees a robed man waving his hands around and practically screaming out unintelligible words. You would be told either "you've spent enough time with party wizard (or are party wizard) to recognize that these sounds and motions are completely unconventional for typical magic casting, you are confident that he's not actually casting a spell" on a success or "With as hectic as this fight has been, and the fact that you have seen him casting magic before, you can't be sure if this is a spell you haven't seen before or a bluff." on a failure.

If your trigger is trying to pre-empt the result of those actions, you don't get to know the result before you react.

Which is why I'm against the idea that you could set this type trigger in most cases*, it's metagamey and relies far too much on the structure of the game rules to define your in world actions and not enough on the potential realism of the scenario you're in.
*except in the cases where your readied action would reasonably be able to pre-empt the expected trigger without breaking the established chain of events, such as casting Silence instead of shooting an arrow.

[Rukelnikov]

And I would ask you to make a check to see if you can recognize a spell or not, your PC doesn't have the knowledge of you as a player, he sees a robed man waving his hands around and practically screaming out unintelligible words. You would be told either "you've spent enough time with party wizard (or are party wizard) to recognize that these sounds and motions are completely unconventional for typical magic casting, you are confident that he's not actually casting a spell" on a success or "With as hectic as this fight has been, and the fact that you have seen him casting magic before, you can't be sure if this is a spell you haven't seen before or a bluff." on a failure.

If your trigger is trying to pre-empt the result of those actions, you don't get to know the result before you react.

Which is why I'm against the idea that you could set this type trigger in most cases*, it's metagamey and relies far too much on the structure of the game and not enough on the potential realism of the scenario you're in.
*except in the cases where your readied action would reasonably be able to pre-empt the expected trigger without breaking the established chain of events, such as casting Silence instead of shooting an arrow.

If you can recognize the signs well enough to cast counterspell, you can recognize them well enough for it to be the trigger of a readied action.

[CorporateSlave]

Because you (the PC) doesn't want to be the first to act in a hostile manner but won initiative? And to me that sounds more like the DM screwing with you really. I would literally be like 'dude, is he casting a spell or not?'

I think more the point he is trying to make is; this is how screwy making up phases of spell casting can be. A player creating a trigger by imagining up something outside the rules such as "when he starts casting a spell" probably deserves to have a DM rules lawyer him right back. After all, does the fighter have time to make an Arcana check to figure out if a spell is definitely being cast? That probably ought to be an Action in and of itself...

And we haven't even touched on the fact that so far we've only discussed spells with a casting time of 1 Action. Is that going to be your trigger? "When the enemy wizards starts casting a spell with a casting time of one Action?" A bit Meta isn't it? A wizard casting a spell with a Reaction is still casting a spell aren't they? Yet the PHB describes a spell with a casting time of 1 Reaction as taking only a fraction of a second to cast.

How exactly do you justify with a "common sense" argument that this can be detected and interrupted in time(by anything that doesn't specify it actually can, such as the CounterSpell Spell)? Probably the reason the game designers only sub-divided spells with a casting time of greater than 1 Action...

If you can recognize the signs well enough to cast counterspell, you can recognize them well enough for it to be the trigger of a readied action.

The problem is, this is an extrapolation to general from a specific exception, which is the exact opposite of how 5e is supposed to work. (PHB Introduction, specific beats general) You've turned it into "specific becomes general."

I mean hey, if that's how you want your table to run, then house rule that you can disrupt spell casting by doing just about anything. But no need to pretend it's RAW, which it isn't.

[Rafaelfras]

This, I suppose.

If the game designers had wanted a spell to be cast in "parts" that lent it to easy disruption (which is what we are talking about here lets face it...allowing any Ready Action trigger to disrupt a spell makes it insanely easy to counter anything but a Subtle Spell), they could easily have added those rules. Where are they? Well, in the part about spells with a casting time greater than one Action, sort of. That casting is broken down into parts...so to speak. But only that extended duration of casting.

As a DM I would not for one second buy the argument...which has zero RAW support...that a viable trigger for a Ready Action's Reaction use is "starting to cast a spell," outside of saying, ok, so your Reaction goes off as soon as the spell is cast, which is pretty clearly the RAI...but the spell is still cast, not interrupted.

Just because something doesn't have any particular RAW contradiction/forbiddance doesn't make it "according to the rules."

Spells with a casting time of 1 Action, 1 Bonus Action, or 1 Reaction simply are not divided up by RAW into any meaningful "phases" such as "beginning to cast, casting, finished casting." However, there are specific things that are able to interrupt. This tells me the RAI is that only these specific things can interrupt the casting. Everything else waits until the spell is cast.

This, there isnt any situation in the book that calls for a concentration check to cast a 1 action, BA or Reaction spells. Stormy weather, high speed horses or charriots, boats on river or sea, etc. Nothing of those old situations is able to disrupt a caster on 1 action spells

[Rukelnikov]

The problem is, this is an extrapolation to general from a specific exception, which is the exact opposite of how 5e is supposed to work. (PHB Introduction, specific beats general) You've turned it into "specific becomes general."

I mean hey, if that's how you want your table to run, then house rule that you can disrupt spell casting by doing just about anything. But no need to pretend it's RAW, which it isn't.

It is. Ready requires you to specify a "perceivable circumstance" as trigger, the only possible argument against "begins casting a spell" as a trigger is that not being a perceivable circumstance. We know for a fact it is, since Counterspell is a thing, and reinforced by the fact that Subtle Spell, makes the casting of a spell not perceivable anymore and thus not a valid target for counterspell.

If you don't wanna allow it at your table, its another matter, but the rules allow for "begins casting a spell" to be a valid trigger.

[JoeJ]

If you don't wanna allow it at your table, its another matter, but the rules allow for "begins casting a spell" to be a valid trigger.

But for the purpose of the Ready action, they don't make any distinction between beginning to cast a spell and casting a spell