But you can interrupt spells in the 5E rules too, just not as often or easily and I have had PCs spells interrupted in game.
Maybe I've missed something, but as far as I am aware, there are only three ways to interrupt spells in 5e.
The first method is "Counterspell" - but that isn't something martials can generally do and is thus moot for the purposes of this discussion.
The second method is to deal damage to a caster while the caster is maintaining a spell that requires concentration - but that only fizzles the ongoing effects of an already-cast spell and my understanding of the O.P. was whether or not interrupting spells should be an option during casting, so again, it's moot for the purposes of this discussion.
The final method is to ready an action ("attack when he starts casting a spell") and deal enough damage with that reactive attack to incapacitate the caster (in which case, why not simply attack on your turn and incapacitate the caster before s/he starts casting the spell, though I suppose in this case that would be "preventing" and not "interrupting").
The underlying question in the original post is not "CAN a martial character interrupt spellcasting" - the OP concludes the RAW do not support that in 5e - but rather "SHOULD a martial character be able to interrupt spellcasting." There are really two approaches to deciding a "right" answer for this question, the first revolves around game balance (i.e., is it balanced for martials to be able to interrupt casters) while the second revolves around verisimilitude (i.e., does it make logical sense for martials to be able to interrupt casters).
ECMO3, my sense is that you have already made up your mind about the conclusion you would like to reach ("casters should be uninterruptable") and are ignoring all arguments to the contrary, because you are not actually addressing the lines of reasoning people are using to come to a "yes they should" conclusion. I don't think there is a "right" answer to the question of "Should this be possible" because the "Right" answer depends very much on the premises you start with.
ARGUMENT IN FAVOR BASED ON GAME BALANCE (VERBOSE AS I DON'T THINK ECMO3 HAS ENGAGED IT YET - APOLOGIES IF HE HAS AND I MISSED IT BUT I'M PUTTING THIS HERE BECAUSE I HAVEN'T SEEN THE WHOLE THING ARTICULATED START TO FINISH)
Observation 1: Spellcasters have access to many spells capable of rendering martial characters unable to utilize their abilities in a single action (casting the spell).
Observation 2: Martials have access to only one method of rendering casters unable to utilize their abilities ("reducing HP to zero") which can only succeed with a single action in very limited circumstances ("caster HP lower than martial's single-action damage-dealing capacity").
Obvservation 3: Casters also have access to many spells capable of rendering other casters unable to utilize their abilities in a single action (Feeblemind, Hold Person, etc.).
Observation 4: Martials also have access to only one method of rendering other martials unable to utilize their abilities (reduce HP to zero) which can only succeed with a single action in very limited circumstances (and these circumstances are less likely to be true for martial targets than caster targets (martials usually have better AC and HP and thus are both less likely to be hit and less likely to have HP lower than the attacker's single-action damage-dealing capacity).
Premise 1: Game balance ought to be defined as equality of potential - in other words, "if one character is capable of rendering the other character unable to use their abilities in a single action, ALL characters should be capable of rendering all other characters unable to use their abilities in a single action."
Conclusion 1: Based on Observations 1 and 3 and Premise 1, Caster vs. Caster interactions are balanced.
Conclusion 2: Based on Observations 2 and 4 and Premise 1, Martial vs. Martial interactions are balanced.
Conclusion 3: Based on Observations 1 and 2 and Premise 1, Caster vs. Martial interactions are NOT balanced.
Conclusion 4: Based on Conclusion 3, the rules as written provide for power imbalance between Casters and Martials and are therefore unsatisfactory if a balanced game is desired.
Proposed Solution 1: No character should have the ability to render another character unable to use their abilities with a single action. Since Martials do not have this ability, no changes to martials are needed under this solution. This means casters' spells must be reduced dramatically in power so they are no longer capable of rendering others unable to use their abilities or, more likely, these spells must be stripped from casters' spell lists entirely - by definition, this is a drastic overhaul of the magic system.
Proposed Solution 2: All characters should have the ability to render another character unable to use their abilities with a single action. This means not only do martials need to be given the ability to incapacitate casters with a single action, they ALSO must have the ability to incapacitate martials with a single action. This would require a radical overhaul of the weapon combat and HP system.
Proposed Solution 3: Add "counterplay" whereby there is an opportunity to prevent a character who is capable with a single action of rendering another character unable to use their abilities from successfully executing the use of that action... in other words, you don't have to overhaul either the magic system or the weapon combat and HP system; instead, you simply introduce a way for the character on the disadvantaged side of the problematic interaction (caster throwing an incapacitating spell at the martial character) to prevent the interaction from occurring.
Problems with Proposed Solutions 1 and 2: Overhauling any complex system is a huge lift and, more importantly, is likely to introduce a number of unintended consequences (some of which may be worse than the original problem we are trying to solve). Solution 1 is problematic as it essentially reduces combat to "whacking on a big bag of hit points until they fall to zero" because all other methods of incapacitation of a foe are removed - which makes for dull gameplay. Solution 2 is problematic as it is likely to reduce any fight to "who wins initiative wins the fight" since every character is capable of incapacitating any other character on their first action. That leaves Solution 3 as the only viable solution.
And... Solution 3 looks VERY MUCH like "casting a spell (i.e., initiating the problematic, unbalanced interaction) should grant (provoke) the disadvantaged side an ability to prevent the interaction (an opportunity to disrupt the spell, probably by an attack)."
Potential Caveats:
- Adding additional counterplay requires additional bookkeeeping.
- The martial character is using an unlimited resource (attacks) to counter the caster's resources (spell slots) and this may make the counterplay difficult to balance - if countering the caster's spells is too easy, it swings the balance in favor of the martial since s/he can deny the caster ALL their spell slots; similarly, if countering spells is too hard, we have not solved the initial imbalance we were looking to solve.
ARGUMENT AGAINST BASED ON GAME BALANCE (MUCH BRIEFER SINCE I SENSE ECMO3 INTUITIVELY GETS THIS ONE)
Observation 5: Casters have a limited set of opportunities (spell slots) available to them to incapacitate enemies.
Observation 6: Martials generally do not have limits on their ability to attempt to incapacitate foes (i.e., martials only run out of their primary mode of incapacitation - attacks - when they themselves are incapacitated).
Premise 2: Game balanced ought to be determined by expenditure of scarce resources - expenditure of limited resources should be more powerful than expenditure virtually unlimited resources.
Conclusion 5: Based on Observations 5 and 6 and Premise 2, casters ought to be more effective when using their limited resources - in particular spell slots - than martials are when using virtually unlimited resources - i.e., attacks. Therefore it should be possible for a caster to incapacitate a foe using fewer actions than a martial provided those actions are coming from limited resources.
Potential Caveats:
- Currently, "fewer actions" for spell casters may be as few as "one" - it may be argued that "one action" is too few and some other metric ought to be used (e.g., half as many spell slot levels as a martial must use attacks should be required to incapacitate a foe).
- If encounters (either in length or frequency) never meaningfully deplete the caster's spell slots, they are effectively an unlimited resource (i.e., Short/Long Rests must be made less frequent and/or eliminated or parties must be put through enough encounters between them that the caster enters some encounters noticeably depleted so the martial's "unlimited" resources actually matter because the martial is still functioning at peak levels while the caster is not).
ARGUMENTS FOR AND AGAINST BASED ON VERISIMILITUDE:
I'm... actually not going to make those here. Instead, I will point out that "Verisimilitude" is another way of saying "subjective" because it's really about what you believe in your head that magic is, and thus
it's extremely unlikely that you'll get people to come to an agreement on a set of premises around verisimilitude for MAGIC so there's not going to be any common ground to build on (e.g., we've already seen argument in this thread on exactly how long one must hold one's thumbs together with fingers in a fan for Burning Hands to trigger). For the most part, people will put forth ideas about how magic "must" work that justify their preconceived notions about how they WANT magic to work (probably to justify their own "coolness" or "game mechanic" perspective). Since it's MAGIC and there's little to compare it to in the real world, it's very difficult to use "verisimilitude" to justify most statements about the exact workings of magic.
The only time "verisimilitude" should really come into the discussion is in order to attack a game balance premise as being something that completely strains the limits of credulity (as a concrete example, others have noted in this thread that even if there is nothing in the rules as written about how to stop a caster, it strains verisimilitude to assert that casters can NEVER be prevented from casting spells - gag the caster to prevent use of verbal components, hogtie the caster to prevent use of somatic components, put a bag over his head to keep him from having line of sight to anything, and take away the caster's component pouch to remove material components makes sense to me as the way of the world at large would go about dealing with casters as there would have to be some way of "safely arresting" rogue casters - arguing that even doing this won't work to stop the caster just because the rules don't explicitly spell out a way to stop casting completely strains verisimilitude far past the breaking point for me; if there's absolutely no way common people can prevent casters from doing magic at all, even the weakest first-level casters are for all intents and purposes unstoppable, unaccountable gods).
D&D is "high fantasy" but "first-level casters are gods" is a little TOO high fantasy for me (and locks out even the most powerful of martials).
SO WHAT?
There is a history in D&D of martials being able to spoil spells. In 1e, 2e, and BECMI, even taking a single point of damage made a caster lose their spell (though a caster that wins initiative usually got their spells off unmolested). In 3E, casters got the ability to "Cast through damage" with Concentration checks. In 5e, Concentration checks were only applied to ongoing effects that require concentration (I didn't play 4e much, so I don't know what the rule is there). The unassailable fact is that casters have been moving from "easy to interrupt casting" to "hard to interrupt casting" to "what's interrupting casting?" in the rules-as-written over time.
Whether or not you think Casters are overpowered compared to martials (or vice versa) is probably based on whether or not you agree more with Premise 1 or Premise 2 above (complete with caveats). It has been my experience that in most campaigns (and in the RAW), Short and Long Rests are available at a frequency such as to make caster spell slots largely an unlimited resource... thus I tend to agree more with Premise 1 being the case in the "average campaign" - which is why I agree that for the average campaign, "let martials interrupt spellcasting" is a thing that probably needs to happen.
As I mentioned in a previous post, however, there is one more factor that ought to be considered... the "fun" factor. It's generally really fun and exciting when you get to disrupt the Big Bad's spell. It's also NOT generally fun when some mook disrupts YOUR spell. I also think there is some fun when there is uncertainty involved in actions - finding a way to recover even though the dice roll went against you can make for a much better story than everything going exactly to plan like clockwork. However, some players think it's more fun when everything goes their way and are disappointed at unexpected failure (this is a whole different discussion I won't delve deeply into here except to note that there are some players that DON'T find a chance of failure fun while for other players, the thrill of succeeding when success is NOT guaranteed is the big draw to an RPG with randomness).
No, personally I
DON'T think guaranteed success is fun, and because of that I don't think D&D's move from "easy to interrupt casting" to "impossible to interrupt casting" in the RAW is the right choice (my views fall more along the lines of what I outlined in the Game Balance case above and that's why I think allowing spells to be interrupted is good - I don't like how easy it was in 1e/2e, and I do think 3e got it about right in terms of Concentration checks with DC equal to damage sustained since martial damage and Concentration checks both scaled with level and usually you wound up needing to roll between an 8 and a 12 on your d20 once modifiers were accounted for to succeed - but it also prompted tactical thinking because if you're going to allow martials to attempt to disrupt a caster, you ALSO ought to give casters some ways to deny martials attempts to disrupt),
Most importantly, I also think at the end of the day, the most important thing in all of this is not necessarily balance - it's the "fun" factor. Since that's subjective, this is a discussion that needs to be had at each gaming table so the group can be given a chance to decide what they want to do with this game balance question (again, I find this much like Counterspell, which is the caster vs. caster equivalent to this dilemma). Do they want to add tactical wrinkles allowing spells to potentially disrupted (including Counterspell, but also including whacking a caster to try to spoil their spell)? If so, under what conditions should this occur, and what are the rules for doing so (and how can casters maneuver to try to avoid this)?
Full disclosure: I make the conscious decision to limit Short and Long Rests when I DM so my campaigns generally function more like balance Premise 2 and thus the martial/caster disparity issue is less pronounced - meaning that while I feel some way for martials to counter casters is necessary in most campaigns, I take my players' preferences into account and make the conscious decision to adjust my own campaign in other ways so that it is
not needed for things to feel balanced; if my players chose to implement "interruptable casting" in Session 0, they'd probably see more opportunities for Short/Long rests as I wouldn't have to resort to other ways for balancing casters and martials... but since they usually choose not to do so, I deliberately adjust my campaigns' Long and Short Rests to make sure the martials don't feel like the casters always do everything.
I guess that's just another way of saying, "there ARE other dials you can twist to fix the martial/caster problem if you don't like letting spells be interrupted... so allowing interruption of spells is NOT a 'mandatory' fix."
MY PERSONAL PREFERENCES (IF ANYONE CARES)
The below is my personal preference, though it should be noted none of the games I have run recently have wanted to use these rules (as I noted before, in every case they have opted for "I don't want my stuff to be interrupted so I'm willing to give up my chance of interrupting the BBEG" - in other words, rules as written plus a ban of Counterspell).
Note these are NOT points I want to argue about - they are my OPINIONS on how I like to set things up at my table and thus unless you happen to play at my table, do NOT apply to you at all (though you are welcome to steal them, tweak them, ignore them, whatever).
1. A caster that takes damage during their turn (either from an effect that deals damage at the start of their turn or from someone else using a reaction to deal damage to them) that attempts to cast a spell on that turn must make a Concentration Check with a DC equal to the damage dealt or the spell is interrupted and the spell slot lost. (This check is made
after they declare their action to attempt the spell; no checking first to see if you can cast, then choosing to do something else if you fail.)
Note this rule also makes areas of effect that deal damage on the start of a character's turn (usually spells) a little more potent.
2. If a caster receives damage from multiple sources during their turn, they check separately against each amount of damage, not once against the total damage; if any of these checks fails, the spell is interrupted and the spell slot lost. (Others may wish to use a single check against the total damage)
3. The act of casting a spell does NOT generally provoke an Attack of Opportunity.
4. Any character may ready an action (weapon attack or spell) that allows them to react to a designated target "starting to cast a spell" in an attempt to interrupt the spell (note this is NOT an AoO; it is a reaction triggering a previously-declared "Readied Action").
5. Martials may spend a Feat (Mage Slayer?) to gain a reaction that is triggered by a spellcaster within their melee reach starting to cast a spell with somatic or material components (not "verbal only" spells); this reaction lets them attack the caster (and if they hit, the caster must make a Concentration check or lose the spell) - this is the exception to #3 above.
6. For using Counterspell, I prefer the following tweaks to the RAW:
- When you attempt to cast Counterspell, you make a Knowledge: Arcana check (a Knowledge: Religion check may be substituted for spell cast by a Divine Caster) with a DC of 10 plus the spell's level to see if the character recognizes the spell. If the Counterspeller succeeds on the Knowledge check, they are told what spell being is cast (though they are NOT told whether or not the spell is being upcast).
- OPTIONAL: The DM makes the Knowledge roll in secret; on a Critical Failure the player is told the wrong spell; on a Failure the player is told they do not recognize the spell, on a Success the player is told the spell, and on a Critical Success the player is told both the spell and the level of the spell slot used (i.e., revealing if upcast).
- If the Counterspeller has the ability to cast the spell themselves (e.g., a Wizard with the spell in their spellbook qualifies whether or not he actually has that spell currently prepared, but a Wizard that does NOT have the spell in the spellbook does NOT even if he can cast spells of the appropriate level) OR if the Counterspeller has seen the target cast this spell before, the Knowledge check is made with Advantage.
- Following the Knowledge check, the Counterspeller declares the spell they are attempting to counter and the level of spell slot they are using to cast the Counterspell (i.e., at this point they declare whether or not they are upcasting Counterspell).
- If they do not correctly name the spell the caster is attempting to cast, the Counterspell automatically fails; if they do name the spell, resolve the Counterspell attempt as written (this does allow PCs to "get lucky" and "guess the spell" even if the Knowledge: Arcana check fails - particularly if going against an opponent known for a certain spell; it also provides a nice incentive for characters to research/develop their own custom spells, thus having the side effect of giving a mechanical reason in-world for wizards to be searching up new knowledge).
- OPTIONAL: A character who knows Counterspell and uses prepared spell slots may substitute an appropriate "offsetting" spell in lieu of Counterspell (DM's discretion; for example, a spell that deals cold damage could be used to counter a spell that deals fire damage, a spell that deals radiant damage might be used to counter a "darkness" or "blindness" effect, and a spell that induces magical silence might be able to counter a spell that deals thunder damage).
