So i will ask... where does it say a reaction can? "If the reaction interrupts another creature's turn, that creature can continue its turn right after the reaction." So we know for sure it can interrupt a TURN, be we do not know if it can interrupt an action, right? I think something important like that would be stated there, if it could apply to all reactions!
Let's look at the ONLY 2 examples of reactions that DO interrupt something...
Counterspell and AoO.
AoO : "The attack interrupts the provoking creature's movement, occurring right before the creature leaves your reach."
Counterspell : " You attempt to interrupt a creature in the process of casting a spell."
Good! 2 Cases in that can interrupt! Let's look at another! I'll let you guess which one it is...
"When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature."
And suddenly there's no interruption going on, except for the turn.
So, BY FACTS, unless there's a specific description the rule holds... You cannot interrupt anything unless a specific description states so. Are we clear now?
Good, because the other point is that the Ready action let's you fire your prepared Action (in combat) or move for 30' iirc using your reaction. What makes use of a reaction able to interrupt anything? The fact that you use a reaction to interrupt a TURN. And that's it. That said, you are using a reaction, but what you are taking is the Action you prepared.
That's a lot of words which only supports things that we agree on (cannot interrupt trigger) which you
then use as 'evidence' of something those words do
not show (cannot interrupt
actions).
Cause
must come
before effect. Therefore, 'trigger'
must occur
before 'reaction'. So far so good.
But the 'trigger' is 'VSM components',
not 'spell effect'.
By having the spellcasting process (the VSM components) as our trigger, our 'reaction' to that trigger can happen
after that trigger (in accordance with the above), and that reaction
still occurs
before the subsequent spell effect.
No. We know the components are part of the spellcasting process. We do not know if it's in the beginning, in the end, or anywhere in between. WE HAVE NO CLARITY. We just know that we are prevented from taking the Action if we miss some of them or there are condition that prevents their appearance. Your "before" and "after" are absolutelty arbitrary, and a RULING, not a RULE.
Balderdash! Just as before, 'cause' must come before 'effect'. If the VSM components are not completed successfully for whatever reason, then they have not been provided, therefore the spell fails. The actual quote is, "If you can't provide one or more of a spell's components, you are unable to cast the spell".
This does not mean that if you are able to provide them, you don't need to bother actually providing them! You have to actually provide them!
The verbal component is the chanting of mystic words. "The words themselves aren't the source of the spell's power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion."
Lets imagine that the mystic words are, "My country, 'tis of thee!" Not only must these words be spoken aloud, they must also have a precise pitch and resonance. Therefore, until you have uttered the final word 'thee', you have not yet provided the verbal component. Therefore, if you only get as far as, "My count..." before being hit with a readied action, at this point you have not provided the verbal component so the spell has not yet been cast.
The somatic component is a specific combination of gestures. "Spellcasting might include a forceful gesticulation or an intricate set of gestures. If a spell requires a somatic component, the caster must have the free use of at least one hand to perform these gestures."
Therefore, until the final gesture has been made, the somatic component has not yet been provided and the spell has not yet been cast. Therefore, if the caster is only halfway through the intricate set of gestures when his hands are amputated, he cannot complete (provide) the somatic component, and the spell will not be cast!
So yes, the VSM components must not only begin, they must be completed
before the spell even begins! It is literally
impossible for the spell to begin before the components are complete, therefore the two things (casting and effect)
cannot overlap. First 'casting',
then 'effect'.
Which means that if you choose 'casting' as your trigger, this trigger does not
need to be 'interrupted' in order for your reaction to occur
before the spell duration begins.
Furthermore, as each spell has a unique set of words and a unique set of gestures, they are identifiable,
perceivable as belonging to a particular spell. This allows an observer to tell what spell is being cast while these components are being performed but before they are completed, although the observer might fail to recognise it in time (before the effect begins), which is where an Arcana check comes in.
These components have a beginning, middle, and end. When someone
begins to cast a spell, this is perceivable even if you do not know which particular spell it would be. So yes you can have 'begins casting' as your trigger, and when he has 'begun casting' then the trigger has happened. Your reaction to that does not interrupt the
beginning of the casting so it is legal. And since the 'beginning' has already happened, the 'beginning' is already complete. The reaction can then occur
before the 'completion' of the casting, and therefore the start of the spell effect, without interrupting the trigger in any way.
Counterspell allows you to interrupt the trigger
No it bloody well doesn't!
There is the 'casting' of a spell, which is the performing of the VSM components, and there is the 'effect' of the spell. 'Casting Time' refers to the VSM components; 'Duration' refers to the spell effect.
The Casting Time occurs
before the Duration even begins, as proved above. During this Casting Time, the spell hasn't even started yet! It is a mundane process, unaffected by what the resulting spell might do.
For example, if the spell effect turns you invisible, you are not yet invisible as you are chanting the magic words and performing the mystic gestures required to cast it! The invisibility, the spell effect, does not affect the casting process of that spell.
Casting Time might be one or more Actions, rounds, minutes or hours. It might be a bonus action. It might be a reaction, but without exception every spell whose casting time is a reaction also tells you what the trigger for that reaction is. We know that this Casting Time must occur
before the spell even begins, therefore that not-yet-in-existence spell can have no influence whatsoever on the trigger that occurs to cast it.
Therefore, any trigger for any spell with a casting time of 1 reaction
must be a valid trigger with or without the existence of any spell, even its future self.
The trigger for
counterspell is, "when you see a creature within 60 feet of you casting a spell". Therefore, 'seeing a creature casting a spell' is a valid trigger
no matter what it is a trigger for! The fact that it may be a trigger for
counterspell is neither here nor there; the trigger is not part of the magic of the spell!
The reason that
counterspell works at all is that the trigger (spellcasting being perceived) occurs
before the spell duration even begins. The trigger (spellcasting) is not itself interrupted. The sequence of events is: trigger (spellcasting) -> reaction (
counterspell) -> spell effect (if it still can).
So we know, for a cast-iron fact, that the sequence: trigger (spellcasting) -> reaction (Readied action) -> spell effect (if it still can) is
also a valid sequence.
We also have Counterspell, that we know only asks for a spell beign cast, that is not a readied action, so does not follow the same rules, that is also an "exception" to the reaction rules, stating clearly that it can interrupt an action while reactions are only stated to be able to interrupt turns. Apples to Oranges.
Counterspell is not an exception to the rules for either valid triggers or for the timing of the reaction.
There is nothing in the Ready rules which states that a readied action cannot interrupt an 'action'; you made that up! What it says is that it cannot interrupt its 'trigger', and you already agree that the trigger need not be an actual Action In Combat. We merely choose a perceivable trigger that occurs
before the part we don't want to happen even begins. So we don't choose 'when a spell begins' as our trigger if we don't want the spell to begin! We choose 'when I see a creature casting a spell' as the trigger, and we
know that this is a valid trigger, that it refers to the VSM casting process not the spell effect, and that the reaction occurs
after the trigger (spellcasting) but
before the spell effect begins; and we know this is how it works because this is how
counterspell is able to function; its trigger conditions are not altered by the magic of
counterspell.
