Using a trigger like 'when a caster starts to cast a spell' does a couple of things wrong:
- It requires that the interrupter can perceive such an event. The rules say a spell starts in the mind of a caster, gathering focus and energy and the perceivable events of somatic and verbal components are implemented only at the end of the casting.
- It makes the wrong assumption that the start of casting a 1 action
spell can be distinguished from the rest of the casting. An action is
a discrete thing (in game) and cannot be subdivided any old way you
All of these parsings and semantics seem to be oblivious to the
This game has turn based combat with the discrete unit of
time being 1 action.
Yes, bonus actions may be a little faster and reactions most
often occur during someone else's turn, but they are also discrete
things. In reality, everything would happen all at once
with the discrete quantity of time being measured in Planck units. Reality
is a mess that doesn't lend itself to game-play. In D&D, time is
atomic, otherwise you don't have a game.
A couple of things to keep in mind:
- A caster can be holding their concentration on one spell and,
on their next turn, cast a 1 action spell without breaking
concentration on the first. Losing concentration on the first because
of casting another spell occurs only when casting a spell that
requires concentration, something that is tagged in the spell
That implies that single action spells don't require concentration.
This is buoyed by the very existence of concentration tags.
Casting a Bigby's Hand on one turn and then casting a fireball on the
next does not interrupt concentration on the Bigby's Hand. The Hand can hold someone/thing away from the party until the fireball does its damage. The very next turn, the Hand can go about doing a slam attack (provided that nothing else interrupts concentration on the Hand between turns).
- Claiming that a 1 action spell without the concentration tag
requires concentration seems to be pulled out of thin air and is not in any rule. It also contradicts the RAW for concentration. [PHB pg. 203]
A single action must be considered indivisible, without
distinction between starting to take the action and taking the
The game risks being wrecked if single action spells are
interruptible with readied actions. Allowing the ready action to have
a chance of interrupting spells with casting times of one action completely
castrates spellcasters in the game. For instance, a single Animate Objects spell could deliver 10 readied melee attacks per round with +8 to hit and each hit inducing a DC10 constitution save.
A caster would never get off another spell (besides shield and counterspell). A ninth level wizard who had his Animate Objects ready and got lucky on initiative could beat a 20th level Wizard without losing a single hit point. Even 15 rats have better-than-even chance. So much for heroic play!
P.S. This has been edited to remove Witch Bolt as an example as it was pointed out by another commenter to explicitly forbid its use in this way. The Witch bolt explicitly and specifically says that the spell ends if you use your action to do anything else. Specific over general implies a rule that needed to be overridden. Otherwise why have the explicit exception?