So the RAW has two somewhat competing notes:
"When you ready a spell, you cast it as normal but hold its energy, which you release with your
Reaction when the trigger occurs."
You do cast the spell as a part of the readied action, that much is crystal clear. You begin that casting at that point, and not later on when you release the energy. However, now let's look at the text for counterspell:
"Reaction: A creature in the process of casting a spell"
So the text does not say "a creature that begins to cast a spell", it effectively says that at any point of the casting, you could activate the reaction.
And so the question then becomes, "Is the release of spell energy in any way part of the casting of a spell?". Your interpretation of that informs how you would rule as a DM.
I would argue that the release of the spell energy IS a part of the casting the spell. If you don't agree with that, there are a few consequences that you may not like:
- The spell's target must be in range at the time you ready a spell. Since a spell's target is required to be in the range at the time of casting, if you rule that casting is "concluded" before the release of energy step, then in theory the only way the spell can function is if your ultimate target is already in range, and if its a target, must be in line of sight and effect.
- The spell's duration begins to tick before the spell's effect kicks in. Duration also begins after the casting of a spell, and so any rounds spent holding a spell for a readied action count against the duration of the spell.
However, ruling that casting is "ongoing" while your holding the energy also comes with its own baggage:
- Components: You could interpret that since components are required during a spell's casting....they must be maintained during the "holding of energy". Example if you move into a silence effect and are holding a verbal spell, that spell is no longer a viable cast and you would not be able to cast it.
