What ways are there to disrupt spellcasting in 5e?

[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