Can I cast reaction spells like Shield or Counterspell when I'm in the middle of casting a spell with a long casting time and don't stop casting it?

Question

Can I use reaction spells like shield or counterspell when I'm in the middle of casting a spell with a long casting time and don't stop casting it?

The Sage Advice Compendium says:

Can you cast a reaction spell on your turn? You sure can! Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius also has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.

Counterspell has only Somatic components. What about casting shield? That has Verbal and Somatic components. Do you have to stop casting a spell with a longer casting time in order to cast a reaction spell that has a Verbal component?

Answer 1

Casting a Reaction spell does not interrupt a spell with a long cast time, regardless of spell components. (As long as it isn't a Concentration Spell)

The rules for spells with a long casting time are...

Certain spells (including spells cast as rituals) require more time to cast: minutes or even hours. When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so. If your concentration is broken, the spell fails, but you don't expend a spell slot.

Source

So, you can see that there are two things that can cause a long-casting spell to fail

1. Using your Action for something other than casting the spell
2. Losing Concentration

So, as long as you keep using your Action to cast the spell, and do not lose Concentration--you can cast whatever Reaction spells you'd like. There are no secret rules.

Do note: If your DM wants to change this because they don't think it makes sense, that is their privilege as DM.

Answer 2

Yes, you can use your reaction to cast another spell so long as it does not require concentration

When you cast a spell with a longer casting time than one action, you have to concentrate on the spellcasting process, and use your action every round to continue casting the spell:

When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so. If your concentration is broken, the spell fails...

So your casting is interrupted if, for whatever reason, you don't or can't use your action on your turn to continue casting, or your concentration is broken. Assuming you keep using your action, you only have to worry about your concentration being broken, which can be caused by a handful of effects but most importantly by:

• Casting another spell that requires concentration. You lose concentration on a spell if you cast another spell that requires concentration. You can't concentrate on two spells at once.

Notably, casting a different spell is not enough to break your concentration; it must be a spell that requires concentration itself.

This means that you can cast other spells while you're casting a spell with a long casting time, so long as you don't have to use your action to do so, and they're not concentration spells.