Casting a spell mid-spellcast

[ATHATH]

Casting a spell with a casting time of longer than 1 action requires your action each turn (and your concentration), but doesn't seem to prohibit you from casting reaction spells or the like.

I have two questions:
1. Does casting a bonus action spell while partway through the casting a 10 minute spell break the casting of the latter, even if it's not the turn you started casting the latter on?
2. Does a 10 minute spell with verbal components prevent you from casting other spells with verbal components while you're casting it? What about somatic components? Material components?

[follacchioso]

You can't cast other spells.

Casting spells with a casting time longer than one action requires you to cast it every turn.

Now, that leaves your bonus action free.
However, if you use your BA to cast a spell, you can't cast anything other than a cantrip:


A spell cast with a bonus Action is especially swift.
You must use a bonus Action on Your Turn to cast the spell, provided that you haven't already taken a bonus Action this turn.
You can't cast another spell during the same turn, except for a cantrip with a Casting Time of 1 Action.

In theory, if there were any cantrips with a casting time longer than one minute, you could cast cast BA cantrips as well. But there are no such cantripts in the books.

[PhoenixPhyre]

In theory, if there were any cantrips with a casting time longer than one minute, you could cast cast BA cantrips as well. But there are no such cantripts in the books.

I'm not sure. From what you quoted, I gather
IF (did cast spell as bonus action)
THEN (other spell, if any, must be a 1 action cantrip).

So a cantrip with a longer cast time (e.g. mending) does exclude a BA cantrip, by necessity. Because it's not a cantrip with a 1 action cast time (definitionally, it has a longer cast time).

[follacchioso]

That's right... So no cantrips, neither :-)

[apepi]

I think you should be able to counterspell though? Just cost a reaction.

[RSP]

Reactions can be cast while casting another spell.

RAW, watch your hand use, though:

“If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures.”

“A spellcaster must have a hand free to access these components—or to hold a spellcasting focus—but it can be the same hand that he or she uses to perform somatic components.”

So if casting a spell with S and/or M components, and during that casting, you need to cast Counterspell, I think you’d need both hands free to do so (one holding the M and doing the Action spell S; the other doing the S for Counterspell).

[claypigeons]

I think you should be able to counterspell though? Just cost a reaction.

You can counterspell the counterspell used to counterspell the spell you were already casting.

You: cast fireball
Enemy: counterspell fireball
You: counterspell the counterspell
You: finish casting fireball

Perfectly RAW

[Selion]

Casting a spell with a casting time of longer than 1 action requires your action each turn (and your concentration), but doesn't seem to prohibit you from casting reaction spells or the like.

I have two questions:
1. Does casting a bonus action spell while partway through the casting a 10 minute spell break the casting of the latter, even if it's not the turn you started casting the latter on?
2. Does a 10 minute spell with verbal components prevent you from casting other spells with verbal components while you're casting it? What about somatic components? Material components?

Two possible interpretations:
"Subsequent actions to maintain concentration in spell casting are not themselves spell casting, thus you are allowed to cast bonus action spells"
"subsequent actions are spell carting, and the case is already covered by the bonus action spell rule"

A better question would be: can I use action surge to cast a different spell while I'm casting a spell maintaining concentration? RAW it appears it's possible.

BTW all these scenarios are extremely rare, unless your DM set up a encounter in the last rounds of a ritual

[tiornys]

A better question would be: can I use action surge to cast a different spell while I'm casting a spell maintaining concentration? RAW it appears it's possible.
I agree, this should work. You've spent your action casting the long-cast spell, you've maintained concentration, you just happen to have another action with which to do something else. That something else includes casting a different (non-concentration) spell. Arguably you don't even need a second hand free as you've already done any S/M component manipulation needed for the long-cast spell with your previous action.

[ATHATH]

Two possible interpretations:
"Subsequent actions to maintain concentration in spell casting are not themselves spell casting, thus you are allowed to cast bonus action spells"
"subsequent actions are spell carting, and the case is already covered by the bonus action spell rule"

A better question would be: can I use action surge to cast a different spell while I'm casting a spell maintaining concentration? RAW it appears it's possible.

BTW all these scenarios are extremely rare, unless your DM set up a encounter in the last rounds of a ritual
I was more thinking about Reaction spells, like Silvery Barbs or Feather Fall.