When can you counterspell a spell with long casting time?

Question

The spell counterspell in D&D 5e has a casting time of 1 reaction, "which you take when you see a creature within 60 feet of you casting a spell". It has the basic effect of:

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.

Most spells have a casting time of 1 action or less, in which case the timing your counterspell shouldn't affect anything outside of weird corner cases involving multiple reactions on the same spell. I'm not asking about those corner cases. However, some spells have a casting time longer than a single action, such as alarm with a casting time of 1 minute (10 rounds).

Alarm is a 1st level spell so should be countered by counterspell without any need for a check. However, I'm not certain what time within the 1 minute window you can actually do so. Specifically, all other reactions I can find specify a "trigger" that is valid only for an instant as far as D&D is concerned; there's no noteworthy duration on "hit by an attack" for example. The general description of a reaction in the basic rules state

A reaction is an instant response to a trigger of some kind

While the more specific rules under spellcasting say

cast in response to some event.

And the rules for readied actions, although not applicable here, give a "use it or lose it" precedent

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

All emphasis mine.

As long as the triggering events are "instant" there isn't much room for ambiguity, but the "trigger" specified for counterspell can be interpreted more as a "condition" or "situation" because "seeing", "casting" and "being within 60 ft" are ongoing rather than instantaneous. I can think of a few ways to reconcile this:

• You can only counterspell when the casting starts, which is the most similar to how counterspell interacts with readied spells.
• You can counterspell at any point during any character's turn as long as all the conditions are met, which is the natural reading but (in my opinion) doesn't fit with the concept of reactions.
• You can counterspell only at the end of the casting. This is closest to the "readied action" rules of "after the trigger", and it also causes a fairly natural distinction between getting countered, which consumes the spell slot, and getting their concentration broken which does not.
• You can cast counterspell at the moment you see the caster. Interpreting "when you see" as an instantaneous event happening right when the caster comes into vision removes the ambiguity completely, and seems (to me) to at least be a valid English reading of the phrase if not the most natural one. It has a lot of weird conceptual consequences though because the spell is counterable through its entire casting, but for any individual it only happens once.
• You have the option to cast counterspell each time the caster takes the "cast a spell" action, but not arbitrarily at other times in the round. This resolves the weirdness of "actively" using a reaction at arbitrary times, while still allowing the entire casting time to be available for countering. I like this interpretation the most, but I also think it has the least textual support.

Which of these is correct? Are there any other reactions where their "trigger" can be valid for more than an instant, which might be used as precedent? When am I able to counterspell a spell that takes more than 1 turn to cast?

Answer 1

You may interrupt the casting of a spell as soon as you see it is being cast, by using your reaction.

The casting time of Counterspell is

1 reaction, which you take when you see a creature within 60 feet of you casting a spell.

If a spell has a casting time of 1 minute, they are casting for that 1 minute and if you see them you may counterspell. If the casting time is 1 action you must immediately spend your reaction to counterspell or miss your window.

You attempt to interrupt a creature in the process of casting a spell.

The key here is interrupt. You may do so at any point during the casting.

Answer 2

Anytime before the cast finishes

The description of Counterspell already gives the needed timing

which you take when you see a creature within 60 feet of you casting a spell

It is written in continuous tense, so you can counterspell the casting anytime before the cast actually finishes.

You can also foil the spell just before it finishes: when the creature declare his action to complete the spell, specifically because Counterspell is one of reaction that allows you to interrupt the trigger.

Answer 3

Rules as Written: As long as a creature is in the process of casting a spell, you can use counterspell against it.

Counterspell says:

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.

In both sentences, the key phrases are "In the process of casting a spell" and "If the creature is casting a spell"... In both cases, the trigger is during the entire process of the spellcasting. In the event of a long casting spell, this gives you the opportunity (on your turn) to move into position (for either line of sight or range issues) to attempt to make a counterspell.

One is unable to ready Counterspell. It must be used as a reaction to an existing condition. For the same reason you can't substitute a reaction (or bonus action) for an action, you can't Ready a reaction either:

To be readied, a spell must have a casting time of 1 action

Please note that a general reaction can also be used on any entity's turn as long as the conditions are met.

This means, theoretically, that if the big evil caster is trying to cast a ritual 9th-level spell and a big fight breaks out, you could attempt a counter spell once every round, using your one reaction per round against his continual spellcasting, at the start of the character's or monster's turn with the highest initiative. And then still have your full casting action on your turn to try and break their concentration with damage.

If you could only have one opportunity to counterspell it would read something like, "The moment you notice a creature casting a spell" or "At the moment of spell completion, you can attempt to..."

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Note that if you use counterspell during your turn using your reaction for the round, you can't cast a bonus-action spell on that turn. If you cast a spell with a casting time of 1 bonus action, then:

You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

However, if you don't cast a bonus-action spell on a turn, your ability to cast other spells (including reaction spells) is not restricted on that turn. You can cast an action spell and reaction spell (such as counterspell) all on the same turn - see the Sage Advice Compendium:

Can you also 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 has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.

Please note, you cannot use your action to cast a spell that utilizes a reaction - the same way that you cannot use your action to cast a bonus action spell. This is also addressed in the Sage Advice Compendium:

Can a bonus action be used as an action or vice versa? For example, can a bard use a bonus action to grant a Bardic Inspiration die and an action to cast healing word? No. Actions and bonus actions aren't interchangeable. In the example, the bard could use Bardic Inspiration or healing word on a turn, but not both.

Answer 4

I'd say that option B is accurate. The spell says nothing about beginnings or ends. You counter it at the point that you cast counterspell. If that's six turns into that ten turn Alarm casting, or one turn in, or if it's done on turn nine, it's countered at that point.

Also, if you're aware of the creature you 'see' it the entire time, which satisfies even that 'strict' reading you mentioned.