If you start casting a new concentration spell while concentrating on Haste, does the wave of lethargy interrupt the casting of the new spell?

Question

Say a spellcaster has previously cast haste on themselves and is currently concentrating on it. However, they see a situation unfolding that desperately requires them to concentrate on another spell (e.g. hypnotic pattern).

They move and take their action to start casting the new spell. However, concentration on a spell begins once you start casting it, which would immediately end haste, causing "a wave of lethargy" that makes the spellcaster unable to move or take actions until after their next turn.

The spellcaster has already finished moving, and has already chosen an action to take (casting a new concentration spell), and has actually started taking that action.

Does haste's lethargy effect interrupt the casting of the new spell, or does the new spell go off and then the caster becomes lethargic immediately afterwards?

Answer 1

The spell goes off

Unless it has a casting time longer than 1 (bonus) action

The caster has already “taken” the Cast a Spell action even if the action is not yet completed. They can’t “take” another one until after their next turn.

Answer 2

The spellcasting is disrupted

In general terms, you have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.

So the question is this: if you are in a condition that you could not do an action, can you (just) finish/complete it? It would make no sense to answer yes. Consider the following:

Lets say you start casting and are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious from the damage. It would be unreasonable to say that you then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.

While moving is not an Action, we can also look at the way the Sentinel feat works. Your attack of opportunity is triggered by someone trying/starting to move away from you. If you hit, their speed becomes 0. Should they then continue to follow their stated course? Of course not, they stop without actually moving any. The whole point of the feat is that you can pin enemies down, and it does so by afflicting the creature with a condition between it starting to do something and finishing it.

If you acquire a status that prevents you from taking an action while taking it, you fail to complete it.

The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)

Answer 3

The second spell is cast successfully and the caster suffers lethargy until the end of their next turn

The purpose of Haste's Lethargy is to balance its benefit with the offsetting risk of essentially losing a turn.

If the lethargy were to take effect before the new spell is cast, the caster would be deprived of not one but two turns, which is an outsize cost.

As Dale points out in his answer, it is the casting of the second spell that causes Haste to end. Haste can't prevent something from happening that is already happening.

Haste ends after the second spell is cast and it is only then that the lethargy sets in. This means that the caster's next turn is effectively lost as normal, though they benefit from the fact that they are able to concentrate on the new spell for the duration of the lethargy.

Answer 4

You can cast the spell

Xanathar's Guide to Everything (p. 77) offers the following rule regarding simultaneous effects:

If two or more things happen at the same time on a character or monster's turn, the person at the game table – whether player or GM – who controls that creature decides the order in which those things happen.

The wave of lethargy occurs at the same time as you are casting the second spell. As such, since it is happening on your own turn, you can choose to have the second spell be cast first, and then have the lethargy take effect afterward.

Answer 5

It would interrupt the spellcasting.

The PHB (p. 203) and basic rules explain how concentration works:

[...] The following factors can break concentration:

• 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.

The introduction to Xanathar's Guide to Everything (p. 5), under "Ten Rules to Remember", reiterates/clarifies this core rule on concentration (emphasis mine):

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly.

Here's how it would go:

Start casting spell > Haste ends > Lethargy happens > Spell is interrupted

If the rulebook stated that the new spell is interrupted when the spell is cast (i.e. when the casting is finished), it would be possible to cast the spell before the lethargy occurred; but as it uses the words 'start' and 'instantly', it quite clearly indicates that it happens as the caster begins casting the spell. You immediately enter a state where you cannot take actions, and as you are still starting to take an action (it is instant, and I am not aware of a ruling indicating that the casting of spells occurs instantaneously), you are unable to take that action, and must drop the spell.

I don't see how RAW could be any clearer about this without directly addressing this particular interaction, something 5e avoids when at all possible.

Answer 6

Logically, the concentrator must be able to cast another concentration spell.

Otherwise, we're caught in a loop of cast → lethargy → didn't cast → no lethargy, etc.

If the same character were to move and take an action, then drop concentration, the lethargy doesn't undo actions they started/started. I'll even give a concrete example:

Let us use a sufficiently high level Eldritch Knight to caste Haste (and also have multiple attacks), with Haste already active. They move up to an enemy and take the Attack Action. Through some means, after the first attack, they drop concentration (let's say Hellish Rebuke and they fail their save). What happens to the Fighter's remaining available attacks? They're not taking an additional action, but simply finishing an action that was already begun.

What if you wanted to Ready a spell (any spell)? Would you begin to take the Ready action and prepare the spell, but the lethargy undoes your Ready and the next spell? Or perhaps, you CAN Ready spells, which would just bypass this entire problem.

There are too many logical loopholes, pit-falls, and catch-22s for the answer to be anything other than "you may finish casting after you start, even if it breaks concentration".

Answer 7

The spell should be cast, with lethargy happening but not interrupting it.

The lethargy is explicitly defined as not allowing the player to move or take actions until after their next turn. Actions are explicitly defined in the PHB to the extent that we know an attack action and an attack are different things (e.g. Multiattack modifies the amount of attacks in a single attack action). So, in this example, a player takes the Casting a Spell action, which, as a result instantly breaks concentration on Haste and begins the Lethargy. The actual casting of the spell at this point is not an action, and therefore is not considered as the player taking an action and violating the rules of the lethargy. Therefore the spell is cast as normal.

In terms of Sentinel and Counterspell, those are all Reactions changing the situation in some way; Sentinel altering the movement speed of the character result in the movement being an invalid distance they can travel and Counterspell simply negating the effect of the spell.