A spellcaster currently concentrating on haste on themselves sees a situation unfolding that desperately requires them to concentrate on another spell, for example, 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 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?


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.

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    \$\begingroup\$ Is the following effectively your argument: haste prevents you from taking actions, but you've already taken an action, so haste has nothing to stop? \$\endgroup\$ – Medix2 Feb 27 at 20:07
  • \$\begingroup\$ @Medix2 effectively \$\endgroup\$ – Dale M Feb 27 at 20:08
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    \$\begingroup\$ Do you have any rules reference to support this? The idea that "choosing an action for the turn" and "executing the chosen action" are separate things, and specifically that only the former is what the rules mean by "taking an action" is intuitive, but I'm not sure if it's actually correct. (Note that I typically think of actions in this way, so I totally understand this interpretation -- I just don't know if it's supported in the rules text). \$\endgroup\$ – Marq Feb 28 at 11:21
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    \$\begingroup\$ @Marq it's not choosing an action that causes haste to end, but rather casting the spell which ends it. You are already casting when lethargy sets in. \$\endgroup\$ – Randomorph Feb 28 at 14:34

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.)

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Rubiksmoose Feb 28 at 20:31

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.

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    \$\begingroup\$ XGE has actually clarified the timing: "As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly." (XGE 5), and with that, I'd argue your second paragraph would make a better case for an answer in the opposite direction if you can back it up more. \$\endgroup\$ – Randomorph Feb 27 at 20:08
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    \$\begingroup\$ Also keep in mind, the lethargy will still cost you an entire turn. The wording of lethargy is that it lasts until the end of your next turn, making lethargy even worse when triggered on your current turn. \$\endgroup\$ – Randomorph Feb 27 at 20:09
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    \$\begingroup\$ @Rand The more I think about it, my answer originally hinged on the argument that ignoring the lethargy during the casting turn was overpowered and therefore couldn't be the intent. But the fact is that the caster would lose two turns, not one, and that is unbalanced in the other direction. I think it takes fewer mental gymnastics with this interpretation and so Dale's answer is correct. I'm changing mine to match. \$\endgroup\$ – Rykara Feb 27 at 20:55

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".

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    \$\begingroup\$ Your first non-bold line is not corect. You drop concentration when you start to cast another concentration spell, not when you finish it. The previous spell would drop even if eg. you are counterspelled. \$\endgroup\$ – Szega Feb 27 at 22:14
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    \$\begingroup\$ @Szega yes, but if the lethargy somehow retroactively removes the action that you started, then you never stopped concentrating, because you didn't have an action to use to cast the other spell, so you don't have lethargy, so...... etc \$\endgroup\$ – goodguy5 Feb 27 at 23:28
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    \$\begingroup\$ Removing and disrupting are different things. \$\endgroup\$ – Szega Feb 27 at 23:34
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    \$\begingroup\$ You are welcome to vote with an answer that agrees with that, but I believe this interpretation to be correct. \$\endgroup\$ – goodguy5 Feb 28 at 0:17
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    \$\begingroup\$ "Start to cast" and "Spell is cast" are two different things, which resolves your loop. You should explain how they are, in fact, the same thing (problematic for counter-spelling) or how "start to cast" doesn't end concentration (but see that one comment above about XGE having explicit rule that it does). \$\endgroup\$ – WakiNadiVellir Mar 2 at 5:56

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