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NOTE: this does not refer to the case of readying a Dispel magic to stop a spell from being cast as in this question, but rather using Dispel magic to stop a readied spell from being cast.

While writing this answer, I ran into an interesting corner case regarding Dispel magic. Given that a readied spell is cast as normal on your turn, but its energy is held (requiring your concentration) until the trigger occurs, after which is released with your reaction, would this classify it as an ongoing magical effect (explicitly called out in the Dispel magic description)?

The rules on spell duration (PHB p. 203) explicitly split spells into instantaneous and concentration, mentioning that (empasis mine):

Concentration (PHB p. 203)

Some spells require you to maintain concentration in order to keep their magic active.

Official rules answers for March 2016

Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend. Here’s why: the effects of an instantaneous spell are brought into being by magic, but the effects aren’t sustained by magic (see PH, 203).

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  • \$\begingroup\$ Related, as the two situations complement each other: Can a readied spell be Counterspelled after it is cast, but before the trigger occurs? \$\endgroup\$ – Olorin May 30 '16 at 22:23
  • \$\begingroup\$ I'm not seeing a difference. \$\endgroup\$ – Christopher May 30 '16 at 23:39
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    \$\begingroup\$ @Christopher if nothing else, Dispel Magic having an Action casting time and Counterspell having a Reaction casting time make the two cases distinct to me. \$\endgroup\$ – nitsua60 May 31 '16 at 0:44
  • \$\begingroup\$ Oh, I see. Dispel magic vs. counterspell. Two different spells. Gotcha! \$\endgroup\$ – Christopher May 31 '16 at 0:59
  • \$\begingroup\$ @Christopher the two questions are indeed tightly coupled, as an yes answer to this one should imply a no answer to the other one, and a yes answer to the other one should imply a no answer to this one. However, they are two separate cases, each with its implications, and discussing both in the same question would be ill fitted. \$\endgroup\$ – Olorin May 31 '16 at 6:38
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Yes, because the spell is no longer instantaneous

Per PH 193, "When you ready a spell, you cast it as normal…and holding onto the spell's magic requires concentration…[i]f your concentration is broken, the spell dissipates without taking effect."

Since the spell is now a concentration spell, it should be susceptible to dispel magic like any other concentration spell—including being dispelled before the trigger.

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    \$\begingroup\$ I'm not sure about this. "Instantaneous" refers to the duration of the spell's effects. The ready action requires you to concentrate, but doesn't change the spell's duration to "concentration". \$\endgroup\$ – Dan Henderson May 31 '16 at 21:19
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    \$\begingroup\$ While the duration of the effect is instantaneous, there is still a duration between when the spell is cast and when its effect occurs. \$\endgroup\$ – Christopher May 31 '16 at 21:58
  • \$\begingroup\$ I think the best way of stopping such a spell from being cast is actually neither dispel magic nor counterspell, but instead the spell anti-magic field. drop that on the caster, and he can't even trigger the spell, because the magic gets negated by the field instantaneously. \$\endgroup\$ – khaoliang Jan 8 '18 at 12:21
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No.

Dispel Magic is not intended to be used until the spell has actually taken effect. Consider the Sage Advice Compendium regarding the timing of Dispel Magic. It's pretty clear that the RAI are that Dispel Magic cannot be used until the spell has actually resolved:

Can you ready dispel magic to stop another spell from taking effect? The easiest way to stop a spell is to cast counterspell on its caster while it’s being cast. If successful, counterspell interrupts the other spell’s casting, and that spell fails to take effect. Counterspell works against any spell, regardless of a spell’s casting time or duration. With the Ready action, dispel magic can be cast in response to another spell being cast, yet dispel magic can’t substitute for counterspell. The main reason is that dispel magic removes a spell that is already on a target, whether that target is a creature, an object, or some other phenomenon. Dispel magic can’t pre-dispel something. If a spell isn’t already present on a target, dispel magic does nothing to that target. The best that a readied dispel magic can do is dispel a spell immediately after it’s been cast to prevent it from having any e ect after the action used to cast it. For example, on your turn you could say something like this: “I ready dispel magic, and if the high priest casts a spell on anyone, I cast dispel magic on the target if the spell takes hold.” If the high priest then cast hold person on your companion who fails the save against it, you could unleash your readied dispel magic and end hold person.

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  • \$\begingroup\$ According to your own rules answers quote: "The easiest way to stop a spell is to cast counterspell on its caster while it’s being cast"; according to the ready action: "When you ready a spell, you cast it as normal". Once your turn is over, as the spell has a casting time of 1 action, it has finished casting as normal, and you're simply holding on to it's effect through concentration, so you can't counterspell it. Do you mean to imply there is a period of time during the readied spell's lifetime that it can't be counterspelled nor dispelled? \$\endgroup\$ – Olorin Jun 3 '16 at 20:21
  • \$\begingroup\$ No, I don't think this is the case. I think the rules clarification makes it clear that the spell, though cast, is not considered "resolved" or "finished" until the effect takes place. For instance, if something causes the caster to lose concentration, the spell fizzles. It doesn't really finish, though the slot is still consumed. It took you an Action to set up the Readied spell (reflected in the casting time). Until it takes effect, it hasn't really been cast, and can be counterspelled. I think this is the only interpretation that makes sense given the rules errata quoted. \$\endgroup\$ – keithcurtis Jun 3 '16 at 21:57
  • \$\begingroup\$ A metaphor would be: It takes you an Action to draw an arrow, pull back the string, and aim. You can Ready to let the arrow fly. You've done all the time-consuming work and have an arrow shot that only takes a reaction to release. Counterspell is the equivalent of someone snatching the arrow out of the bow (and breaking it). Dispel Magic is the equivalent of pulling it out of the target (and maybe casting a heal spell). \$\endgroup\$ – keithcurtis Jun 3 '16 at 22:01
  • \$\begingroup\$ "The spell, though cast, is not considered resolved or finished until the effect takes place" - I fully agree with you. However, as you say, the spell has been cast, and the spell slot consumed. Counterspell specifically says that "You attempt to interrupt a creature in the process of casting a spell" which is not the case here any longer. As an example, just like any summoning spell (which can be dispelled once cast), you finish casting the spell then concentrate on its effect. In the case of readied spells, that effect exists (on the caster, held by him), but is not resolved/finished yet \$\endgroup\$ – Olorin Jun 4 '16 at 8:25
  • \$\begingroup\$ By this reasoning, dispel magic cannot work either, according to the quote above, since it cannot be used on the spell, but only on the target affected by the spell. Since there is no spell that does this—using a very strict interpretation of RAW—I think the only sensible alternative is to use RAI as presented above to allow Counterspell to stop a readied spell. The only alternative would be to create a third type of abjuration spell-stopper, and I don't think anyone wants that. \$\endgroup\$ – keithcurtis Jun 4 '16 at 15:52

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