Can I choose to fail the ability check for Dispel Magic?

Question

Suppose I want to cast Dispel Magic (at 3rd level) on a target that has one beneficial spell on it and one harmful spell on it. I only want to dispel the harmful spell and leave the beneficial spell intact. Clearly, if the beneficial spell is 3rd level or lower, I am out of luck, since it is dispelled automatically. However, if the beneficial spell is of a higher level, it is only dispelled with a successful spellcasting ability check. Can I choose to fail this check on purpose to ensure that the beneficial spell is not dispelled while still attempting to dispel the harmful spell?

Another less likely context where this might come up would be if I was being forced under duress to dispel a spell, and I wanted to pretend to try and fail. This seems somewhat similar to another question about destroying a spell scroll by intentionally misreading it, but it also seems different enough that I can't quite tell how to apply that answer here.

Answer 1

You can't opt to fail a dice roll

Check this related answer.

You can, though, describe your action as "trying to fail something in a specific way", and then the DM might want you to roll to see if you "succeed at failing".

Remember, D&D5e has the following pattern:

1. The DM describes the environment.
2. The player describes what they want to do.
3. The DM narrates the results of the adventurers' actions.

So, instead of the PC's action being "I cast dispel magic" and opting to fail it, their action can be "I try to fail-cast dispel magic". It's up to the DM how to resolve the results of this action. It is clearly a different action than actually trying to cast the spell.

First Scenario

For your first scenario, you could actually just target the harmful magical effect that you want to remove, the way I read Dispel Magic. If you expand it to two harmful effects and one good, you are out of luck, though. The best you can do is to describe your action as "I try to dispel only the harmful effects" - it's up to the DM if you can do it. By RAW, spells do only what they say and you can't force-fail a check, so you can only dispel (or try to) everything on the creature using the action "I cast dispel magic on this creature".

Remember the actions are something the characters do. "Failing a check" is not something a character does, so "I cast the spell and fail the check on the good effect" is not a description of an action.

Second Scenario

For the second scenario, remember that if a spell has worked or not is only known through its effects. The way I understand the rulings, there is no difference between you randomly doing some gestures and casting a spell with Somatic components, except if the spell works, which will be noticed by the effects. This question has some answers that deal with this "perception of failing a spell".

If you want to pretend to cast a spell, this is more like a Performance Check (doing something close to the Verbal and Somatic components, without actually casting the spell) contested by an Arcana or Insight check than actually casting the spell and failing. This is certainly something you can do, but it might not be the best option if you are a Wizard with low Cha and without Performance proficiency.

Instead of pretending, you could describe it on the lines of "I slightly change the spell in a way I know it won't work", which might be an Arcana check.

The point is: the rolls are supposed to check if you can succeed at your action. "Failing deliberately" is an action, and you might have to succeed at the dice roll to do it correctly. The DM can decide it is trivial to "succeed at failing" and not require the roll, though.

Answer 2

Jeremy Crawford tweeting a response a while back in regards to optionally failing a savings throw stating.

No rule lets you opt to fail a save. As DM, I might allow it, assuming you aren't incapacitated or dominated.

Using that as precedent, there are no rules stating you can choose to fail an ability check, so I would say the RAW ruling is "no".

However, like Jeremy I would personally give the player some leash on generic ability checks as long as their approach isn't unreasonable.

However, When it comes to dispel magic. I would probably stick with the "no". This is because I have a hard time reconciling the idea of half-heartedly willing magic into existence to selectively cancel magical effects.