Do you know that Detect Magic is being suppressed if it intersects with Antimagic Field?

Question

If you cast detect magic and it is suppressed by an anti-magic field, do you know that the spell was suppressed or does it give no feedback whatsoever?

The exact scenario that came up was something like this:

The wizard is standing and looking at a door. She casts detect magic to see what she can see. There is a beholder on the other side of the (thin, wooden) door, far enough away that it would not be in the detect magic spell, but that its anti-magic cone would be on the side of the door opposite the wizard. Does her spell detect the anti-magic cone?

Answer 1

What a neat question!

The answer to your stated question is Yes, you would be able to detect it, but not for the reason you might suppose.

The Beholder's cone behaves as the spell antimagic field, with a few exceptions (shape, range, and at-will-ness). Since this cone is an area effect spell, it follows those rules. Specifically, PHB 204 says

A spell's effect extends in a straight line from its point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in Chapter 9.

And it turns out that your simple wooden door does, in fact, provide cover. As such, the Antimagic Cone doesn't penetrate the door, and our daring hero is not affected by the spell.

On the other side of the door, detect magic is a self-targeted spell. Since the wizard is not under any particular effects, such as that field, the spell goes off without a hitch. Note that because it's a self-targeted spell, it isn't bound by the area rules. Its rules are somewhat special.

you can use your action to see a faint aura around any visible creature or object in the area that bears magic

with the exception

The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

This means the wizard can, in fact, sense through our (presumably less than 3 foot thick) wooden door, and could notice the antimagic area on the other side of it.

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However, it didn't seem to be the question you meant to ask. So, let's assume the beholder's side of the door is in total darkness (to prevent the wizard from seeing it), the wizard had opened the door, and the wizard hadn't cast the spell.

This basically just changes the scenario to be the wizard unknowingly casting inside an antimagic field.

According to the description for antimagic field,

Within the sphere, spells can't be cast

That is, if you're already in the field, the spell will simply fail. It's unclear to me from this description what you'd notice as a caster.

Presumably, as the area is "divorced from the magical energy that suffuses the multiverse", and as arcane casters are particularly in tune with that energy, you'd notice something was wrong.

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Let's go back to the original question, and open the door (for some reason).

If you already had detect magic up, you'd have noticed an Abjuration around the area. Then, upon entering, detect magic would be suppressed and you would no longer sense the area as magical (along with anything else magical you could sense).

Interestingly, I wasn't able to find anything in the PHB or DMG about what you do, or don't, sense when a spell you're concentrating on is either suppressed or falls on an invalid target. As such, I believe it would be a DM's judgement call.

Answer 2

You won't see anything.

Detect Magic states,

If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic

Antimagic field specifies that

even magic items become mundane.

This means that none of the objects in the antimagic field would have the visible aura. There is no "background magic" that you might use to notice that the magic is missing in a particular area.

Answer 3

The magic user would detect the cone as magic, but nothing more when focusing on it

If the magic user is never inside the magic field, the magic user would detect the Abjuration magic of the spell effect. If he takes an action to focus on it, he will not see anything, as the dispel magic spell description indicates objects must be visible. The door is blocking this part of the spell.

Furthermore, as clarified in Sage Advice (February 2016), a beholder's anti-magic eye is magical, as the text indicates it creates "the effects of a spell that’s mentioned in its description."

Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Does its description say it’s magical?

If your answer to any of those questions is yes, the feature is magical.

The beholders anti magic cone emulates Antimagic Field:

Antimagic Cone. The beholder's central eye creates an area of antimagic, as in the anti magic field spell...

Therefore it would detect as magical.

Finally, would the detect magic work beyond the point of intersection with the field, and would the wizard be aware?

If the sphere overlaps an area of magic, the part of the area that is covered by the sphere is suppressed.

So detect magic would cease to function at the point of intersection with the field. Since the spell has not ended, and concentration has not been broken, there is no rule to suggest the wizard would be aware of that.

It is a moot point anyway, as taking the action for detect magic doesn't work through the door, so there should be nothing to clue off the wizard that there is a beholder on the other side of the door. He would just detect that there is magic somewhere within 30', which itself is useless as he probably has magic items on his person.