Omniscient Magic Mouth?

Question

As a departure gift to a member of our party, I wanted to give a pendant with Magic Mouth on it, the trigger condition being "when a creature that you're having a conversation with lies to you". The DM argued that the spell gives the object the ability to "see and hear", but cannot make rolls, and as such, it cannot know when a creature is lying. Which I'm cool with, since he is the DM.

I'm DMing an upcoming Ravnica campaign and a player expressed their interest in using Magic Mouth sorta like Programming which I know nothing about but it seems both creative and fun enough for me to allow. Thing is, the lie detector thingy bought up a bunch of questions in my head about how much "sensitivity" or "objective knowledge" about the world the Magic Mouth could actually work with, and I would like a frame of reference for the upcoming Magic Mouth shenanigans.

Question time!

• The Magic Mouth spell never states that it's providing the object with "the ability to see and hear", just that it triggers when a thing happens and such trigger must be based on visual/audible conditions. The spell does not specify whether those perceptible conditions are or are not relative to the caster's senses. How much abstraction would you feel is correct to get away with when casting the spell? If in darkness, would a "when a creature approaches" condition would still trigger, even if it's not technically possible to see for a caster who lacks darkvision? If it does so, what about conditions increasingly more specific such as "when a creature breathes" / "when a heart beats" / "when the spectrum of color is reflected on a surface" (which is pretty much always). The fact that humanoid creatures have certain limitations in regards of how their senses interact with the world doesn't seem to implicitly impose a limitation on the spell, and since, for example, atoms always posses energy, you can technically always have a "visual condition" to be triggered, when would you put a limit to this arbitrarily high perception?
• The spell allows for conditions as general or complex as the caster wants. How would you go about, for example, the "lie detector pendant" thingy in regards to how much "objective knowledge" the mouth can obtain, since while the spell does not implicitly provide the mouth with these skills, it also does not disregard them.

I dunno, maybe kinda overreacting here since it's my first time DMing and you know how that goes.

Answer 1

The rules don't specify how well the Magic Mouth sees/hears.

In the absence of a clear statement by the rules, it's up to the DM to decide how much the Magic Mouth can detect. A reasonable baseline is that the Mouth can see and hear as well as a normal human (or other player race). Does it have darkvision? Can it detect invisible creatures? These are not answered, but you should decide how to answer them. One option is to use the caster as your basis. If the caster could have seen something, then the Mouth can also.

In prior editions, magic mouth had darkvision but nothing more complicated than that, so I'd go with either that or baseline PC senses, and use the caster's spell save DC for any contested checks (like say a person trying to sneak past the magic mouth without triggering it).

Lying is not a physical quality.

This lie detecting amulet is definitely off the table; your DM correctly notes that the spell can only respond to what it can perceive, and it has no way to determine truth. It knows a person is speaking, but only that. Determining if somebody is lying is a complex task that requires interpretation and comparison against previously known truths.

However you rule, just be sure your ruling doesn't mean Magic Mouth takes over for other spell. There are many 2nd level spells that can potentially act as a lie detector -- detect thoughts should be able to tell you if a person lies (possibly a contested roll would be appropriate here), and zone of truth actually stops them from lying (if they fail a save). So that's a good clue that magic mouth shouldn't be able to do anything on the same scale of power, particularly because the magic mouth spell has no time limit. Similarly, magic mouth shouldn't be able to take the place of see invisible and make Stealth checks largely pointless.

It should be obvious from a first look that a permanent always-on lie detector is not a balanced effect for the cost of a single 2nd level spell slot, even if it's openly audible and dispellable. It would largely obviate an entire class of skill rolls. Sorry, just a hard no, and I would expect any experienced DM to rule the same way.