D&D 5e has a handful of spells and features meant to protect the user against lie-detection magic. The examples I'm aware of are the Glibness spell and the Mastermind Rogue's Soul of Deceit subclass feature. Specifically, Glibness says:
[…] no matter what you say, magic that would determine if you are telling the truth indicates that you are being truthful.
Soul of Deceit provides an identical feature, but then goes a bit further:
[…] no matter what you say, magic that would determine if you are telling the truth indicates you are being truthful if you so choose, and you can’t be compelled to tell the truth by magic.
At first glance, these "anti-lie-detector" features seem designed to defeat Zone of Truth. I certainly thought so when I wrote this answer. However, as the comments on that answer rightly pointed out, Zone of Truth is not precisely a lie-detection spell, and the precise interaction between that spell and these features is not clear. I'm going to quote the full text of the spell since any of it might turn out to be important:
You create a magical zone that guards against deception in a 15-foot-radius sphere centered on a point of your choice within range. Until the spell ends, a creature that enters the spell's area for the first time on a turn or starts its turn there must make a Charisma saving throw. On a failed save, a creature can't speak a deliberate lie while in the radius. You know whether each creature succeeds or fails on its saving throw.
An affected creature is aware of the spell and can thus avoid answering questions to which it would normally respond with a lie. Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth.
In short, a failed save causes the following:
- The affected creature can't lie
- The affected creature is aware of the compulsion
- The caster knows the creature failed their save
while a successful save only alerts the caster that the save was passed. Importantly, every creature in the spell's area makes this saving throw even if they are immune to all effects of the spell.
The problem is that Zone of Truth doesn't directly tell the caster whether the affected creature is lying or not. It tells the caster whether or not the creature has failed their saving throw against the spell. So despite the name of the spell, it's not clear whether this is "magic that would determine if you are telling the truth". The extra clause of Soul of Deceit seems like it should clearly defeat the spell's compulsion against lying, but beyond that it's not clear what other effects either Soul of Deceit or Glibness protect against.
So, assuming the creature fails their saving throw and is affected, to what extent does either Glibness or Soul of Deceit protect the affected creature from the effects of Zone of Truth? Specifically, do either of these allow the affected creature to speak a deliberate lie, and if so, what information does the caster of Zone of Truth receive when they do so, and how does it differ from the information the caster would receive if the affected creature did not have the effect protecting them?
Note that I am not asking about the affected creature's ability to be evasive or or avoid answering a question as described in the last paragraph of Zone of Truth. Being evasive is always allowed by the spell. Assume that the affected creature wants/needs to tell a deliberate lie.
Best Answer
The rules do not make this clear, so it's up to the DM.
Glibness protects you from magic that detects whether you are telling the truth, and the Mastermind's Soul of Deceit feature does the same and also prevents you from being magically compelled to tell the truth, so the essential questions we need to answer are:
Let's also look at whether there are any relevant rulings from the game's designers, and then think about which answers to these questions would be most fun for the game.
Does Zone of Truth detect whether you are telling the truth?
The spell does not alert the caster or anyone else to whether the speaker is telling the truth. So the DM could rule that since the spell is not "magic that would determine if you are telling the truth", and doesn't "indicate" whether you are being truthful, Zone of Truth does not interact with the anti-lie-detection features of Glibness and Soul of Deceit in any way.
On the other hand, The DM could rule that while under the effects of Glibness, or having the Soul of Deceit feature, the Zone of Truth spell itself cannot tell whether you are lying or telling the truth and thus cannot have any effect on you even if you fail your saving throw.
Does Zone of Truth compel you to tell the truth?
On one hand, the DM could rule that Zone of Truth only prevents you telling a lie, but does not compel you to speak. Since it doesn't compel you to speak, it doesn't "compel you to tell the truth". Contrast with Suggestion, for example, which could absolutely be used to compel a target to tell the truth, and Soul of Deceit unambiguously defeats that.
On the other hand, the DM could rule that since Zone of Truth prevents you from lying, you must tell the truth if you speak at all. Therefore, it is in fact "compelling you to tell the truth" and Soul of Deceit defeats it.
Relevant ruling from the game's designers
There is a relevant unofficial ruling on Twitter by Jeremy Crawford (the lead designer of the 5th edition D&D rules) about the Ring of Mind Shielding:
From the Ring of Mind Shielding entry in the DMG (emphasis mine):
This creates a strong case that the anti-lie-detection features of Glibness and Soul of Deceit are in fact not designed to defeat Zone of Truth.
As a DM, you might interpret this to mean that a Ring of Mind Shielding doesn't interact with a Zone of Truth because the ring specifically protects against magic that allows other creatures to determine whether you are lying, and so you could rule that Glibness and Soul of Deceit would still make you immune to Zone of Truth.
What things in the game unambiguously affect Zone of Truth? What other lie-detection features exist in the game?
I'm not aware of any magic in the published materials that directly allows a creature to determine whether another creature is lying or telling the truth, nor any magic that would allow you to uncontroversially tell a lie within a Zone of Truth, short of a Wish.
So the question sort of becomes: if these abilities can't defeat Zone of Truth, what are they even for?
Rule of cool
I know you're not asking for opinions, but I certainly think it's a more interesting and dramatic ruling if Glibness and Soul of Deceit (and the Ring of Mind Shielding) can defeat a Zone of Truth. Particularly since Glibness is an 8th level spell and the Mastermind with Soul of Deceit has to be at least 17th level. It would be weird for these powerful abilities to be circumvented by a technicality of a 2nd-level spell like Zone of Truth.