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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:

  1. The affected creature can't lie
  2. The affected creature is aware of the compulsion
  3. 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.

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    \$\begingroup\$ I feel like several recent questions have been dancing around this issue without directly addressing it, but I may have missed one. So apologies if this question turns out to be redundant. \$\endgroup\$ – Ryan Thompson May 2 at 13:51
  • \$\begingroup\$ Related on Useful spells in 5e for a legal trial \$\endgroup\$ – NautArch May 2 at 13:52
  • \$\begingroup\$ I"m also not 100% sure I understand what you're asking because of what you have noted about Zone of Truth's mechanics regarding telling truth and the freedom you have within it. \$\endgroup\$ – NautArch May 2 at 13:53
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    \$\begingroup\$ @NautArch Can you explain what's not clear about the question? I've tried to be as precise as I can. \$\endgroup\$ – Ryan Thompson May 2 at 13:57
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    \$\begingroup\$ I'm not asking anything about being evasive. I'm assuming a situation where the creature wants to deliberately and unambiguously lie. For example, they want to answer no to a question when they absolutely know the true answer is yes. \$\endgroup\$ – Ryan Thompson May 2 at 14:02
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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:

  1. Does Zone of Truth detect whether you are telling the truth?
  2. Does Zone of Truth compel you to tell the truth?

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:

A ring of mind shielding and a zone of truth have no effect on each other.

From the Ring of Mind Shielding entry in the DMG (emphasis mine):

While wearing this ring, you are immune to magic that allows other creatures to read your thoughts, determine whether you are lying, know your alignment, or know your creature type.

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.

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.

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You asked:

What information does the Zone of Truth caster receive, and how it does it differ from if they didn't have Glibness or Soul of Deceit?

They learn whether or not you pass the saving throw, it doesn't differ at all. That's all Zone of Truth says they learn, so that's all they learn. The other answers cover this pretty well, too.

Do Glibness and Soul of Deceit protect you from Zone of Truth?

Glibness does not help against Zone of Truth specifically, while Soul of Deceit defeats it.

As you point out, these two effects have different wording. Glibness simply causes magic that says whether or not you're telling the truth to say that you are. But Zone of Truth doesn't check whether you're telling the truth; if you failed your save, it compels you to tell the truth. Glibness would help if you could lie and ZoT only reported if it was truth or a lie, but that's not what happens. You're simply forced to tell the truth...

...which is exactly what Soul of Deceit protects against.

...and you can’t be compelled to tell the truth by magic.

Even if you fail your save against Zone of Truth, it will not force you to tell the truth if you have Soul of Deceit. So the caster will learn you failed your save, and assume everything you say is true, because you cannot lie. But you can lie. As much as you want. In this case, it's actually better to fail your save, because it makes your lies incredibly believable.

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  • \$\begingroup\$ I like this concise answer, it would be even better if you provided links. \$\endgroup\$ – Akixkisu May 2 at 18:54
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    \$\begingroup\$ Zone of Truth does not compel you to tell the truth, it only prevents you telling a lie, no? \$\endgroup\$ – Apocalisp May 2 at 19:03
  • \$\begingroup\$ @Akixkisu links to what? Glibness, Soul of Deceit, and Zone of Truth are already linked in the question, along with their relevant text quoted. \$\endgroup\$ – gatherer818 May 3 at 13:36
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    \$\begingroup\$ @Apocalisp I hate to admit, you may have a point there. The wording's not as tight a match as I'd like. I think "you can't lie" means "you are compelled to tell the truth (if you speak at all)", but I can see some wiggle room, you're right. \$\endgroup\$ – gatherer818 May 3 at 13:37
  • \$\begingroup\$ @Apocalisp "Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth." Also, consider these four necessary conditions for lying: 1. lying requires that a person makes a statement. 2. lying requires that the person believes the statement to be false: that is, lying requires the statement to be perceived as untruthful. 3. lying requires that the untruthful statement is made to another entity. 4. lying requires that the person intends that other person to believe the untruthful statement to be true. \$\endgroup\$ – Akixkisu May 3 at 13:51
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RAW some of them, to some extent; Rules-As-Sane all of them, to some extent

This answer will have some strict readings and some personal interpretations. The personal interpretations will be overall based on the following paradigm:

  • As seen in the answers to What truth-detecting magic (excluding Zone of Truth) does a Ring of Mind Shielding protect against?, Zone of Truth is quite literally the only spell or non-background magic (other than Wish, which can do anything) in the game in the category of detecting, compelling, or restricting truth or lies.
  • The magical truth/lie detection/compulsion clauses in the Ring of Mind Shielding, Glibness spell, or Soul of Deceit feature simply must have some mechanics in game that they apply to, otherwise that is a lot of meaningless rules text.
  • Therefore, given a sensible interpretation of the rules, those clauses should in some fashion affect Zone of Truth. I refer to this as "rules-as-sane".
  • This is despite the fact that, given a strict reading of the rules, most of them do not.

Now let's go through all the anti-lie-detection features one by one.

Ring of Mind Shielding (item): RAW, ZoT defeated, caster may know something's up

While wearing this ring, you are immune to magic that allows other creatures to read your thoughts, determine whether you are lying, know your alignment, or know your creature type. [...]

All we need to answer this one is to answer whether Zone of Truth is "magic that allows other creatures to determine whether you are lying".

When Alice casts Zone of Truth, and Bob fails the save, Alice knows that anything Bob says in that area for the next 10 minutes is the truth. So Zone of Truth has allowed Alice to determine whether Bob is lying (for that period of time) - he isn't. It's true that Zone of Truth could only ever have provided a "he isn't", rather than a "he is"; but that is still a determination, and one that provides information.

So Zone of Truth is indeed such magic. This means that the wearer of a Ring of Mind Shielding is immune to Zone of Truth.

What does "immunity" mean for what the caster knows about the success/failure/other of the saving throw? Well, this answer suggests that immunity means no save would take place, and the caster would know that. I believe the RAW isn't clear here.

In my personal view the linked answer seems a sensible ruling since it is thematically in line with the other features of the Ring; e.g. it blocks mind-reading, but doesn't allow you to create false thoughts - so the caster of Detect Thoughts knows something odd is up with the wearer of the ring, and under this ruling so would the caster of Zone of Truth.

(Note that Jeremy Crawford unofficially disagrees with my RAW interpretation here. But I think he's wrong, as explained above.)

Glibness (spell): RAW, no interaction; RAS, undetectably defeats ZoT

[...] Additionally, no matter what you say, magic that would determine if you are telling the truth indicates that you are being truthful.

As we've established above, Zone of Truth is "magic that would determine if you are telling the truth". However, as also established above, it does that solely by indicating that you are being truthful. Glibness forces it to indicate truthfulness, but you already can't lie. So RAW Glibness has no effect on Zone of Truth.

But, as discussed in the intro, that's a bit ridiculous since it would mean this clause of the spell has no effect on anything at all. So I would personally rule that Glibness would allow you to lie under a Zone of Truth. And because this is not "immunity" to such magic, you can still fail the save and lie, and the caster is none the wiser.

Soul of Deceit (Mastermind Rogue feature): RAW, no interaction; RAS, undetectably defeats ZoT

[...] Additionally, 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.

RAW, this suffers from the same issues as Glibness, given they share the same first clause. It has an extra clause which might help - but strictly speaking, Zone of Truth prevents you from telling a lie rather than compelling you to tell the truth, so Soul of Deceit doesn't protect from Zone of Truth through that clause either.

Nevertheless, the extra clause makes it even more obvious this should by all rights apply to Zone of Truth. So I would personally rule the same as for Glibness - you can lie, even when you fail the save.

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  • \$\begingroup\$ Since "rules-as-sane" isn't a common term, you may want to clarify in your answer that you're just trying to use it to refer to "a sensible/logical interpretation of the rules". \$\endgroup\$ – V2Blast May 3 at 22:03
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The language of the rules does not make this transparent, so it's may be up to the DM, but Soul of Deceit defeats it strictly RAW.

What is a lie and what is compelling the truth?

There are four necessary conditions for lying:

  1. lying requires that a person makes a statement.
  2. lying requires that the person believes the statement to be false: that is, lying requires the statement to be perceived as untruthful.
  3. lying requires that the untruthful statement is made to another person.
  4. lying requires that the person intends that other person to believe the untruthful statement to be true.

Based on these conditions it is reasonable to argue that RAW you are compelled to speak the truth under the effects of Zone of Truth, but there is also semantics to argue that you are merely compelled to not lie under the effects of Zone of Truth.

The following part of the RAW of Zone of Truth makes the best case for compelling truth:

Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth.

The creature is not only compelled to not lie but also compelled to stay in the boundaries of the truth.

To what extent does either Glibness or Soul of Deceit protect the affected creature from the effects of Zone of Truth?

On the basis of this thread that indicates that there are no official rules that let you fail a saving throw and the exact wording in your question we can make the following assumptions:

  1. The caster knows whether the creature failed their saving throw or succeeded. Following this, if you do not fail your saving throw the caster always has reason for suspicion.

Therefore only If you failed your save Glibness gives you protection from suspicion. You are still compelled to say the truth if you fail your save, therefore the caster of Zone of Truth always has reason for suspicion if you succeed, and you are compelled if you fail. This results in Glibness being almost no protection at all if the caster is suspicious.

  1. Soul of Deceit protects you from being compelled to tell the truth. Therefore, if you fail your save, you have full protection from the caster's suspicion. If you succeed your saving throw, the same suspicion can be raised against you.

So, assuming the creature fails their saving throw and is affected:

  1. Under the effect of Glibness you are compelled to say the truth if you fail your save.
  2. Soul of Deceit protects you from being compelled to not lie. Zone of Truth doesn't directly tell the caster whether the affected creature is lying or not, therefore Soul of Deceit protects you from suspicion and all relevant effects as long as you fail your saving throw.

what information does the caster of Zone of Truth receive when they do so

The caster of Zone of Truth does not gain any further information through magic means if you failed your saving throw. The only information is the result of that throw. The caster may make logical deductions.

how does it differ from the information the caster would receive if the affected creature did not have the effect protecting them?

The only information is the result of the saving throw combined with the logic of deduction under the assumption that the creature is under the effect of compulsion if they fail their saving throw or is not compelled if they succeed their throw.

Your DM may rule that if you are also compelled to tell a deliberate lie, as implied here:

Assume that the affected creature wants/needs to tell a deliberate lie.

To be under the protection of Soul of Deceit, you have to be evasive, because you are only compelled to stay in the boundaries of truth if you are evasive, which leads to Soul of Deceit not protecting you.

If this is a question of want, then Soul of Deceit would protect against Zone of Truth in this reading.

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  • \$\begingroup\$ Comments that provide reasons for your downvoting would be appreciated. \$\endgroup\$ – Akixkisu May 2 at 15:11
  • \$\begingroup\$ Why would glibness not work? Also, please see this meta about downvote reasons. I get wanting to know but don't expect to necessarily get that info. \$\endgroup\$ – NautArch May 2 at 15:16
  • \$\begingroup\$ Glibness works, there is no line in my post that indicates that it doesn't work. Glibness does not protect you from the suspicion of failing your saving throw if you fail the affected creature can't lie. \$\endgroup\$ – Akixkisu May 2 at 15:20
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    \$\begingroup\$ @NautArch I think you have it backwards? If you succeed on the save, the caster knows you might be lying, but if you fail the save, the caster believes you are compelled not to lie. \$\endgroup\$ – Ryan Thompson May 2 at 15:32
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    \$\begingroup\$ ZoT doesn't' compel truth - it prevents deliberate lies. \$\endgroup\$ – NautArch May 2 at 20:25

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