How can the Zone of Truth spell be defeated without the caster knowing?

Question

The Zone of Truth spell contains the following clause:

You know whether each creature succeeds or fails on its saving throw.

In a scenario of checking someone's loyalty, any countermeasure that allows you to always succeed your save against this spell will alert the caster that you have succeeded the save. They will keep you in the area of the spell until you fail, or until it's clear something is stopping you from failing - at which point the caster will tell you to remove whatever that effect is doing this, or have it assumed you are disloyal.

What are all the countermeasures to this spell which:

1. don't allow the spell to affect you, and
2. don't alert the caster you're unaffected?

Note that to fulfil (1), answers should avoid "you can lie by telling the truth in a ZoT". A competent interrogator will simply ask direct yes/no questions and assume disloyalty if they are not answered directly.

Answer 1

Mechanically, Zone of Truth can be defeated only by high-level features

As you say, any attempt to resist or evade the effects of Zone of Truth alerts the caster. And in the context of a loyalty test, avoiding the effect is synonymous with admitting disloyalty. The only exceptions to this are a handful of high-level features: the Mastermind Rogue's Soul of Deceit feature (as mentioned in Louis Wasserman's answer) defeats truth-detection magic, and the 8th level spell Glibness provides a similar ability:

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

Note that while Glibness causes Zone of Truth to read whatever you say as truthful, it doesn't necessarily protect you from being forced to tell the truth. If you intend to use it, you should ask your DM ahead of time how it will work with Zone of Truth to avoid arguments when it actually happens. (In contrast, Soul of Deceit explicitly says you can't be forced to tell the truth by magic, so there is no ambiguity there.)

In any case, unless you have access to features or spells available around 15th-17th level, you'll have to make it through your loyalty test by telling what your character believes to be the truth.

Believe your lies

However, there is at least one small and difficult-to-exploit loophole that doesn't require any powerful abilities: Zone of Truth only prevents a creature from speaking a deliberate lie. If you can manage to believe your lies, you will be able to speak those lies while under the effect of Zone of Truth. For example, if the loyalty question is something like "Are you loyal to the Empire?", then you would be able to answer with a simple "yes" even if you were plotting to assassinate the emperor, as long as you truly and honestly believed that doing so would help the Empire. Obviously, this is a very situational workaround, but I want to emphasize that this is materially different from being evasive or "lying with the truth". This is a case where you give exactly the truthful answer that is expected of you, but for a completely different reason (while praying that you are never asked to elaborate on that reason).

In practice, when you as a player attempt to use this, you should probably expect some push-back from your DM, asking if your character really believes this. This "loophole" has a heavy character focus rather than a mechanical focus, so if you are planning to do this, be prepared with a solidly fleshed-out backstory to justify your honesty. And of course, be willing to accept that a slightly different question might reveal that your interpretation of loyalty differs from that of your interrogators. In the above example, you would not fare well if you were instead asked "Are you loyal to the emperor?"

Unfortunately, I don't have any experience to share of using this "loophole" in an actual game. I'm just pointing out that it's the only loophole I can think of.

Answer 2

Modify Memory

Depending on the circumstances and the precise information you need to keep secret, you may able to effectively defeat a Zone of Truth by having your memory modified in advance by an ally using the Modify Memory spell. Affected creatures in a Zone of Truth cannot deliberately lie, but that doesn't mean that whatever they believe to be true actually is. After interrogation, someone surreptitiously casts Remove Curse or Greater Restoration on you and your original memory returns, at which point you presumably mutter "keikaku¹ doori".

Given the restrictions of Modify Memory this works best for concealing knowledge of specific events; the longer ago the events are or the more material you have to cover, the technique swiftly becomes impractical.

This doesn't strictly meet your constraints since it requires you to actually be affected by Zone of Truth and does not conceal that from the caster, but I am not sure there actually is a reliable way to render oneself immune to the spell without tipping off or having to compromise the caster. Either they know you're passing your saves, or by lack of information they know you're not even making saves and therefore aren't affected.

_{1. keikaku means plan}

Answer 3

The Mastermind archetype for the Rogue class from Xanathar's Guide to Everything has a class feature, Soul of Deceit, which is explicitly about defeating truth magic: it says that you can't be compelled to tell the truth by magic and that magic that determines if you're telling the truth says you are.

This isn't succeeding on the save; it doesn't say anything about a saving throw. I would read the effects as that you make the saving throw as normal, but can ignore the results and lie as you like.

Answer 4

Get Dominated

So you cannot speak a deliberate lie?

deliberate, adj. done consciously and intentionally

If you are not in control of your own actions, you cannot lie deliberately. You are forced to say what you say. Get an ally (standing outside the spell radius if possible to make sure) to cast dominate person on you and

take total and precise control of the target. [..] the creature takes only the actions you choose, and doesn't do anything that you don't allow it to do. (PHB 235)

You will still roll the saves against the zone of truth and the results will be known by the caster. It is just that this makes no difference regarding what you actually say.

Answer 5

Minor Illusion

You say yes (or even don't speak at all), they hear no. You told the truth (or at least spoke no lie), but defeated the Zone of Truth with a cantrip.

If you need more certainty of success, whether to make a more complete illusion or to fool more observers, just go with a higher level illusion spell.

Of course, you will need a way to cast it without detection, but once you have the general principle, this is a much easier task to tailor to the specific situation, restrictions, and resources available, ranging from metamagic, to items, to allies casting it for you.

Answer 6

The Avoid Approach

Besides other people's answers which mostly confront the spell directly (and are very interesting), the target can avoid answering the questions directly just like most politicians do.

From Zone of truth's spell entry (emphasis mine):

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.

A competent Interrogator can assume whatever he wants, however that says nothing about the actual truth. Not answering directly is not enough evidence to conclude treachery (as long as you don't lie directly while you failed a save) and by assuming anything they may be wronging someone (as a GM I'd consider this possibility as it's a nice conflict for good characters).

Consider these scenarios:

1. If the target doesn't know about their own disloyalty or that answer they are giving is false, then they can be convincing while technically lying, because they don't know the truth. And the spell will confirm that he wasn't allowed to say falsehoods or not. So, an opposing enemy mastermind can manipulate others into speaking falsehoods without their knowledge (if the mastermind is not the one being questiond), another option is that the target simply doesn't remember (drunk, brainwashed, any form of mind control that happens without the awareness of target, etc.), therefore the spell can't stop them from saying: I don't remember, I was drunk.

2. The target has enough status to not submit to this interrogation. Nobles can avoid the law if they have enough clout and even if the PCs use force of arms nobles could keep answering with: "I will not submit to your magic interrogation", specially if the target is knowledgeable about magic. Do note that it doesn't matter if the target knows the truth or not, people in a position of power like nobles will use their power to protect themselves from a threat, torturing or killing a noble could have serious consequences regardless of his status as a traitor, so even if your interrogators assume the target is a traitor, unless it can be proven, it's meaningless because the target has enough power to avoid the consequences, you could even make the npc simply tell the truth and then use his "power card" (status) and show the PCs that they can't try a noble.

3. The target has been disloyal, but not the specific disloyalty that the players want, like allowing enemy to retreat when he was ordered to kill them, or he doesn't pay taxes (or even steals from the treasury) or did a bunch of other disloyal actions, but none of these are related to what the players actually facing. (These can works as hooks for more conflicts), so the target is presumably safe as long as people don't ask him directly about their specific disloyalty, but even if players do it, the spell still allows the target to not answer or evade answering, as investigators can't force the target to answer with yes or no. The questions may be simple, but answers don't have to be.

The Interrogation Interference Approach

Because I do love come conspiracies in my games here it goes:

Let's suppose the target is in fact disloyal. He could have kidnapped or otherwise displaced the caster before the interrogation (if the caster is not a player, but that can work even if it is) and whoever is casting Zone of Truth is actually an ally of the target in disguise.

So he can say whatever he wants, since the caster is an ally of the target, nobody else will know what the zone of truth revealed.

Some ways to do this:

1. The caster may be bribed.

2. Before the Interrogation someone could have used Suggestion on the would be caster of Zone of Truth and made him ride a horse for hours away from the place where the interrogation would happen and then, an ally of the target that nobody knows will either substitute or impersonate the caster using alter self or some other way.

3. If the caster of Zone of Truth is an NPC he may be an ally of the target and this a plot to gain the truth of the PCs anyway, so the whole interrogation could be staged without the players knowing.

4. The target is actually an expendable impersonator whose sole purpose is to be killed so the PCs think they have killed the real traitor, but didn't. In this case, the target may just say whatever as he is supposed to die anyway.