Can you prevent players using cantrips to tell whether a creature is disguised as an object?

Question

As it is established in the question below that there is some distinction between a cantrip that targets a creature and targeting an object:

What happens when a caster targets an object that looks like a creature with a spell that targets only creatures?

Considering cantrips have unlimited use this allows players to methodically target all objects in an area until the cantrip (that only targets creatures) has a success. This is bad as it reduces engagement with the game, there is no need to analyse the situation to determine if an apparent corpse is someone playing dead or a chest is a mimic.

They can just say "I cast Eldritch Blast on every corpse and object in the room, working left to right" with no worry of hitting an object.

The XGE optional rule does not help as cantrips don't consume a spell slot. It only confirms that nothing happens on targeting an object.

What can a DM do to prevent this boring but highly effective tactic?

EDIT: many have asked what is the relevant distinction between this cantrip tactic and weapon attacks, the distinctions are:

• It is ranged with no cost in resources such as ammunition, so DM cannot limit this by running out of projectiles.
• A weapon attack does not automatically reveal if the "thing" targeted is an object or corpse, you'd have to make a very good perception check to see the reaction to being hit. The moment you succeed in casting Eldritch Blast you instantly know you targeted a creature, which is far more reliable than investigating.
• This avoids you desecrating corpses so I cannot have NPCs object to dead bodies being mutilated as an incentive to carefully investigate. It only appears they are continually casting a spell with no effect.
• This prevents the use of area traps that are triggered by an object being jolted (such as being shot by an arrow) as an incentive to investigate. As when such cantrips target objects the spell fails.

Answer 1

Don't use the rule in the first place.

If you are not running adventures league you have great leeway to adapt the rules. Object vs. creature vs. illusion and whatever else is one of the rules ripe for ignoring.

Eldritch Blast (as an example) is just sending a beam of force in a certain direction, just let it hit whatever they aim for. Problem solved.

I have always done this and it hasn't caused a single odd situation in my games, in fact it is far more realistic and adds to the game rather than detracts.

Overall whenever you find a rule and think 'this rule is causing a lot of problems' don't then look for more rules to counteract that rule, just ignore it, improvise and move on.

Answer 2

Disincentivize the tactic

From a RAW perspective, there is no mechanistic way to prevent this. It is how the rules work.

You will need other ways to make this unattractive to the players, so they do not want to use this tactic, if just discussing the issue with the players does not resolve it.

Do not allow standing orders

The behaviour you describe leads to a very boring play pattern if they do this every single room in a dungeon, on the off chance to catch a disguised foe. It likely is boring to them too, similar to play patterns that search for secret doors everywhere, or look for traps everywhere.

If you do not allow standing orders for something like this, the players will likely tire of doing it and stop it by themselves. I believe limiting standing orders to something reasonable like always moving stealthily in dungeons is a fair way to simulate the player characters forgetting about checking for something, by demanding the player does remember and actively do it in every new situation.

Create in-world disincentives

As another option, you need to find an in-game solution that makes the tactic backfire, to discourage its use, for example:

• Random Encounters. Casting cantrips takes time, and verbalizing them makes noise. You could use wandering monsters to disencourage the players wasting too much time this way, albeit they do not work as well with the faster healing in 5e.
• Time pressure. Giving them limited time to achieve their goal. Doing this on every object in, say a kitchen will take minutes. However, you would need rooms with large numbers of objects and tight time constraints -- not a solution that will generally work.
• Social Pressure. I suspect the issue is mostly happening during dungeon exploration. If the party also does this in town or tavern, then constantly regurgitating arcane formulas is sure to draw some negative response by the barkeep, populace or authorities.
• Astral Predators. You could design special unsavory monsters from the astral plane that get attracted by lots of spellcasting in a short intervals, and manifest to attack the caster or snatch them away to the astral plane. Or just use existing ones like phase spiders.

Allow this occasionally

Lastly, I feel the situation is different if they do this only because they have reason to suspect something is wrong. For example, they are hit by piercers in a cave, and in the next room, they bombard all the stalactites with fire bolts to check if any of them are creatures. Or, they find an apparent corpse on a dais in a crypt, and they suspect it to be an undead monster. I think this is a fair use of the cantrip in those situations and would not prevent the use.

Answer 3

I would suggest that this is a place for a reasonable DM ruling rather than a strict reading of the rules.

Xanathar's Guide to Everything has this rule for how to deal with spells that target an invalid target:

If you cast a spell on someone or something that can't be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn't attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

Now, I'd suggest that "the spell did nothing" should not be interpreted to mean literally nothing happens; it merely means the spell's effects don't apply. The spell does no damage to objects, and any other effects don't take place. An eldritch blast of energy splashes across the table, the ray of frost makes a patch of frost appear on the corpse, and so on; it merely deals no damage to the object in question, and the player characters can tell.

But how do we prevent "shoot everything just in case it's a mimic"? By the rules as written, that strategy seems to work. I have a few suggestions, and you could use one or all of them.

Option 1: Interpret spells more broadly.

I could argue that many cantrips are written too strictly to deal only with creatures. If an eldritch blast deals force damage, why should it only be able to hit creatures? Why not use it to try to break an object? Is it really so different from trying to break an object with a maul or war-pick?

As a DM, you should feel free to declare that spells with attack rolls are firing energy into the environment, and that energy can have reasonable effects on the world. Eldritch blasts can gouge or splinter stone, shocking grasp can scorch wood, acid splash can eat through a rope, and so on. As always, feel free to rule that specific damage types can't harm certain objects -- it's reasonable to rule that cold damage doesn't particularly harm stone in the same way that you can't cut a rope with a hammer or break down a door by firing arrows at it.

I've always tried to encourage clever and interesting play from my players, and saying things like "Oh, you can't do that, the energy blast doesn't hurt objects" only discourages coming up with interesting uses for magic.

Option 2: Sir, this is a Wendy's.

If somebody walked into a restaurant and immediately stabbed the table to check if it was a mimic, it would rightfully freak out everyone around them. Spells should be the same way, if not more so because when somebody starts casting spells, your average citizen won't know what it's going to do. Doing this in a town can and should make the PCs the object of intense law enforcement interest, as much as somebody waving a gun around in a Wendy's would -- even if they didn't intend to hurt anybody, it's probably illegal and definitely upsetting.

Similarly, walking through a dungeon hitting every object with an axe as you go would be noisy and slow, and using a spell for the purpose isn't any different. Every monster in a hundred foot radius should know exactly where the players are, with plenty of time to prepare for them, and that has implications for how the game goes.

Option 3: Paranoid much?

I have to ask where this paranoia is coming from. Do you use mimics and other "trap monsters" so often that the players feel justified in attacking every random object just in case? I can certainly see using a strategy like this when they find a random treasure chest sitting alone in an otherwise empty room and have reasonable suspicion that it isn't what it seems, in which case the players should be rewarded for having good intuition.

If this really is an issue, first check yourself and make sure you aren't just throwing these mimic-like monsters at your players too often. Assuming you aren't, the easiest fix may be to simply talk to the players to assure them that your job as DM is to give the players a good, fun story, not to destroy them, and explain that that excessive paranoia is both unnecessary and destructive to the narrative you're mutually building.

It may not work, but open communication is always better than trying to manipulate the players though in-game levers. That just turns it into a power struggle between you and players and a game of "who can out-think whom", which is also known as player-versus-DM gaming. You don't want that (partly because it's a fake anyway -- the DM can always "win" if he wants to.)

Answer 4

Maybe you are giving too much information to your players about spells effects.

No.1 : They have no way to tell if the target was affected or not by the spell unless the result of the spell has noticeable/perceivable effects. The Eldritch blast example you are providing is a good one. Nothing says the Eldritch blasts 'fizzles' if the target is invalid. The blast goes and hits the target... there is not a 'HP bar' on top of the target's head and if someone wants to know if there was any adverse effect on the target, a Medecine check is required (which also takes time, at least 1 min each)... They will likely find that after spending 6 hours in the first room that this tactics is really, really inneficient, they will lack food, water, attract dozens of wandering monsters and miss most of their deadlines....

Ref: PHB P205

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.

No.2: There is no way for caster to tell the difference between an invalid target and a valid target that made its saving throw. The resulting perceivable effect is often the same.

ref : XGE P85

If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn't attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

Lets take a look at 2 scenarios.

A) Eldritch blast cast at an object: The spells goes off normally, PC makes an attack roll, hits the object. Then needs to go inspect said object for any effect as it will not be perceivable enough without perception or medecine check DC15. Doing these checks will require at least 2 mins. 1 min for perception, 1 min for medecine. Reveal info based on check results

B) Eldritch blast cast at a Mimic: The spells goes off normally, PC makes an attack roll, hits the mimic you apply damage without telling anything. If the mimic decides to stay hidden, PC will need to approach and touch the creature to perform the Medecine and Perception check DC15 to determine if there is any effect. Mimic attacks then. Game over....

Conclusion: I suggest you review maybe the amount of information you give to your players when their characters cast spells, this issue will then resolve by itself.

Answer 5

Introduce scenarios where this method backfires.

Most other answers accept the premise that this method would reliably produce positive outcomes and focus on either why it wouldn't work or what external pressures might disincentivise it. I would like to suggest a different angle: Even assuming the method works, it might sometimes lead to bad outcomes. For instance:

• There is a very valuable artifact which is sentient and therefore counts as a creature. It becomes damaged after Eldritch Blast is cast on it.
• There is a potentially friendly NPC hidden in the area, but they immediately turn hostile after being attacked.
• A rat jumps on the treasure chest at the exact wrong moment, the Eldritch Blast goes off, the rat jumps out of its way, and the spell and damages the treasure.

Sure, all of these scenarios are unlikely, but so is discovering a hostile creature. If a negative outcome is a possibility, the cost-benefit analysis can shift so that this method is no longer optimal.

Answer 6

As with other time-intensive tactics players use--wandering monsters. Play them with a bit of intelligence, rather than simply attacking they ambush the party when they're done with the room. More than one group might even arrive.

And if they're going to blast every object in the room, consider how broad "object" actually is. The ceiling isn't a mimic, the mimic removed one stone in the ceiling and is occupying the spot where it is.

Answer 7

As a DM, you are allowed and even expected to tweak the monster stat blocks. So the Monster Manual version of mimic has this:

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary object.

You should create a new variant:

Fool Mimic

[...other tweaks, could be stronger, could be weaker...]

False Appearance (Object Form Only). While the fool mimic remains motionless, it is indistinguishable from an ordinary object. When a fool mimic takes damage in Object Form, it can perfectly mimic the appearance of the damage to the object it is. The fool mimic appears as an object to any magical ability or spell of less power than True Seeing, as if it was permanently True Polymorphed into the object.

Now all you need to do is place two chests in an empty room. One is ordinary mimic, another is this new fool mimic. Players detect and kill the first mimic, then happily proceed to loot the fool mimic. Ideally these mimics should be distinguishable during battle or after killed, so the players know something is going on.

Disclaimer: I have not used this particular variant myself, as I haven't encountered the mimic detection problem. But tweaking monsters to get around players knowing too much is true and tried tool of DMs.

Answer 8

After much digging I found this on page 185 of the Player's Handbook:

"Objects are immune to poison and psychic damage, but otherwise they can be affected by physical and magical attacks much like creatures can. The DM determines an object’s Armor Class and hit points, and might decide that certain objects have resistance or immunity to certain kinds of attacks. (It’s hard to cut a rope with a club, for example.) Objects always fail Strength and Dexterity saving throws, and they are immune to effects that require other saves."

I think this is doing more than being redundant with specific rules of spells but laying down a general rule that spells that have either attack rolls or require a strength or dexterity saving throw can target objects if it can target creatures. Really, spells that have saves other than Dexterity or Strength can still target objects, they just automatically succeed.

Nowhere else in the core rules does it say that if spell that says it targets creatures must only target creatures.

So a houserule is not necessary, just cite this page and interpret the rule in the way that breaks the game the least. Allowing any attack cantrip to be used as a "detect creature" spell would be breaking the game.