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

Question

There are many spells which explicitly target creatures:

Choose one creature within range

The target creature is normally supposed to make a saving throw in this case.

What happens when the caster chooses a creature, that is actually an object? For instance, a scarecrow, a mannequin, an illusion, or a corpse.

Examples:

• An evil mage is hiding in the bushes. He casts an illusion of himself on the road, using the Silent Image spell. A PC sorcerer walks down the road, sees the illusion and casts Magic Missile on it.

• A cleric is standing near a pile of dead bodies. He has suspicions that a few corpses are actually undead creatures. He casts Sacred Flame on every corpse, see which if them succeed, hence, reveals the undead.

• A wizard casts Acid Splash on a mannequin, trying to damage it with acid.

Possible outcomes I can think of are:

1. The caster cannot even try to cast the spell, regardless of their information about what the target actually is.
2. The caster tries to cast the spell, the casting fails, not expending the spell slot.
3. The caster casts the spell, it expends the spell slot, then fails.
4. The caster casts the spell, but it hits the original and not the illusion.
5. The caster casts the spell, it flies to the target illusion but deals no damage to it (passes through it).

References in the rules and Sage advice

I was trying to investigate, but it didn't make things clearer:

• Does magic missile hit silent image's illusory creature? has an answer, that Magic Missile won't hit. It says nothing about if it can be targeted at all, it leaves it up to the DM.

• Magic Missile automatically finds the original, even when the caster targets an illusion. At least when the illusion was created by the Mirror Image spell. Now confirmed by Jeremy Crawford. This supports the counter-intuitive option 4.

on the other hand

• Casters don't automatically know when a spell fails, if there were no perceivable effects (let's say you can try to cast the Command spell on an illusion). That assumes that casters can waste their spells on illusion, which means they at least can target it. This supports options 3 and 5.

Answer 1

There are 2 ways to resolve this that have been discussed by WOTC designers in an official capacity: An older option on a Sage Advice segment of one of their Dragon Talk podcasts from January 2017 (specifically at about 13:12 for ~7.5 minutes), and a newer optional rule provided by Xanathar's Guide to Everything's Dungeon Master's Tools chapter.

Sage Advice: Intent is that spell effect doesn't happen, action cost applies, and DM judges whether spell slot is used

To summarize Jeremy Crawford's statements in the January 2017 podcast, "illegal targeting" is a gap in the written rules (as of the date of the podcast) and it's mostly open to DMs to choose how to handle it. That said, Crawford says the intent for how it should be handled is that the spell should still take up the casting time but the spell effect will not occur and not consume a spell slot (I.E., option 2 listed in the question).

There are enough corner cases with this solution at the time of the podcast that Crawford still recommends that a DM adjudicate each individual occurrence on a case-by-case basis until there is eventually an official printed rule. As an example of why, he says spells such as those which require a spell attack probably should still consume their spell slot since there'd be some dissonance with the fact that those spells can miss, unlike saving throw spells which always "hit" but the target can resist their effects.

(He does not clarify what should happen if something like Eldritch Blast, which targets only creatures, actually hits a non-creature in this case.)

The flavor reason for this is that he views spells as essentially trying to make a magical connection between the caster and target. When that connection is established, the energy of the spell is consumed in producing the spell's effect - but if the target isn't one the spell can make a connection to, nothing happens and that energy isn't expended.

He views spell attack spells as a different category; if I had to guess why, it's likely because they mainly produce some effect that then follows standard attack rules in trying to reach the target.

Xanathar's Guide to Everything (optional): No spell effect, action cost applies, and spell slot is expended

As of Xanathar's Guide to Everything's release in November 2017, the (optional) rule for resolving invalid spell targets states (p. 85-86):

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.

This is in slight contrast to Crawford's earlier statements on the topic above, in that the spell still occurs and consumes a spell slot with no apparent effect.

Answer 2

This came up in a recent conversation on a forum about using spells like Sacred Flame as a way of determining if statues were really gargoyles or something like that.

As I recall, the rules do not have a terribly satisfying answer to this. The spells are written the way they are to avoid awkward situations like a continuous stream of acid splash cantrips burning their way through the wall of a castle or something. In practice though, most people seem to find the idea of characters using metagame knowledge like "it only targets creatures" in order to deduce other information as distasteful at best.

The few times I've seen it come up in play DMs have basically handwaved it away, letting the approximate effect take place even if you targeted an object instead of a creature, but not allowing characters to take it to absurd lengths like burning their way through a castle wall with a cantrip.

Answer 3

If a spell has restrictions on what kinds of targets it can have, this tells you which kinds of targets it can affect, and it has no effect on targets who don't meet this restriction. For instance, Charm Person specifies a Humanoid target. You can cast Charm Person on a Beast creature, or on a door, and it will have zero effect on either of these targets, but will still expend the spell slot and will be cast normally. It only has the effect described if you cast it on a Humanoid target.

Similarly, Eldritch Blast specifies it can only target creatures. This suggests that if a beam from Eldritch Blast hits an object, it will not affect it or deal any damage, and the eldritch energy will simply dissipate harmlessly. But you can still aim the spell at said object, intentionally; there's nothing preventing you from doing that.