Do you waste a spell slot if you try to cast a spell without the correct components?

Question

A recent question got answers indicating, among other things, "If you spend 100gp on a pearl but the seller ripped you off and the pearl you got is only worth 50gp, then it doesn't count as a 100gp pearl for spell components." That's all reasonable to me: magic probably doesn't care about the economy, it just has some need for a "good enough" pearl and such a pearl happens to cost 100gp in a fair trade.

However, lets say a Wizard isn't very good at this whole "haggling" thing and buys the first pearl he finds which happens to have a "100gp" tag on it, then trys to cast identify with it. Naturally, he got ripped off and the pearl is too small or too bad, or maybe not even a pearl at all.

Based on answers to the other question, the spell shouldn't succeed, but did he just waste a spell slot on the attempt?

In my mind, the same rule should apply to other components as well; normally a caster would know if they can't move well enough for somatic components or can't speak for verbal components, but if they were wrong (due to an illusion maybe?) and tried when the components weren't available, it seems that should be the same.

Basically, if a character thinks they can cast a spell, and tries to do so, but was wrong and could not supply some of the components, is the spell lost or does it get stopped early enough in the process to just do nothing?

Answer 1

Spell is not cast, no resources lost

This is covered under Casting a Spell rules in Chapter 11 of the PHB(emphasis mine):

A spell's components are the physical requirements you must meet in order to cast it. Each spell's description indicates whether it requires verbal (V), somatic (S), or material (M) components. If you can't provide one or more of a spell's components, you are unable to cast the spell.

If you are unable to cast, then you have not wasted any resources.

Your Action cost

Whether or not this still takes up your Action will be dependent on the GM. But given that you were unable to cast the spell, it is reasonable to say you were also unable to use your Cast a Spell Action.

However, there is an argument for spending the Action as well. You have attempted to actually complete the spell. You thought you had all required components, reached into your component pouch and said your verbal and done your somatic components as well. Functionally, you have spent all your time in the full events of casting, but just never cast it. The time spent was your Action, even though you didn't actually cast or use a spell slot resource.

Compare against Invalid Targets

A similar, but very different, issue is when you look at casting on an invalid target. In this case, the spell was cast, but it had nothing valid that it could affect. In this case, Xanathar's does provide the optional rule of using the spell slot resource. The difference is in the invalid target case the spell was actually cast while in the missing component, the spell was not cast.

Component shenanigans

I do want to mention the core of this question of a DM providing invalid components to a player. Think long and hard about why you may want to do this and also how you plan on doing it. Normally, I think the assumption is that players are getting what they pay for. If you challenge that assumption, then you need to have a mechanic in place for it so that players understand the world they're playing in. Rolling Deception vs Insight is a way to do that at the time of purchase, but starting to go down the road of distrust between players and DMs is one that may be perilous unless everyone at the table has agreed to it.

Answer 2

The way I understand magic in D&D, it's channeled through the MU, and the reagents act as either a focus (to help channel) or catalyst (spark it but not used up) or reagent (used up in casting).

EG: a cleric uses their holy symbol as both a focus and catalyst when turning undead.

EG2: an MU uses the pearl as a focus and reagent to cast Identify, and the pearl is destroyed when doing so.

But, the power channels through the caster. This is why higher level characters can cast more spells. They have more experience.. and by that we mean ... abstraction .. that some how lets them cast more spells before exhaustion. (because most MU's regain spells after a sufficient night's rest, and clerics regain them after sufficent rest and meditation).

So.. if you use a spell component that doesn't meet the requirements of the spell... the spell doesn't go off. And if the spell doesn't go off.. technically it means the spell's power has also not channeled through you. So, it doesn't count as a successful spell casting / spell count drain.

Role-playing this out, the GM would simply say "you raise the pearl up to use your magic to identify the objects, and ... nothing happens. The pearl doesn't glow like it usually does, and you don't feel the channeling of power like you normally do."

(from there the GM may have you roll a skill or something to see if you can tell that the spell component doesn't meet req's)

(roll dice) "You sense that the pearl is the problem. And upon further inspection, you realize it wasn't prepared correctly to be used for this spell. The guy at Bob's Reagent Emporium has clearly ripped you off."

What skill that would be..I have no clue. Played in one campaign where the GM let MU's build up an "appraisal" skill, sort of like how Thieves would have appraisal and use it to determine value of items, but MU's could use it to determine quality of spell components they found (to keep from getting screwed over by hack vendors) and to know how to create their own spell components in order to cut down on the cost of keeping an MU's spells going. (b/c it can be a major hassle having to run back to towns for reagents and other stuff if your MU is a workhorse.)