At what point does a caster define the target of a spell?

Question

The new School of Invention tradition for Wizards (Unearthed Arcana) while still in playtesting (and indeed a bit silly in my opinion, more of a "chaos wizard"), got me thinking about its reckless casting ability. This allows the caster to cast magic they do not have prepared and get a random result. The random spells have different targeting criteria (Wall of Stone, Fireball, Feign Death , Mass Cure Wounds) and so it is not possible to decide the target of the spell until it has been cast, as you don't know what you are going to get until it is cast.

This suggests that the designer of the subclass has made the assumption that you don't actually target until you have cast the spell and release it, which makes sense from one point of view, strongly supported by the way a prepared action casting works, PHB p.193 where you cast the spell and hold the energy until the condition is met for the readied action at which point you target the spell.

While the Unearthed Arcana is not RAW or finalized and released, my question is not about this subclass, but about the official rules as a whole.

Is there was any direct RAW, or other supporting or opposing RAW, that states at what point a caster defines the target of a spell?

As a sub-question: are there any circumstances where it actually matters, other than the already defined Readied Action spell casting, in the official rules?

Answer 1

No explicit RAW, but evidence suggests target is selected when spell is released

General Rule

As you mention in your question, the part of the text dealing with readying the spell is the most compelling piece of evidence the book has:

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs.

Combined with the Sage Advice, Rules Answers: June 2016:

For readying a spell or other action, does the target have to be in range?

Your target must be within range when you take a readied action, not when you first ready it.

Together, these pieces create a compelling case for a general rule.

While readying a spell (per the SA ruling) you can specify the target of the spell when the spell is released not when the spell is cast (related). And, because the description says that you "cast [the spell] as normal" there is solid evidence that that is the normal behavior of spells in general.

Thus, the general rule we glean from this is: a spell's target is determined when the spell is released but after it is cast.

What about spells with longer casting times?

They follow the same ruling above because they are not considered to be cast until you have completed the casting process for the requisite amount of time:

Certain spells (including spells cast as rituals) require more time to cast: minutes or even hours.

So there is really no difference between these spells and spells of any other casting time. At the end of the specified casting time, the spell is cast. When it is then released, targets must be selected for the spell.

When does it matter? Never (or extremely rarely)

Edge cases not covered by current rules are few if not non-existent as far as I can tell right now.

The only time this distinction matters is when there is a difference in time between the casting and the releasing (or the effects) of the spell. The only current cases of that are readying an action to cast a spell and specific spells (eg Glyph of Warding(Spell Glyph)) that specify the rules within the spell description.

The fact that spellcasting has worked seemingly perfectly well despite the lack of an explicit ruling in this case lends further support to the fact that such a general rule is probably not necessary (outside of what has already been explicitly said). It doesn't seem like there are an abundance or even any edge cases that aren't covered under the current rules.

It doesn't matter for the UA School of Invention Wizard

I know you did not need or want an answer focused on the UA part of your question. However, it is worth noting that even the UA Invention Wizard that inspired the question would not fall into the category of a case in which this distinction actually matters. Per the current description of reckless casting:

Starting at 2nd level, you can attempt to cast a spell you don’t have prepared. When you use this ability, you use your action and choose one of the following options:

• Roll on the Reckless Casting table for cantrips and cast the resulting spell as part of this action.
• Expend a spell slot and roll twice on the Reckless Casting table for its level, or the 5th-level table if the slot is 6th level or higher. Pick which of the two results you want to use and cast the resulting spell as part of this action.

Both options have the player roll and then have the wizard cast the spell that results. The only difference to normal casting is the random roll. There is no separation in time between the casting and the effect of the spell that would make targeting unclear.

Answer 2

Two-for-one:

The Sage Advice Compendium v. 2.5 says:

[NEW] If a spellcaster is affected by slow and takes two turns to finish casting a spell, what happens if their target has moved out of range or out of sight? You choose the targets of a spell when you complete casting a spell, not when you start.

This answers both "are there RAW" and "are there any cases in which it matters".

Answer 3

A caster defines the target of a spell while resolving the spell's effects, after the casting of the spell has completed.

The rules for casting a spell indicate that all text that's not part of a spell's basic information is the description of a spell's effect:

Each spell description begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.

Generally speaking, the rules tell you to pick a target within that spell effect description, meaning that the act of choosing a target for the spell is itself part of the spell's effect. For example, hold monster's spell effect description includes the following:

Choose a creature that you can see within range. The target must succeed on a Wisdom saving throw or be paralyzed for the duration. [...]

So when casting hold monster, you find out if the spell successfully is cast, then choose your target after the casting is completed and the effect is coming into play.

This would matter if, say, you had multiple eligible targets for hold monster, and one attempts (and fails) to counterspell you when you go to cast it. The casting would then successfully complete, and you would have additional information available to use in deciding which target to pick- you now know one of the targets is a spellcaster, and they may be a higher priority as hold monster's target.

Another example scenario is the one @Kirt points out in their answer from the Sage Advice Compendium, which also reinforces this interpretation of the rules:

If a spellcaster is affected by slow and takes two turns to finish casting a spell, what happens if their target has moved out of range or out of sight?

You choose the targets of a spell when you complete casting a spell, not when you start.