When must the wizard choose to overchannel?

Question

The Evocation specialist wizard's overchannel ability says:

When you cast a wizard spell of 5th level or lower that deals damage, you can deal maximum damage with your spell.

Does the "that deals damage" clause mean the wizard can decide to overchannel once they are sure the spell is going to deal some damage? i.e., that it is not counterspelled or evaded by a successful save, etc.

(This would be similar to paladins being able to choose to apply smite damage after they know they have hit, so this kind of post-hoc add on is not unprecedented.)

Answer 1

No, in this case you need to decide whether you're going to use Overchannel when you cast the spell.

The timing for using Overchannel is "when you cast", and a spell you can use it on is "a wizard spell of 5th level or lower that deals damage". This is merely meant to specify that you can only use it on spells that include damage as part of their effects (like fireball), as opposed to spells that don't deal any damage (like invisibility).

If it was meant to be decided after it had dealt damage, it would have to use wording like "When you cast a wizard spell of 5th level or lower and deal damage".

Answer 2

Like the text

When you cast a wizard spell

says, you decide when you cast the spell, not when you know if the spell hits or the target fails their save.

Compare this to the paladin ability Divine Smite

[W]hen you hit a creature with a melee weapon attack, you can expend one spell slot to deal radiant damage to the target, in addition to the weapon’s damage.

which specifically states that you choose when you hit.

Answer 3

You decide when you cast the spell, not after waiting to see if it hits.

First, the spell explicitly says (emphasis mine):

When you cast a wizard spell of 1st through 5th level that deals damage, you can deal maximum damage with that spell.

This means that you can choose to Overchannel on-cast of a wizard spell of 1st-5th level which does damage as part of its effect, not that you choose to Overchannel when a wizard spell of 1st-5th level that you cast does damage.

If Overchannel allowed you to decide after hitting or after rolling your damage, it would be worded similarly to the Channel Divinity: Destructive Wrath feature of the Tempest Cleric:

When you roll lightning or thunder damage, you can use your Channel Divinity to deal maximum damage, instead of rolling.

On the topic of "that deals damage"

To clarify the ambiguity of how "when you cast a spell that deals damage" should be interpreted, look at the wording of the "Heart of the Storm" feature of the Storm Sorcerer:

In addition, whenever you start casting a spell of 1st level or higher that deals lightning or thunder damage, stormy magic erupts from you.

If "that deals damage" meant "when the spell you're casting deals damage" then Heart of the Storm would never trigger, as no spell does damage when you start casting it. Therefore, "that deals damage" must be a property of the spell itself (meaning it can do damage as part of its effect), rather than triggering only if a specific casting does damage (by hitting with an attack roll, for example).

Answer 4

The more i look at it the more I lean towards after the fact because it seems both are open to interpretation but if we take only what they wrote and don't make any assumptions or interpretations than the "that deals damage" is the only thing we're left with.

"When you cast a Wizard spell that deals damage" is the only requirement. The only way to fulfill that requirement is with a successful spell attack roll. A missed roll does not deal damage and does not fill the only stated requirement.