Does a “Recharge X-Y” ability need to recharge after it is Readied (but not used)?

Question

This question was prompted by recent discussions on dragon breath weapons and cover. It occurred to me that a dragon could potentially use the Ready action to use its breath weapon once a certain number of foes were within its area of effect (waiting for a hiding enemy to come out of full cover). But what if the enemies stayed in cover for the entire round (if the trigger never happened)? Could the dragon then use its breath weapon on its next turn, or would it have to roll a d6 to see if it recharged (rules for recharging found in MM, p. 11)?

My inclination is to say that simply readying a breath weapon doesn't use it up, since nothing in the rules specifically says it does. But on the other hand, the closest equivalent in the rules would probably be readying a spell (since, although a dragon's breath weapon isn't magical, it does have an area of effect and saving throw). And when you ready a spell, you use any resources that the spell consumes, whether you end up releasing it or not.

Does a "Recharge X-Y" ability need to recharge after it is Readied (but not used)? Or does it only need to recharge after it is triggered?

Answer 1

Dragon breath isn't a spell

Dragons can cast spells if the DM activates that optional feature in the MM; by default a dragon's breath is not a spell attack. Trying to apply the spell rules linked to the Ready action to an action that isn't a spell can be a table rule or a house rule - you could do it that way, but the rules don't call for it.

Breath weapons are a unique ability of dragons that are part of that creature's mix of unique features, as is a Gorgon's breath (et cetera). Per the "Limited Usage" heading on Monster Manual p. 11 (or the corresponding section of the Basic Rules):

Recharge X–Y.

The notation "Recharge X–Y" means a monster can use a special ability once and that the ability then has a random chance of recharging during each subsequent round of combat. At the start of each of the monster's turns, roll a d6. If the roll is one of the numbers in the recharge notation, the monster regains the use of the special ability. The ability also recharges when the monster finishes a short or long rest.
For example, "Recharge 5–6" means a monster can use the special ability once. Then, at the start of the monster's turn, it regains the use of that ability if it rolls a 5 or 6 on a d6.

The dragon does not have to expend its breath when readied. The Ready action states:

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round. (Ready, SRD, p. 93)

The text that follows covers spells and those features which makes the spell provision for Ready (burn the spell slot) a case of specific over general. The dragon's breath is not a spell, so it is not covered in that specific exception to the Ready action mechanic.

Since the description in the Monster Manual does not indicate ability loss when not used under a Ready action (temporary loss of that ability is folded into the Recharge mechanic), and since other abilities (like attacks, lair actions, etc.) are not lost by not activating them when Readied, there is no reason to expend the breath when it was readied but not used.

If you view the breath as a special attack, like a wing attack, or a tail attack, this is consistent.

Answer 2

Only after it is triggered.

When you do a Ready action, you choose a trigger and an action (Attack, Ability, Cast a Spell, etc). However, you might not use that action at all, and so don't use its resources. For Recharge X-Y abilities, you only recharge them after you use them.

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

Spells are an exception, since

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

So in their case, you use up the spell slot immediatly.