Does a cantrip fired with the Ready Action require concentration?

Question

The SRD states the following for the Ready action:

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.

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration.

It explicitly states that you can use this to cast a spell. Can you use this to cast a cantrip as well? To me it sometimes seems unclear whether a cantrip is considered a 0th level spell for these scenarios.

Furthermore, does this also require concentration for the cantrip, even if it does not normally require concentration?

Answer 1

Per the SRD on spells:

Spell Level
Every spell has a level from 0 to 9. A spell's level is a general indicator of how powerful it is, with the lowly (but still impressive) magic missile at 1st level and the earth-shaking wish at 9th. Cantrips — simple but powerful spells that characters can cast almost by rote — are level 0.

So yes, Cantrips are spells.

And even though the Cantrip itself does not require Concentration, the character is "holding onto the spell's magic" and therefore "requires concentration". So the casting of the spell does not change, it's because you need to keep the energy swirling until the triggering event that they are concentrating.

Say Merlin states he will Ready casting the Firebolt spell until the trolls are in range. But before the trolls have a chance to move, an orge throws a javelin at Merlin and hits. So on top of the damage, Merlin must make a Concentration check. If he fails, he loses the spell's energy and will not cast it when the trolls move. If Merlin succeeds, then the spell still goes off without a hitch when the trolls are close.

Answer 2

Cantrips are spells, so yes, they require concentration when being readied as well.

A cantrip is a spell that can be cast at will... ^{PHB, pg.201}

The fact that they are 0th level spells does not matter, they are still spells.

Answer 3

Yes and Yes

To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration.

Any spell with casting time of 1 action can be readied.

Concentration is required.