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?
Best Answer
Per the SRD on spells:
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.