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?