The most recent Sage Advice Compendium erratas the find familiar spell so that familiars are now stripped naked when dismissed to a pocket dimension. The Pact of the Chain warlock can summon a Sprite familiar; when initially summoned, it is wearing leather armor, which its statblock lists as the source of its armor class.
Is there a system in place for calculating what the AC is if the warlock dismisses the sprite to a pocket dimension, and then later causes the sprite to reappear as an action?
It looks like using the formula for player characters in leather armor (11 + Dex), would be consistent with the published Dexterity, but we know that the rest of the sprite's equipment (longsword, longbow) work differently because it's a monster, so I'd prefer DM-facing monster adjustment rules for this.