The most recent Sage Advice erratas find familiar so that familiars are now stripped naked when de-summoned. The Pact of the Chain’s Sprite familiar is initially summoned wearing leather armor, which is listed in its description as the source of its armor class. Is there a system in place for calculating what the AC is if the warlock desummons and then later resummons the sprite?
It looks like using the formula for player characters, viz. 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 its a monster so I’d prefer DM-facing monster adjustment rules for this.