Cut each string cheese stick in half to make 24 pieces. Place them in the freezer for 30 minutes to prevent excessive melting during cooking.
Set up three shallow bowls: one with flour, one with beaten eggs, and one with breadcrumbs mixed with garlic powder.
Remove cheese sticks from freezer. Coat each piece in flour, then dip in egg, and finally roll in the breadcrumb mixture. For extra crunch, dip in egg and breadcrumbs again for a double coating.
Place breaded mozzarella sticks in a single layer in the air fryer basket, making sure they don't touch. Spray generously with cooking spray.
Air fry at 390°F for 6-8 minutes, flipping halfway through, until golden brown and crispy.
Let cool for 2-3 minutes before serving with warm marinara sauce for dipping.