Wipe the 12 mushrooms with a damp cloth to remove any dirt. Remove and discard (or save for another use) the stems; keep the caps whole for stuffing.
In a medium bowl, combine 4 ounces softened cream cheese, 1/2 cup shredded cheddar cheese, 1/4 cup breadcrumbs, 1 tablespoon chopped fresh parsley, 1/2 teaspoon garlic powder, 1/4 teaspoon salt, and 1/4 teaspoon pepper. Stir until the mixture is evenly blended.
Using a small spoon, divide the filling evenly among the 12 mushroom caps. Press the filling gently into each cap so it stays in place and mounds slightly on top.
Arrange the stuffed mushrooms in a single layer in the air fryer basket with the filling side up. Do not overcrowd the basket; work in batches if needed.
If your air fryer requires preheating, preheat it to 375°F. Cook the stuffed mushrooms at 375°F for 8 minutes, or until the tops are golden brown and the filling is heated through.
Carefully remove the mushrooms from the air fryer and let them rest 1–2 minutes before serving.