Prep the potatoes: Peel the sweet potatoes if you prefer, or leave the skin on for extra fiber. Cut into even sticks about 1/4 to 1/3 inch thick. Thinner sticks crisp better.
Soak briefly (optional but helpful): Add the cut fries to a bowl of cold water for 10–15 minutes. This pulls out surface starch and helps them crisp. Drain and pat very dry with a clean towel.
Preheat the air fryer: Set it to 380°F (193°C) for 3–5 minutes. A hot basket helps prevent sticking and promotes browning.
Season: In a large bowl, toss fries with oil. Sprinkle on cornstarch, salt, pepper, garlic powder, smoked paprika, and cayenne. Toss until evenly coated. The cornstarch should disappear into a light, dry-ish coating.
Load the basket: Arrange fries in a single layer with a little space between pieces. Cook in batches if needed. Crowding leads to steaming, not crisping.
Air fry: Cook at 380°F (193°C) for 12–15 minutes total, shaking the basket or flipping halfway. Start checking at 10 minutes. Fries are done when edges are browned and centers are tender.
Finish and season: Transfer fries to a bowl or tray and sprinkle with a pinch more salt while hot. Repeat with remaining batches.
Serve: Enjoy right away with your favorite dip. If holding for a few minutes, keep finished fries on a wire rack to stay crisp.