Pat chicken breasts dry with paper towels. Slice into ¼-inch strips perpendicular to the muscle fibers (against the grain) to maximize tenderness.
In a small bowl, combine chili powder, cumin, garlic powder, onion powder, smoked paprika, oregano, salt, black pepper, and cayenne pepper (if using). Mix thoroughly to ensure even distribution of spices.
Place chicken strips in a medium bowl, drizzle with ½ tablespoon oil, and sprinkle with 2/3 of the seasoning mix. Toss until evenly coated. Allow to rest at room temperature for 15 minutes to optimize flavor penetration.
In a separate bowl, combine sliced bell peppers and onions. Drizzle with remaining ½ tablespoon oil and sprinkle with remaining seasoning mix. Toss until evenly coated.
Set air fryer to 400°F (204°C) and allow to preheat for 3-5 minutes depending on your model's specifications.
Arrange seasoned chicken strips in the air fryer basket in a single layer with minimal overlap. Cook for 6 minutes.
Open the air fryer and add the seasoned vegetables on top of the partially cooked chicken. Gently toss to integrate.
Continue cooking for an additional 6-8 minutes, opening halfway through to toss ingredients for even cooking.
Check that chicken has reached an internal temperature of 165°F (74°C) using an instant-read thermometer and that vegetables have developed light charring on edges while maintaining some firmness.
While the chicken and vegetables finish cooking, warm tortillas according to package directions or in a dry skillet for 20-30 seconds per side.
Transfer fajita mixture to a serving plate. Serve with warm tortillas and desired toppings.