Add halved Brussel sprouts into a bowl with oil, salt, and pepper. Mix to combine.
Place Brussel sprouts into the air fryer and set the bowl aside. It's okay if they overlap, just do not fill your air fryer more than halfway.
Cook brussels sprouts for 8-10 minutes, shaking the basket halfway through.
Remove brussel sprouts from the air fryer and put them back into the bowl. Toss in sweet chili sauce and add red pepper flakes if using. Mix to combine.
Put brussel sprouts back into the air fryer and cook for 1-2 minutes to heat up the sauce.