Preheat the oven to 425°F (220°C) and line a baking sheet with foil or parchment if desired.
In a small mixing bowl, whisk together 1/4 cup soy sauce, 1/4 cup sweet chili sauce, 1 tbsp sriracha (if using), minced garlic, and 1 tsp cornstarch until smooth.
Place the cut chicken, broccoli florets, chopped bell pepper, and snap peas on the prepared baking sheet in a single layer.
Drizzle the sauce evenly over the chicken and vegetables and toss on the sheet with tongs or a spatula until everything is coated.
Bake for about 20 minutes, stirring or flipping halfway through, until the chicken is cooked through and vegetables are tender-crisp.
Sprinkle with sesame seeds, if using, and serve immediately with rice or noodles, or divide into four meal-prep containers.