Make the Marinade: Whisk garlic, sweet chili sauce, soy sauce, brown sugar, rice vinegar, and coconut milk until smooth.
Marinate the Chicken: Add chicken to a zip-top bag, pour in the marinade, seal, and toss to coat. Let it sit while you prep everything else.
Cook the Pasta: Bring a large pot of water to a boil, salt it, then cook spaghetti until al dente. Drain and set aside.
Cook the Chicken: Heat oil in a large skillet over medium-high heat. Add chicken (leave most marinade in the bag) and cook until browned and cooked through. Transfer chicken to a clean bowl.
Boil and Reduce the Sauce: Carefully pour the remaining marinade into the skillet. Bring it to a full boil, then reduce to a simmer and cook 3–5 minutes, stirring, until slightly thickened.
Toss the Noodles: Add drained spaghetti to the sauce and toss until evenly coated.
Add Crunch + Herbs: Add cabbage, cilantro, and 1/2 cup of the basil. Toss just 30–60 seconds—enough to soften slightly but keep crunch. Add chicken back in and toss. Sprinkle in about 1/4–1/3 cup peanuts.
Make the Salad Dressing: Whisk lime juice, sesame oil, soy sauce, and rice vinegar.
Finish the Cucumber Salad: Toss cucumber with dressing, remaining basil, and remaining peanuts.
Serve: Plate noodles with cucumber salad on the side. Add lime wedges if you want it brighter and less “sweet-chili-forward.