Cook the spaghetti in a large pot of boiling salted water until al dente. Reserve 1/2 cup pasta water; drain, rinse spaghetti with cold water, drain, and set aside.

Meanwhile, in a large bowl, whisk together peanut butter, soy sauce, sesame oil, ginger, sugar, garlic, and sriracha. Add the cooked spaghetti, 2-4 tablespoons of the pasta cooking water, and toss. Mix in the chicken and grapes. Top with cabbage, scallions, and peanuts, then serve.