1. Cook spaghetti according to package instructions. Drain, briefly rinse under cold water, then drain again.
2. Meanwhile, in a bowl, whisk together peanut butter, soy sauce, chili paste, and 1/3 cup warm water until smooth.
3. Toss spaghetti, carrots, and cucumbers with peanut butter mixture, adding 1 Tbsp. or more of warm water to thin sauce if necessary. Sprinkle each serving with toasted sesame seeds, if desired.
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