Warm broth in the pot over medium-low until steaming (about 160°F). Add peanut butter, soy sauce, and rice vinegar. Whisk 60–90 seconds until completely smooth.
Add neutral oil, garlic, ginger, and scallion whites. Cook over medium heat 2 minutes, stirring.
Increase heat to a gentle boil. Add spaghetti (break in half), bell pepper, and shredded carrots. Stir to separate noodles.
Cook over medium, stirring every 2 minutes, for 12–14 minutes until spaghetti is al dente. Add snap peas in the last 3 minutes.
Remove from heat. Stir in sesame oil, sriracha, scallion greens, and juice of ½ lime. Taste and adjust.
Divide among bowls. Top with crushed peanuts, cilantro, sesame seeds, and lime wedges.