Bring a large pot of water to a boil. Cook the soba according to the package, usually 4–5 minutes, stirring occasionally so the noodles do not stick.
Drain the noodles in a colander and rinse them under cold running water, rubbing them gently with your hands to remove the surface starch. Drain well.
In a bowl, whisk the peanut butter, soy sauce, rice vinegar, lime juice, maple syrup, sesame oil, ginger, garlic and chili flakes. Add warm water a tablespoon at a time until the dressing is pourable.
Over a bowl, cut the grapefruit into segments, removing the membranes. Squeeze any leftover juice from the membranes into the dressing.
Toss the noodles with the bell pepper, edamame, carrots, scallions and most of the dressing.
Top with the grapefruit segments, cilantro and chopped peanuts, and drizzle with the remaining dressing.