Toast the pecans in a dry skillet over medium heat for 3-4 minutes, stirring frequently, until fragrant. Set aside to cool.
In a large bowl, whisk together mayonnaise, Dijon mustard, lemon juice, salt, black pepper, and cayenne (if using).
Add the shredded chicken, diced celery, halved grapes, and toasted pecans to the bowl.
Stir gently until all ingredients are evenly coated with the dressing.
Cover the bowl and refrigerate for at least 30 minutes to allow flavors to meld.
Serve chilled or at room temperature on bread, croissants, lettuce wraps, or as a dip with crackers.