Heat coals or gas grill for direct heat. Cut four 18x12-inch pieces of heavy-duty aluminum foil. Spray with cooking spray. Place 1 chicken breast half, one-fourth of tomatoes and one-fourth of the zucchini on one side of each foil piece. Spoon 2 tablespoons pesto over chicken mixture on each sheet.
Fold foil over chicken and vegetables so edges meet. Seal edges, making tight 1/2-inch fold; fold again. Allow space on sides for circulation and expansion.
Cover and grill packets 4 to 5 inches from medium heat 20 to 25 minutes or until juice of chicken is no longer pink when centers of thickest pieces are cut. Place packets on plates. Cut large X across top of each packet; fold back foil.