Tear two large sheets of heavy-duty aluminum foil, each about 12x18 inches.
In a bowl, toss the chicken thighs with olive oil, rosemary, salt, and pepper.
Place one chicken thigh in the center of each foil sheet. Scatter the halved grapes around the chicken.
Fold the foil over the chicken and grapes, then crimp the edges tightly to seal. Make sure there's a little room for steam inside.
Place the foil packets on a campfire grate over medium coals or on a camp stove over medium heat. Cook for 12-15 minutes, flipping once halfway.
Carefully open one packet to check that the chicken is cooked through (165°F internal temperature). Let rest for 2 minutes before serving directly from the packet.