A manually-operated window treatment system may include a roller tube, a covering material, and a semi-rigid chain assembly. The covering material may be attached to the roller tube and may be operable between a raised position and a lowered position via rotation of the roller tube. The semi-rigid chain assembly may be configured to be operated by a user to rotate the roller tube. The semi-rigid chain assembly may be configured to limit the size of a loop that can be formed by the semi-rigid chain assembly. The semi-rigid chain assembly may be operatively coupled to a drive pulley of the window treatment system. The semi-rigid chain assembly may include flexible outer housing and a stiffening rod. The flexible housing may surround and operate along the stiffening rod. The hollow chain may include multiple links that are connected together to form a continuous loop.