A surgical instrument includes a housing, a shaft coupled to and extending distally from the housing, an end effector assembly disposed at a distal end of the shaft, a movable handle coupled to the housing, a drive assembly coupled to the end effector assembly, and an extension mechanism disposed within the housing. The extension mechanism is configured to couple to the shaft and between the movable handle and the drive assembly for moving the shaft and drive assembly between a retracted position and an extended position relative to the housing. The extension mechanism is coupled between the movable handle and the drive assembly in each of the retracted and extended positions such that the movable handle is operable to effect manipulation of the end effector assembly in each of the retracted and extended positions.