Modular surgical instruments and related methods are disclosed herein, e.g., for performing rod reduction, derotation, and/or set screw insertion during spinal surgery. An exemplary system can include an instrument body configured to couple to a bone anchor assembly to provide a modular platform for carrying out various steps of a surgical procedure. For example, the instrument body can receive a reduction instrument therethrough for reducing a spinal rod into the bone anchor assembly. A derotation instrument can be attached to the reduction instrument for performing derotation maneuvers or applying other manipulation forces. A modular driver or handle adapter can be attached to the reduction instrument and/or to the derotation instrument to facilitate rod reduction. Any of the instrument body, the reduction instrument, and the derotation instrument can include a working channel therethrough. A set screw or closure mechanism, and a driver instrument for applying the set screw or closure mechanism to the bone anchor assembly, can be inserted through the working channel.