We propose to develop, fabricate, and test a small-scale medium-energy (0.2 - ~500 MeV) gamma-ray telescope, optimized for photon detection in both the Compton-scattering and pair-production regimes. The instrument will consist of a double-sided Si-strip tracking detector with energy deposition readout, a composite CdZnTe-strip (CZT) and CsI(Tl)-log calorimeter with high spatial and good energy resolution, and a highly efficient anti-coincidence detector (ACD). This instrument will be a prototype for a potential future MIDEX-scale mission (ComPair) designed to provide a more than order of magnitude increase in sensitivity to the MeV gamma-ray Universe compared to past missions. ComPair will provide a significant improvement in both angular and energy resolution over previous instruments operating in the 0.2$-$100 MeV range, offering a truly new window on this poorly explored energy range. In this proposal, the team proposes to develop and test the key detection elements for ComPair, integrate these elements in a prototype telescope, perform a series of beam tests to demonstrate the performance, and perform a balloon test flight to study the background rejection capability of the prototype instrument. As a result, we will establish the proof of concept for a Si-CZT Compton-Pair space telescope and elevate the TRL for the ComPair technology to 6-7.