The Compton Spectrometer and Imager (COSI) is an upcoming NASA Small Explorer mission focused on exploring the 0.2-5 MeV energy range in the electromagnetic spectrum. This MeV gap has long been underexplored due to technical challenges, in particular the high instrument and astrophysical backgrounds in this energy regime. COSI aims to conduct spectroscopy, imaging, and polarimetry of cosmic gamma-ray sources to study galactic positrons, galactic element formation, accreting black holes, and contribute to multi-messenger astrophysics. COSI utilizes sixteen germanium detectors (GeDs) for high-resolution spectroscopy and an Anti-Coincidence Subsystem (ACS) comprised of twenty-two Bismuth Germanate (BGO) scintillation crystals coupled with Silicon Photomultipliers (SiPMs). The ACS provides active shielding, detects $\gamma$-rays escaping the GeDs, monitors background radiation, and triggers transient alerts. Achieving a low energy threshold for the ACS is crucial for anti-coincidence vetoing and detecting transients. For COSI’s ACS, SiPMs have been selected over photomultiplier tubes (PMTs) due to their low voltage, weight, and volume. Contrary to other scintillators which prefer a large number of SiPMs to read out the signal, we find that a 3$\times$3 SiPM array centered on the narrow side of a BGO scintillator is enough to provide the best possible energy threshold. In this research, we provide an overview of the COSI ACS detector system, and report on results from various performance tests of the BGO detectors and the flight-like readout system.