Solid-state batteries could enable improved energy density and enhanced safety compared to Li-ion batteries. The interfaces between many solid-state electrolytes (SSEs) and lithium metal are (electro)chemically unstable, however, and we lack a detailed understanding of how interfacial transformations relate to electrochemical degradation during cycling. Here, we characterize the (electro)chemical reaction processes that occur at the interface between Li_1.4Al_0.4Ge_1.6(PO₄)₃ (LAGP) and lithium using in-situ transmission electron microscopy and ex-situ techniques, and we show that the growth of a new phase at the interface is intrinsically linked to chemo-mechanical degradation of the cell. Lithium insertion into LAGP at the interface drives the transformation to form an amorphous phase with expanded volume. In symmetric Li/LAGP/Li cells, the evolution of mechanical stress due to this transformation ultimately causes fracture of the SSE and an associated impedance increase. The morphology of the transformed interphase layer is highly dependent on the applied current density, and the interphase morphology significantly influences mechanical stability. Unlike many other SSEs (such as garnets), cell failure is not dictated by short circuiting. Instead, failure in LAGP is governed by the major increase in impedance as cracks are formed and continue to grow. This work shows that the nature of the reaction at the SSE/Li interface plays a crucial role in determining chemo-mechanical degradation mechanisms, with implications for understanding degradation in the whole class of SSE materials with unstable interfaces.