Despite their different chemistries, novel energy-storage systems, e.g. Li-air, Li-S, all-solid-state Li batteries, etc., face one critical challenge of forming a conductive and stable interface between Li metal and a solid electrolyte. An accurate understanding of the formation mechanism and the exact structure/chemistry of the rarely existing benign interfaces, such as the Li/cubic-Li_7-3xAl_xLa₃Zr₂O₁₂ (c-LLZO) interface, is crucial towards enabling the use of Li metal anodes. Due to spatial confinement and structural/chemical complications, current investigations are largely limited to theoretical calculations. Here, through an in situ formation of Li/c-LLZO interfaces inside an aberration corrected scanning transmission electron microscope, we successfully reveal the interfacial chemical and structural progression. Upon contact with Li metal, the surface of the c-LLZO is slightly reduced, and simultaneously receives Li⁺ from the Li metal, thereby maintaining the charge balance. Instead of triggering decomposition reactions, a localized phase transition occurs leading to the formation of a 6 nm thick interfacial tetragonal LLZO phase that is stable over time (Figure 1). Although this interphase is ultra-thin and can be easily overlooked, it may effectively prevent further interfacial reactions without compromising the ionic conductivity. Although the cubic-to-tetragonal transition is typically undesired during LLZO synthesis, the similar structural change was found to be the likely key to the observed benign interface. These insights provide a new viewpoint for designing Li/solid electrolyte interfaces that can enable the use of Li metal anodes in next-generation batteries.