Recent years have witnessed significant advances in all-solid-state lithium batteries (ASSLBs). However, soft breakdown hidden in ASSLBs has been overlooked in most previous research. Moreover, existing assessment criteria are insensitive to detecting soft breakdown. Here, we first discuss the current status of ASSLBs and highlight the challenges of evaluating the soft breakdown phenomenon with the existing evaluation method. A simple but effective strategy—cyclic voltammetry—is then proposed to diagnose soft breakdown in all-solid-state symmetric cells. To establish a standard testing protocol, several critical parameters that have not been well emphasized thus far, including areal capacity, thickness, and porosity of solid electrolytes, are numerically analyzed to understand their significant effect on the energy density of practical all-solid-state pouch cells. With these understandings, we establish a definitive testing benchmark with the aim of guiding the research efforts toward in-depth scientific understanding and practical engineering design. Display omitted To develop next-generation energy storage systems with high energy density and unprecedented safety, all-solid-state lithium batteries (ASSLBs) that replace conventional flammable organic liquid electrolytes with solid-state electrolytes (SSEs) have been revived in academia and industry. However, soft breakdown hidden in ASSLBs has not been well realized, which generally leads to unreliable conclusions and significantly retards ASSLB development. To exclude this phenomenon, we propose a simple but effective method—cyclic voltammetry—to diagnose the soft breakdown in ASSLBs. Moreover, a low-frequency electrochemical impedance analysis is employed to quantify the soft breakdown phenomenon. Finally, a standard testing protocol is suggested, which could help this community obtain reliable and comparable results in the future. This work's fundamental understanding and established assessment metrics are also applicable to other metal-based all-solid-state batteries. Soft breakdown hidden in ASSLBs has been overlooked in most previous research. Here, we propose a simple but effective strategy—cyclic voltammetry—to diagnose soft breakdown in all-solid-state batteries. Moreover, low-frequency electrochemical impedance spectroscopy is employed to quantify the soft breakdown. With this understanding, we establish a standard testing protocol, which could be used to unify future research endeavors if adopted. This work provides new insights into ASSLBs and establishes assessment metrics for this community.