Composite anodes of Li₃PS₄ glass+Li–Si alloy (Type 1) and Li₃N+LiF+Li–Si alloy (Type 2) are prepared for all‐solid‐state batteries with Li₃PS₄ (LPS) glass electrolyte and sulfur/LPS glass/carbon composite cathode. Using a three‐electrode system, the anode and cathode potentials are separated, and their polarization resistances are individually traced. Even under high‐cutoff‐voltage conditions (3.7 V), Type 1 and 2 cells are stably cycled without voltage noise for >200 cycles. Although cathode polarization resistance drastically increases after 3.7 V charge owing to LPS oxidation, LPS redox behavior is fairly reversible upon discharge–charge unlike the non‐composite alloy anode cell. Time‐of‐flight secondary ion mass spectrometry analysis reveals that the enhanced cyclability is attributed to uniform Li–Si alloying throughout the composite anode, providing more pathways for lithium ions even when these ions are over‐supplied via LPS oxidation. These results imply that LPS‐based cells can be reversibly cycled with LPS redox even under high‐cutoff voltages, as long as non‐uniform alloying (lithium dendrite growth) is prevented. Type 1 and 2 cells exhibit similar performance and stability although reduction product is formed in Type 1. This work highlights the importance of alloy anode design to prevent chemo‐mechanical failure when cycling the cell outside the electrochemical stability window.