The stability of Sn−Pb perovskite semiconductors thin films remains a major challenge for their integration into efficient and durable photovoltaic devices. In this work, we demonstrate that the incorporation of methylammonium chloride (MACl) in DMSO-free Sn−Pb precursor solutions significantly enhances both the structural and operational stability of Sn−Pb perovskite films and solar cells. MACl processed films exhibit enhanced crystallinity, environmental stability and photostability, thus tackling the most critical instabilities related to the defect chemistry of tin in tin-based perovskites and halides in lead-based perovskites. We show that Cl⁻ preferentially resides at the Pb_0.5Sn_0.5I-terminated surface, reducing the formation probability of halide interstitials, preventing I₂ loss under illumination and reducing O₂ uptake under ambient air exposition. As a result, solar cells incorporating MACl-treated films maintain stable performance under maximum power point tracking for over 900 h. This work highlights the crucial role of interfaces and paves the way for more durable perovskite solar cells.