Sn-based metal halide perovskites are promising next-generation materials for visible and near-infrared photodetection due to their eco-friendly nature and low-temperature processability. However, the accompanying challenge of Sn2+ oxidation has been restricting the operational stability. Herein, we mitigate this limitation through a competitive molecular strategy targeting solvent oxidation pathways, utilizing amino-functionalized 4-tetradecylaniline (4-TCA) to coordinate with Sn2+ sites. The incorporation of 4-TCA reduces the trap density and enhances the crystallinity of CsSnI3 films, leading to significantly improved optoelectronic properties. As such, we develop high-performance CsSnI3 perovskite photodetectors that retain nearly unchanged performance after 18 h. Our detectors exhibit a high responsivity of 354 mA W−1 under zero bias and a rise time of 2.6 μs, comparable to state-of-the-art Pb2+-free perovskite photodetectors. We further demonstrate the device potential by developing the first perovskite quadrant photodetector, capable of resolving a 30 μm light-spot displacement and highlighting the promise of CsSnI3 for position-sensitive photodetection.