Ammonia-assisted electrolysis can lower the anode potential relative to oxygen evolution, yet realistic anion-exchange-membrane systems necessitate catalysts that enhance ammonia oxidation while maintaining rapid hydrogen evolution. Here, we develop a NiCo-MOF@SnS2 heterostructure, denoted as NCS-2, in which sulfur-vacancy-rich SnS2−x and NiCo species reorganize under bias to generate an active NiCoOOH or SnS2−x heterointerface. X-ray photoelectron spectroscopy and extended X-ray absorption fine structure, together with operando Raman spectroscopy, reveal the bias-driven formation of Ni and Co oxyhydroxides, accompanied by a SnS2-to-SnS2−x conversion, consistent with strengthened interfacial charge transfer. In alkaline electrolyte, NCS-2 delivers overpotentials of 75 mV for the hydrogen evolution reaction and lower potential of 1.30 V for the ammonia oxidation reaction at 10 mA cm−2, along with an ammonia oxidation Tafel slope of 33 mV dec−1 and stable operation. In a full anion-exchange-membrane ammonia electrolyzer with 3 cm2 electrodes, an NCS-2-based cell operates at 1.51 V at 100 mA cm−2 and reaches 1.0 A at 1.90 V in 1.0 M KOH plus 0.5 M NH4OH at 30 °C. These results highlight reconstruction-enabled heterointerfaces as a feasible route to efficient ammonia-assisted hydrogen production.