At high temperatures, oxidation is generally regarded as an unavoidable degradation process for ceramic protective coatings. Here, oxidation is shown to drive dynamic glass evolution and form a robust self-sealing barrier under extreme thermal conditions. An in situ alloyed boride–silicide composite coating is constructed, and the controlled incorporation of HfSi₂ is found to change oxide-scale development by providing additional Hf and Si sources during oxidation and promoting the early formation and continuous reconstruction of a highly sealing Hf–Si–O glassy layer. The optimized coating exhibits exceptional oxidation resistance at 1700 °C, with the oxygen permeability and carbon loss rate reduced by 61.67% and 75.26%, respectively, corresponding to an average protection efficiency of 99.94%. Mechanistically, oxidation-driven glass evolution seals pores and microcracks and slows inward oxygen diffusion, whereas appropriate HfSi₂ incorporation helps compact and stabilize the glassy layer, while excessive glass stiffening reduces high-temperature fluidity and weakens scale integrity. These findings provide a general design method for improving self-sealing oxidation protection at high temperatures.