This article presents a study on the development of ultra-stable self-powered direct X-ray detectors based on submicrometer-thick metal-halide perovskite films. The detectors, fabricated from FAPbBr₃ films deposited on mesoporous TiO₂ scaffolds, demonstrate exceptional operational stability, with negligible signal loss after 26 days of continuous X-ray exposure. The detectors achieve a record bulk sensitivity of 7.28 C Gy⁻¹ cm⁻³ at 0 V, an unprecedented value for thin-film-based photoconductors and photodiodes for "hard" X-rays. The study also shows that these detectors can operate under X-ray beams produced by a medical linear accelerator used for cancer treatment, highlighting their potential for real-world applications. The stability and performance of the detectors are attributed to the combination of self-powered operation and the superior chemical and structural stability of the FAPbBr₃/m-TiO₂ stack. The results demonstrate that submicrometer-thick FAPbBr₃ films can withstand prolonged X-ray exposure with minimal degradation, making them promising candidates for high-performance X-ray detection systems. The study also highlights the role of trap-assisted photoconductive gain in achieving high sensitivity and the importance of internal electric fields in enabling efficient charge collection. The findings suggest that submicrometer-thick perovskite films could offer significant advantages over traditional X-ray detectors in terms of sensitivity, stability, and operational efficiency.This article presents a study on the development of ultra-stable self-powered direct X-ray detectors based on submicrometer-thick metal-halide perovskite films. The detectors, fabricated from FAPbBr₃ films deposited on mesoporous TiO₂ scaffolds, demonstrate exceptional operational stability, with negligible signal loss after 26 days of continuous X-ray exposure. The detectors achieve a record bulk sensitivity of 7.28 C Gy⁻¹ cm⁻³ at 0 V, an unprecedented value for thin-film-based photoconductors and photodiodes for "hard" X-rays. The study also shows that these detectors can operate under X-ray beams produced by a medical linear accelerator used for cancer treatment, highlighting their potential for real-world applications. The stability and performance of the detectors are attributed to the combination of self-powered operation and the superior chemical and structural stability of the FAPbBr₃/m-TiO₂ stack. The results demonstrate that submicrometer-thick FAPbBr₃ films can withstand prolonged X-ray exposure with minimal degradation, making them promising candidates for high-performance X-ray detection systems. The study also highlights the role of trap-assisted photoconductive gain in achieving high sensitivity and the importance of internal electric fields in enabling efficient charge collection. The findings suggest that submicrometer-thick perovskite films could offer significant advantages over traditional X-ray detectors in terms of sensitivity, stability, and operational efficiency.