This study presents a self-powered direct X-ray detector based on a 255-nm-thick FAPbBr3 film deposited onto a mesoporous TiO2 scaffold. The device demonstrates ultra-high operational stability and excellent repeatability, with negligible signal loss after a 26-day continuous X-ray exposure. The bulk specific sensitivity is evaluated at 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 device also exhibits trap-assisted photoconductive gain, enabling a high sensitivity. Additionally, the device is validated under the X-ray beam produced by a medical linear accelerator used for cancer treatment, demonstrating its potential for practical applications. The study highlights the advantages of using stable cations and self-powering strategies to improve the stability and performance of metal-halide perovskite-based X-ray detectors.This study presents a self-powered direct X-ray detector based on a 255-nm-thick FAPbBr3 film deposited onto a mesoporous TiO2 scaffold. The device demonstrates ultra-high operational stability and excellent repeatability, with negligible signal loss after a 26-day continuous X-ray exposure. The bulk specific sensitivity is evaluated at 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 device also exhibits trap-assisted photoconductive gain, enabling a high sensitivity. Additionally, the device is validated under the X-ray beam produced by a medical linear accelerator used for cancer treatment, demonstrating its potential for practical applications. The study highlights the advantages of using stable cations and self-powering strategies to improve the stability and performance of metal-halide perovskite-based X-ray detectors.