IoT uplink performance is crucial for a wide variety of IoT applications such as health sensing and industrial control, which demand reliable delivery of sensor data to the cloud. However, due to the limited transmission power budget imposed on many power-constrained IoT devices, IoT uplinks are highly susceptible to cross-technology interference (CTI) caused by coexisting networks. Previous approaches to mitigating CTI have relied on MAC/PHY designs. They suffer from poor performance and limited generality in the presence of wideband CTI sources such as Wi-Fi and RF jammer, which transmit aggressively on large spectrum chunks using diverse radio technologies. This paper introduces RF-SIFTER, a general and highlyeffective system that protects low-power IoT uplinks against intensive wideband CTI. RF-SIFTER enables technology-agnostic blind beamforming to sift signals based on bandwidth, allowing IoT signals to pass through while rejecting interference wider than the IoT band. RF-SIFTER is designed as a Layer-0.5 that is transparent to IoT MAC/PHY, ensuring general applicability and practical deployability across a wide range of coexistence scenarios. RF-SIFTER is implemented on an FPGA-based software radio platform. Extensive experiments show that RF-SIFTER can improve the SINR of IoT uplink signals by up to 29 dB and increase packet delivery ratio