Introduction: The flexibility and fracture resistance of nickel-titanium (NiTi) files have revolutionised root canal preparation—but file fractures caused by cyclic fatigue or torsional failure are challenging. Thermal pretreatments aim to improve fatigue resistance. This study compared the cyclic fatigue resistance (CFR) of four novel reciprocating NiTi file systems with low- versus high-temperature thermal modification under simulated clinical conditions.