Existing fiber-based fire detectors, while capable of rapid response, often suffer from limited flame retardancy and poor long-term durability. To address these limitations, this work proposes a coaxial MXene fiber (CoaxMXeneFib) inspired by the sunflower stem. It features a polyvinyl alcohol (PVA)/Ti3C2Tx MXene semiconductor core for temperature-responsive detection, encapsulated by an outer flame-retardant PVA sheath for protection. By strategically incorporating boric acid as a crosslinker during coaxial wet-spinning, a gradient interface was formed, enhancing the tensile strength from 0.56 MPa (core only) to 19.9 MPa for CoaxMXeneFib while effectively delaying core oxidation (only 1.23% resistivity increase after 10 days at 80°C). Upon heating, PVA catalyzes MXene oxidation into a C/N-TiO2 network, enabling ultrafast fire warning (<2 s), while the generated TiO2 promotes sheath carbonization, achieving a high limiting oxygen index (LOI) of 44.6%. The fiber demonstrates excellent stability under harsh conditions (acid, alkali, solvent, and UV-humidity) with only minor performance degradation in alkaline environments. It also maintains stable warning functionality after 15 000 friction cycles, 1000 bending cycles, and 30 washing cycles. Integrating rapid response, high flame retardancy, mechanical robustness, and environmental durability, the CoaxMXeneFib offers a promising solution for next-generation flexible fire-safety systems.