Steel-timber hybrid structures have emerged as a low‑carbon alternative to traditional steel-concrete composite systems. However, despite increasing research at ambient conditions, their fire performance remains poorly understood due to complex thermo-mechanical interactions at the steel-timber interface. This study presents a numerical investigation of the composite behaviour of hot-rolled steel beams supporting cross-laminated timber (CLT) slabs. Finite element modelling, validated at the component level against thermal and mechanical benchmark tests, was used to simulate a 9 m unprotected steel-CLT composite beam under ambient and elevated temperatures. The analysis evaluates the influence of varying degrees of composite action, heating regimes, material degradation, and thermal expansion on the global structural response. Results show that while composite action enhances stiffness at ambient conditions, its contribution reduces significantly at elevated temperatures. In fire, different interface conditions (full, partial, and zero composite action) exhibit similar failure temperatures for the case study, all exceeding that of the bare steel beam. The system response is strongly influenced by restrained steel thermal expansion and timber degradation at the steel-timber interface. These findings provide new insight into steel-CLT composite behaviour in fire and support the development of safe hybrid structural systems.