Spherical Tokamak for Energy Production (STEP) requires high-field magnet designs and has therefore adopted REBCO-based high temperature superconductor (HTS). HTS enables the Toroidal Field (TF) coils to be re-mountable, which unlocks STEP’s vertical maintenance approach, however, re-mountable joints, approximately 18 GJ of stored energy, and limited space down the centre of a spherical tokamak, make the TF coils the most challenging . STEP has pursued a passive approach to TF coil quench protection in order to limit coil terminal voltage. Initial results suggest that a solution may rely on tuning internal coil resistance, which requires the development of targeted resistivity ‘partially insulating’ material, coupled with actively powered heaters to deliver TF system de-energisation without hotspots and mechanical load imbalance. The pre-conceptual inter-coil structure demonstrates acceptable stresses and deflections under steady-state operating conditions and preliminary fault scenarios, and loads are distributed to limit the tensile force on the TF centre rod. Finally, HTS must operate reliably in a high radiation environment and endure high neutron fluences to ensure commercially relevant magnet lifetimes. Initial experiments indicate that instantaneous gamma irradiation of HTS has no negative impact on current carrying capacity. Experimental programmes are underway to cold irradiate HTS to fusion-relevant fluences and to develop a cost-effective method of assuring tape irradiation resistance quality using oxygen ions as an analogue for neutrons.