Material extrusion additive manufacturing (AM) of thermoset resins has advanced rapidly in recent years, motivated by the superior stiffness, chemical resistance, and thermal stability of many thermoset resins when compared to thermoplastic polymers that are typically used in fused filament fabrication. Despite these advances, most thermoset feedstocks that have been developed for material extrusion AM still lack sufficient thermal stability for use in autoclave processes or as structural components in hot automotive or aerospace applications. This work addresses that need by designing and characterizing a semi-solid epoxy/nanoclay composite feedstock for material extrusion AM that achieves glass transition temperature above 200 °C while balancing the rheological properties required for printing with the stability required for curing at elevated temperature. A two-zone warm print head enables printing of the semi-solid formulation while maintaining extended pot life. The resulting composite possesses a Tg between 202 °C and 205 °C (measured via dynamic mechanical analysis and thermomechanical analysis, respectively), a heat deflection temperature of 240 °C, flexural modulus of 4.26 GPa, and flexural strength of 130 MPa along the print direction.