Fast electrons in partially ionized plasmas lose energy through inelastic collisions with bound electrons. While the mean energy loss is well described by stopping-power theory, fluctuations associated with discrete excitation and ionization events produce energy straggling and an additional longitudinal diffusion in momentum space. We incorporate this effect into fast-electron kinetics through a derived Fokker–Planck operator whose coefficients are obtained from ab initio quantum many-body simulations. We demonstrate that neglecting inelastic energy diffusion in partially ionized D–Ar plasmas can underestimate primary runaway-electron generation by several orders of magnitude. This research was supported by R&D Program of "Optimal Basic Design of DEMO Fusion Reactor, CN2502-1" through the Korea Institute of Fusion Energy (KFE) funded by the Government funds, and also by the Technology Development Projects for Leading Nuclear Fusion through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. RS-2023-00281276).