<p>The gastrointestinal tract contains complex fluids, such as mucus, chyme, and water, that can significantly influence capsule robot locomotion by reducing friction or introducing hydrodynamic drag. This study presents a bidirectional fluid-structure interaction model that captures the dynamics of a vibro-impact capsule self-propelling through a fluid-filled small intestine. The model couples the motion of the magnetically actuated capsule, the viscoelastic deformation of the intestinal wall, and a gas-liquid two-phase flow field. Numerical predictions were systematically validated against experimental measurements under controlled laboratory conditions. The results show that an increased liquid volume fraction generates stronger resistance to capsule motion, more so than fluid viscosity alone, by causing fluid accumulation and vortex formation, thereby elevating hydrodynamic pressure and drag. Moreover, capsule performance is improved with higher excitation frequencies and duty cycles, enhancing both propulsion and motion robustness. This work provides a validated numerical platform for designing and optimising magnetically driven capsule robots, advancing their potential for diagnostic and therapeutic applications in the gastrointestinal tract.</p>