<p>This manuscript presents an ab initio investigation of the fundamental properties of the full Heusler alloys K₂CdSn and Rb₂CdSn, to assess their structural stability and optoelectronic characteristics. Using density functional theory (DFT), the structural, electronic, and optical properties of these compounds were systematically explored. The structural analysis confirmed their mechanical stability and crystallographic integrity within the full Heusler phase. Electronic band-structure calculations demonstrated semiconducting properties with appropriate band-gap characteristics, suggesting possibilities for solar energy harvesting. The optical properties, comprising the dielectric constant, absorption coefficient, optical conductivity, refractive index, reflectivity, and energy loss function, have been meticulously analysed. The findings demonstrate significant optical absorption in the visible spectrum, an elevated dielectric response, and advantageous charge-transport properties, endorsing their use in next-generation photovoltaic devices, as the material effectively maintains photogenerated charge-carrier transport with minimal energy loss. The findings offer atomistic-level insights into K₂CdSn and Rb₂CdSn, underscoring its potential as candidate materials for next-generation solar systems.</p>