Context <p>Lead-free double perovskites are actively explored as environmentally benign alternatives to lead-based halide perovskites for optoelectronic applications. In this work, the influence of halide substitution (X = I, Br, Cl) on the structural, mechanical, electronic, and optical properties of cubic K₂AgSbX₆ double perovskites is systematically investigated. Halide replacement induces a monotonic reduction in lattice parameters and a widening of the electronic band gap, increasing from the iodide to the chloride compound. Hybrid-functional calculations predict indirect band gaps ranging from 0.60 eV for K₂AgSbI₆ to 1.73 eV for K₂AgSbCl₆. Mechanical analysis reveals ductile behavior for the iodide and bromide phases, while the chloride phase is significantly stiffer and brittle. Optical calculations indicate strong absorption across the visible range, with K₂AgSbBr₆ exhibiting an optimal balance between band gap, absorption strength, and mechanical flexibility, making it particularly promising for optoelectronic and photovoltaic applications.</p> Methods <p>First-principles density functional theory calculations were performed using the CASTEP code. Structural optimization and ground-state properties were obtained using the rSCAN meta-GGA functional, while electronic band structures were refined using the HSE06 hybrid functional. Ultrasoft pseudopotentials generated within the on-the-fly scheme were employed with a plane-wave cutoff energy of 500 eV and a Γ-centered 4 × 4 × 4 Monkhorst–Pack k-point mesh. Elastic properties were evaluated via the Voigt–Reuss–Hill approach, phonon dispersions were calculated using density-functional perturbation theory, and frequency-dependent optical properties were derived from the complex dielectric function.</p>