Lead-halide perovskites deliver outstanding efficiencies but are limited by the toxicity of soluble Pb2+ and emerging regulatory constraints. Fully inorganic Cu2AgBiI6 (CABI) have emerged as a lead-free, wide band gap material with strong optical absorption and solution processability, yet its device performance remains far below their optoelectronic potential. In this work, we synthesized Cu2AgBiI6-xClx with controlled amounts of chloride (x = 0-0.8), revealing chlorine-induced lattice distortion accompanied by shifts in energy level alignment and reduced Urbach energy. The resulting solar cells exhibit increased power conversion efficiency with cumulative Cl content, obtaining the best results for x = 0.6 with 1.18% at 1 sun, and 2.15% under 1000 lux. Electrical characterization reveals reduced Shockley-Read-Hall recombination, more efficient charge extraction and higher shunt resistance for Cu2AgBiI5.4Cl0.6. This particularly benefits the photovoltage and fill factor under indoor light operation, compared to CABI. Stability analyses through in situ XRD at 1000 lux and maximum power tracking, revealed a degradation mechanism linking a progressive lattice expansion and power loss in reference CABI devices. Superior retention of the crystalline structure and slower power decay was found when chlorine is present in the lattice. These results establish halide-site alloying as an effective route to engineer both lattice geometry and defect chemistry, advancing Cu-Ag-Bi halides as efficient, stable and environmentally benign wide-gap absorbers for next-generation indoor photovoltaics and tandem solar cells.