Nanowires are versatile nanostructures, which allow an exquisite control over bandgap energies and charge carrier dynamics making them highly attractive as building blocks for a broad range of photonic devices. For optimal solutions concerning device performance and cost, a crucial element is the selection of a suitable material system which could enable a large wavelength tunability, strong light interaction and simple integration with the mainstream silicon technologies. The emerging GaBi_xAs_1−x alloys offer such promising features and may lead to a new era of technologies. Here, we apply million-atom atomistic simulations to design GaBi_xAs_1−x/GaAs core–shell nanowires suitable for low-loss telecom-wavelength photonic devices. The effects of internal strain, Bi Composition (x), random alloy configuration, and core-to-shell diameter ratio (ρ_D) are analysed and delineated by systematically varying these attributes and studying their impact on the absorption wavelength and charge carrier confinement. The complex interplay between x and ρ_D results in two distinct pathways to accomplish 1.55 μm optical transitions: either fabricate nanowires with ρ_D ≥ 0.8 and x ∼ 15%, or increase x to ∼30% with ρ_D ≤ 0.4. Upon further analysis of the electron hole wave functions, inhomogeneous broadening and optical transition strengths, the nanowires with ρ_D ≤ 0.4 are unveiled to render favourable properties for the design of photonic devices. Another important outcome of our study is to demonstrate the possibility of modulating the strain character from a compressive to a tensile regime by simply engineering the thickness of the core region. The availability of such a straightforward knob for strain manipulation without requiring any external stressor component or Bi composition engineering would be desirable for devices involving polarisation-sensitive light interactions. The presented results document novel characteristics of the GaBi_xAs_1−x/GaAs nanowires with the possibility of myriad applications in nanoelectronic and nanophotonic technologies.