<p>Revealing the atomistic origins of failure in amorphous solids remains challenging. In metal-metalloid glasses, the mechanical response is strongly governed by the breaking and reformation of covalent bonds, which cannot be adequately captured by non-reactive interatomic potentials. In this work, large-scale reactive molecular dynamics simulations using a ReaxFF potential that includes angular constraints for covalent bonding reveal a composition-dependent transition from cleavage cracking to shear banding in model Cu–Si glasses. During uniaxial tension, a highly connected, rigid Si-rich network suppresses strain dispersion, generating highly localized unstable regions characterized by shear-induced reduction in atomic number density and severe distortion of Si–Si–Si bond angles. These unstable regions serve as energetically favorable pathways along which the crack advances. As the Cu content increases, reduced angular rigidity and the enhanced connectivity of flexible Cu-centered polyhedra promote widespread shear transformation and shear band-mediated plasticity. In contrast, a modified embedded-atom method potential, which intrinsically overestimates the angular flexibility of the Si-rich structures, fails to reproduce this transition. Our results suggest that angular rigidity and its spatial heterogeneity are key descriptors of plastic deformation in metal-metalloid glasses.</p>