<p>Ductile fracture under bending is governed by interactions among length scales associated with the imposed deformation field, material microstructure, and defects. Here, we use dual-phase advanced high-strength steels as a model system to examine these interactions through large-scale, microstructure-based finite element calculations. The dual-phase microstructure is modeled explicitly in the near-surface region of bend specimens. The ferrite/martensite feature size and specimen thickness are systematically varied to examine structural–microstructural length-scale interactions. Non-metallic inclusions are introduced to examine defect–microstructural length-scale interactions. The results show that microstructural refinement delays strain localization and crack nucleation, improving bendability for a fixed structural length scale. However, the extent of improvement depends on sheet thickness. The calculations further show that inclusions become especially detrimental when their size exceeds the microstructural length scale. Thus, bendability is governed not by a single material or geometric length scale, but by the coupled effects of specimen geometry, microstructure, and defect length scales. The findings have implications for designing high-strength multiphase sheet materials, where microstructural refinement is often pursued to increase strength and enable thickness reduction.</p>