Hematopoietic stem and progenitor cells (HSPCs) reside in a mechanically distinct bone marrow niche, yet how niche biomechanics shape genome stability and stress responses has been difficult to test because conventional two-dimensional (2D) culture lacks marrow viscoelasticity and uses surfaces that activate platelets, confounding hematopoietic readouts. Here, we show that this methodological gap has masked a basic principle: the marrow niche actively constrains genotoxic stress signaling in HSP