Both computational and experimental studies have shown that crack deflection and arrest can be achieved by introducing heterogeneities such as tougher or stiffer inclusions into brittle materials [1, 2, 3]. However, this strategy typically requires the fabrication of novel composite materials and is not easily applicable to existing infrastructure. In this work, we investigate noninvasive strategies for crack deflection and propagation control in brittle solids using a variational phase-field (PF) fracture framework. Focusing on Mode-I loading in three-dimensional domains, we demonstrate that selectively constraining out-of-plane displacements on the specimen surface can effectively mimic the influence of mesoscale material inclusions on crack path and stability, but without altering the bulk material. We systematically compare crack evolution across a range of displacement-constraint configurations and other external, noninvasive modifications that perturb the stress field under quasi-static conditions. The resulting crack paths, propagation, and arrest mechanisms are contrasted with those obtained from fully dynamic PF simulations, highlighting the role of inertia and radiated elastic waves. Lastly, our numerical results are evaluated against predictions from Linear Elastic Fracture Mechanics (LEFM), demonstrating the dynamic PF formulation’s capacity to capture key theoretical trends given varying degrees of three-dimensional effects on the stress state. Overall, this study suggests a practical pathway for controlling fracture behavior in existing brittle materials through externally imposed boundary modifications rather than material design. Ongoing work and recent extensions of this approach will also be discussed.