The ability of metastatic cancer cells to invade distant tissues requires them to cross a variety of tissue boundaries, each posing distinct structural and biochemical challenges. In particular, the boundary between dense, extracellular matrix (ECM)-rich tumor tissue and surrounding stromal tissue is associated with phenotypic changes in MDA-MB-231 breast cancer cells following transmigration, including increased invasiveness and aggressiveness. It remains unclear whether this transition arises from selective, permissive filtering of pre-existing subpopulations, such as cancer stem cells, or an instructive response of the entire cell population. Here, by combining single-cell migration analysis, heterogeneity analysis of cell proliferation, and computational modeling, it is demonstrated that tumor-tissue boundaries act as instructive interfaces. Using an established 3D fibrillar collagen I matrix model of interfaces, it is shown that all cells can transmigrate the interface with no evidence of selective filtering of subpopulations. Proliferation heterogeneity remains unchanged between transmigrated and non-transmigrated cells, further supporting an instructive mechanism. Simulations confirm that the interface instructively modulates cell behavior. These results indicate that tissue boundaries can reprogram cancer cell phenotypes, representing a potentially targetable mechanism in metastatic progression.