Antigen (Ag) escape is a frequent mechanism of relapse after CAR-T therapy, even though only ∼1% of leukemic and ∼0.1% of lymphoma cells are Ag⁻ at baseline. In this study, we modeled extreme Ag heterogeneity (>20%) to define how Fas/FasL-dependent bystander killing contributes to tumor clearance. Across patient cohorts, Fas expression predicted survival after CD19 CAR-T therapy, particularly in CD19-low disease. In both murine and human systems, Fas-dependent bystander killing required Ag stimulation and cell contact, operated within a defined therapeutic window, and could eradicate large fractions of Ag⁻ tumors in vivo . Pharmacologic potentiation with inhibitor of apoptosis protein antagonists or genetic stabilization of CAR-T membrane-bound FasL enhanced bystander killing but simultaneously induced CD4⁺ T cell fratricide, which was rescued by CAR-T Fas knockout. Importantly, Fas sensitization also enabled bispecific antibody-redirected T cells to mediate bystander killing in resistant tumors. Finally, targeting tumor-associated macrophages triggered Fas-dependent clearance of neighboring tumor cells. These findings establish Fas-mediated bystander killing as a generalizable and therapeutically actionable axis to prevent Ag escape and broaden the scope of targeted T cell therapies.