Rowe and colleagues discover that one-carbon (1C) metabolism rewiring occurs upon T-cell activation to support proliferation and cytolytic activity in CD8+ T cells and that supplementation of 1C donor formate rescues the dysfunctional T cells and their responsiveness to anti-PD-1 in selective tumor-infiltrated T-cell subsets. This finding represents an attractive strategy to overcome a metabolic vulnerability in the tumor microenvironment and improve the efficacy of immune checkpoint blockade. See related article by Rowe et al., p. 2566 (8).