The clinical success of chimeric antigen receptor (CAR) T cell therapy requires scalable, non-invasive strategies for in vivo T cell engineering. Although mRNA delivery offers a promising alternative, lipid-nanoparticle-based carriers show limited efficiency for in vivo T cell transfection and typically require antibody conjugation. Here we report an inherent T cell-activating polymer–lipid nanoparticle that enables ligand-free, efficient mRNA transfection and activation of T cells in vivo. This mRNA delivery vehicle, composed of p-toluenesulfonyl arginine (RT)-modified oligoethylenimine-based lipid nanoparticles (ERTLNPs), preferentially mediated mRNA transfection in the spleen following systemic administration. Without exogenous stimulation, ERTLNPs intrinsically activated T cells, triggering robust mRNA expression and proliferation. Mechanistically, ERTLNPs engaged the PI3K/AKT/mTOR signalling axis to reprogram T cell metabolism, promoting expansion and restraining exhaustion. The systemic delivery of mRNA encoding fibroblast activation protein CAR via ERTLNPs contributed to the in situ generation of functional CAR T cells, which efficiently eliminated pathological fibroblasts in models of cancer and fibrosis, with minimal off-target effects. This ligand-free, metabolically reprogramming mRNA delivery system provides a clinically translatable approach for in vivo CAR T cell generation.