Therapeutic cancer vaccines deliver tumor antigens to elicit cytotoxic T cells capable of killing tumor cells. For maximal tumoricidal activity, the T cells require robust activation, precise trafficking to tumors, and sustained function within the tumor microenvironment. Here, we developed lipid nanoparticles (LNPs) that co-deliver mRNAs encoding a tumor antigen and an αPD-L1xαCTLA-4 bispecific checkpoint engager (Bi-chE). This platform enables in situ arming of tumor-specific T cells with Bi-chEs, resulting in potent activation of tumor-specific T cells while effectively mitigating immune-related adverse events (irAEs) associated with systemic checkpoint blockade. The armored T cells bypassed immune checkpoints and fortified T cell-tumor engagement, achieving enhanced antitumor efficacy in murine melanoma and breast cancer models. Furthermore, Bi-chE arming extended the therapeutic reach of cancer vaccination to untreated distal tumors, whereas co-delivery of tumor antigen mRNA with individual αPD-L1 and αCTLA-4 single-chain fragment mRNAs produced therapeutic effects limited to the injection site. Overall, arming tumor-specific T cells via targeted anchoring of αPD-L1xαCTLA-4 Bi-chEs represents a compelling approach to potentiate the antitumor efficacy of cancer vaccines.