T cells are triggered when the T-cell receptor (TCR) encounters its antigenic ligand, the peptide-major histocompatibility complex (pMHC), on the surface of antigen presenting cells (APCs). Because T cells are highly migratory and antigen recognition occurs at an intermembrane junction where the T cell physically contacts the APC, there are long-standing questions of whether T cells transmit defined forces to their TCR complex and whether chemomechanical coupling influences immune function. Here we develop DNA-based gold nanoparticle tension sensors to provide, to our knowledge, the first pN tension maps of individual TCR-pMHC complexes during T-cell activation. We show that naïve T cells harness cytoskeletal coupling to transmit 12-19 pN of force to their TCRs within seconds of ligand binding and preceding initial calcium signaling. CD8 coreceptor binding and lymphocyte-specific kinase signaling are required for antigen-mediated cell spreading and force generation. Lymphocyte function-associated antigen 1 (LFA-1) mediated adhesion modulates TCR-pMHC tension by intensifying its magnitude to values >19 pN and spatially reorganizes the location of TCR forces to the kinapse, the zone located at the trailing edge of migrating T cells, thus demonstrating chemomechanical crosstalk between TCR and LFA-1 receptor signaling. Finally, T cells display a dampened and poorly specific response to antigen agonists when TCR forces are chemically abolished or physically "filtered" to a level below ∼12 pN using mechanically labile DNA tethers. Therefore, we conclude that T cells tune TCR mechanics with pN resolution to create a checkpoint of agonist quality necessary for specific immune response.