Engineering biosensors that sensitively recognize specific biomolecules and trigger functional cellular responses is a holy grail of diagnostics and synthetic cell biology. Biosensor design approaches have mostly focused on binding structurally well-defined molecules. Coupling the sensing of flexible compounds to complex cellular responses would considerably expand biosensor applications, but remains challenging. We developed a computational strategy for designing highly sensitive receptor biosensors of flexible peptides. Using the method, we created ultrasensitive chemotactic GPCR:peptide pairs capable of eliciting potent chemotaxis in human primary T cells. Through mutual induced fit, our flexible structure design approach enhanced peptide contacts with binding and allosteric sites in the ligand pocket to achieve unprecedented signaling efficacy and potency. The approach paves the road for designing peptide-sensing receptors and expanding the biosensor toolbox for basic and therapeutic applications.