This paper derives a boundary correction to the effective action governing massless propagation in relativistic quantum field theory. Propagation along null worldlines lacks a proper-time parameter, obstructing intrinsic transport of internal degrees of freedom, a limitation usually treated as conceptual rather than quantitative. We show that this structural feature admits a concrete expression as a boundary contribution that supplements, but does not modify, the standard bulk action. The correction preserves gauge and Lorentz invariance, introduces no new degrees of freedom, and leaves dispersion relations and bulk equations of motion unchanged. Its effects appear only as suppressed modifications of global phase and polarization transport along null propagation. All asymptotic scattering amplitudes and experimentally tested predictions of quantum electrodynamics remain unaffected. The result provides a precise quantitative marker for the conditional applicability of particle-based descriptions to massless modes within a conventional field-theoretic framework.