Conditional action selection from optical cones in nonlinear electrodynamics

Characteristic cones specify propagation speeds but leave constitutive information undetermined. We give two conditional inverse results for derivative-free nonlinear electrodynamics on fixed spacetime. For existing normalized real C³ actions with positive distinct vacuum-Hessian eigenvalues, exact cone functions on a neighborhood and one regular polarization anchor determine at most one local action. An exceptional anchor with nonzero characteristic-intercept gradient supplies such a polarization constraint. The proof combines the classical characteristic kernel, action exactness and uniqueness of a linear differential system. A corollary bounds the parity-even class by two action germs; the displayed pair attains that bound. For an analytic pair related by established Fresnel-preserving transformations, a specified unit-normalized magnetic transport coefficient bounds distance from the exceptional reference with asymptotically sharp amplification 2√2 B⁻³. The bound covers all actual action sections in the declared cone class, with a second action furnishing a sharpness witness. Polarization gives a simpler discriminator, and differential constitutive data have B⁻² conditioning under their stated normalization. We also identify classical reductions of the exclusion and magnetic-calibration examples. The conclusions concern exact-data action selection and the conditioning of a specified observation record; they do not establish a new cone-ambiguity mechanism or a measurement advantage. Includes the main manuscript and technical supplement. File-specific licenses: Conditional_Action_Selection.pdf is licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Conditional_Action_Selection_Supplement.pdf is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). Each license applies only to the named file.