We formulate a horizon-regular double-null criterion for regular inner marginal horizons in spherically symmetric metric $f(R)$ gravity. Using normalized outgoing and ingoing radial null vectors ${\ell }^{\mu }$ and $n^{\mu }$, with $n^{\mu }$ affinely parametrized, we derive an exact evolution law for the area-weighted outgoing expansion $r^2{\theta }_{(\ell )}$. Its source is controlled by the scalaron $F\equiv f_R>0$ and by a mixed quantity ${𝒫}_{\ell n}$ containing matter, scalaron derivatives, and the curvature potential. If ${𝒫}_{\ell n}\le F/r^2$ along a regular ingoing null segment issuing from a nondegenerate future outer marginal sphere, then the outgoing expansion cannot return to zero, and no second regular marginal sphere of the same family can occur on that generator. Conversely, a nondegenerate future inner marginal sphere requires the reverse inequality, so an outer–inner pair necessarily entails a source reversal and an exact integral balance. No trapped-region assumption is required. In the static limit, the criterion reduces to a horizon-regular relation involving the radial derivative of the metric function and remains valid in the degenerate case under the stated regularity conditions. It reproduces the Reissner–Nordstr"om classification and is verified in an exact charged, nonconstant-curvature $f(R)$ black hole with a nonconstant scalaron. {The source ratio is shown to coincide with a parallelly propagated component of the Einstein tensor, so the source ratio has a finite one-sided limit as $F\to 0^{+}$ when the geometry and the transported frame extend regularly. The obstruction is realized explicitly for constant-curvature vacuum black holes in any $f(R)$ model, including Starobinsky and Hu–Sawicki-type models, and for a neutral nonconstant-scalaron black hole in which the mixed source stays below threshold throughout the interior.} The resulting Cauchy-horizon statement is conditional and applies only when the candidate boundary is also a regular nondegenerate future inner marginal horizon.