We derive an exact radial monotonicity law for static, spherically symmetric spacetimes in metric $f(R)$ gravity. For $d{s}^2=A(r)d{t}^2-d{r}^2/B(r)-r^{2}d{\mathrm{\Omega }}^2$, the matter contribution and the scalaron Hessian combine into an effective radial-convergence numerator that fixes the derivative of $\mathcal{Q}=B/A$. On every connected static interval with $A>0$, $B>0$, and $f_R\equiv df/dR>0$, its sign therefore determines the monotonicity of $\mathcal{Q}$. The integrated identity retains the finite, generally nonzero value of $B/A$ at a regular nondegenerate Killing horizon, consequently, a fixed-sign convergence condition orders the horizon end point ratios rather than excluding two horizons. A zero-integral obstruction arises for equal end point values, including boundaries where $B\to 0$ while $A$ remains finite and nonzero. Saturation is equivalent to $B/A=\mathrm{const}$, and in vacuum requires a scalaron profile linear in the areal radius. We illustrate the strict nonsaturated branch, within the general relativity sector, using the exact constant-density stellar interior, and apply the equality and consistency diagnostics to the constant-$X$ and power-law solutions of Multamäki and Vilja [Spherically symmetric solutions of modified field equations in $f(R)$ theories of gravity, Phys. Rev. D 74, 064022 (2006)]. In particular, in the Schwarzschild–de Sitter two-horizon parameter range, one constant-$X$ solution crosses $f_R=0$ inside the complete static patch, while a direct substitution into the original radial equation exposes an unresolved exponent mismatch in the displayed power-law family. The results provide a model-independent static-sector diagnostic and a precise starting point for a future horizon-regular treatment of black hole interiors.

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