In this Letter, we develop a numerically efficient framework for evaluating parameters in metric theories of gravity, and apply it to constrain the horizon-scale magnetic field in the Kerr-Bertotti-Robinson spacetime using the latest Event Horizon Telescope observations. The method’s adaptive ray-tracing strategy achieves near-linear computational efficiency without loss of numerical accuracy. This efficiency allows for high-precision black hole shadow modeling at a minimal computational cost and, for the first time, supports statistically robust parameter inference for arbitrary stationary black holes from horizon-scale observations. Applying the framework to the exact magnetized and rotating Kerr-Bertotti-Robinson solution [Phys. Rev. Lett. 135, 181401 (2025)], we evaluate the horizon-scale magnetic fields of M87* and Sgr A*. The analysis yields a field strength of ${93.3}_{-23.8}^{+14.7}\mathrm{G}$ for Sgr A*, consistent with the 71 G equipartition estimate from polarized Atacama Large Millimeter/submillimeter Array observations, thereby supporting the predictions of Einstein’s gravity.

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• Classical black holes
• General relativity
• Gravitation