Observable level consistency of M87* and Sgr A* from EHT ring diameters: a minimal forward model

We develop a fully reproducible observable level framework to test horizon scale models with Event Horizon Telescope (EHT) ring diameter constraints for M87* and Sagittarius A* (Sgr A*). Rather than reanalyzing interferometric visibilities, we use EHT data products for the ring diameter [Formula: see text] and the calibrated gravitational angular scale [Formula: see text] [Formula: see text] and map them to a baseline general relativity shadow diameter prediction. We quantify any net mismatch between the observed ring diameter and the baseline shadow scale by an effective shift parameter [Formula: see text] – 1, which aggregates emission and propagation effects at the observable level without specifying a microscopic plasma model. Calibration uncertainty in [Formula: see text] is propagated by treating [Formula: see text] as a nuisance parameter constrained by EHT and marginalizing it in the likelihood, including a split normal representation for asymmetric errors. Using EHT constraints, we obtain [Formula: see text] = 0.064±0.135 and [Formula: see text] = 0.038±0.232, both consistent with [Formula: see text] = 0 within current uncertainties. We further provide joint constraints in a common difference parameterization enabling compact cross target consistency tests that can be straightforwardly updated as improved EHT constraints become available.