  We develop a universal approach to the one-loop effective field theory (EFT)
using the Covariant Derivative Expansion (CDE) method. We generalise previous
results to include broader classes of UV models, showing how expressions
previously obtained assuming degenerate heavy-particle masses can be extended
to non-degenerate cases. We apply our method to the general MSSM with
non-degenerate stop squarks, illustrating our approach with calculations of the
coefficients of dimension-6 operators contributing to the $hgg$ and
$h\gamma\gamma$ couplings, and comparing with exact calculations of one-loop
Feynman diagrams. We then use present and projected future sensitivities to
these operator coefficients to obtain present and possible future indirect
constraints on stop masses. The current sensitivity is already comparable to
that of direct LHC searches, and future FCC-ee measurements could be sensitive
to stop masses above a TeV. The universality of our one-loop EFT approach
facilitates extending these constraints to a broader class of UV models.
