  We derive causality constraints on the simplest scalar-tensor theories in
which black holes differ from what General Relativity predicts, a scalar
coupled to the Gauss-Bonnet or the Chern-Simons terms. Demanding that time
advances are unobservable within the regime of validity of these effective
field theories, we find their cutoff must be parametrically of the same size as
the inverse Schwarzschild radius of the black holes for which the non-standard
effects are of order one. For astrophysical black holes within the range of
current gravitational wave detectors, this means a cutoff length at the km. We
further explore the leading additional higher-dimensional operators potentially
associated with the scale of UV completion and discuss their phenomenological
implications for gravitational wave science.
