Higher-derivative inflationary EFTs in the Horndeski/DHOST class can harbor hidden ghost,gradient, or non-adiabatic instabilities that evade naive truncations. We formulate an FLRW-adapted Causality–Stability Tester (CST) gate suite by augmenting the Minkowski gate set withthe adiabatic gate G7 and apply it to an exact derivative-coupled attractor in ( 12 M 2P l+γX)R+αX2.In the constant-kinetic attractor sector (X = 12 ϕ20) the scalar stability window is K > 0 andG > 0, and the tensor-speed bound |cT − 1| ≲ 10−15 yields |γ| ≲ 5 × 10−16 M 2P l/|X| in thesmall-|γX|/M 2P l regime. Gate G7 excludes otherwise ghost-free and gradient-stable models withrapid variation ˙ωk/ω2k̸ ≪ 1, i.e. strong particle production in the adiabatic basis.
SIKX HILTON (Tue,) studied this question.