We study primordial perturbations in Higgs–$R^2$ inflation in the presence of nonminimal kinetic mixing between the Higgs field and the scalaron. By numerically solving the multifield background and linear perturbation equations, we identify distinct dynamical regimes controlled by the Higgs nonminimal coupling ${\xi }_h$. For ${\xi }_h\sim \mathcal{O}(0.1)$, transient turning of the inflationary trajectory leads to a transfer between adiabatic and isocurvature modes, generating localized features in the primordial curvature power spectrum. In contrast, in the weak-coupling regime ${\xi }_h\ll 1$, the curvature spectrum remains nearly featureless while isocurvature perturbations do not fully decay, resulting in a residual isocurvature component at the end of inflation. We compute the associated cosmic microwave background angular power spectra and discuss the observational implications of these regimes. Our results highlight the role of multifield dynamics in shaping primordial perturbations and provide constraints on viable realizations of Higgs–$R^2$ inflation.

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• Alternative gravity theories
• Cosmic microwave background
• Cosmology
• Inflation
• Quantum fields in curved spacetime