We formulate the late-transition interacting thawer (LTIT) as a late-activating, variable-coupling realization of coupled quintessence in which a canonical scalar field couples conformally only to cold dark matter (CDM). The construction is a nonuniversal dark-sector interaction: baryons and radiation remain minimally coupled, while CDM follows a conformally related metric. This structure ties three observational sectors to one microscopic mechanism—the prerecombination calibration scale, the low-redshift expansion history, and the coupled growth response. We derive the exact background equations, the exact CDM scaling identity ${\rho }_{\mathrm{c}}(a)={\rho }_{c0}{a}^{-3}C[\varphi (a)]/C({\varphi }_0)$, and the coupled CDM-scalar perturbation equations in synchronous gauge in a form suitable for Einstein-Boltzmann solvers. In the representative benchmarks, ${\mathrm{\Omega }}_{\varphi }(z_{*})\sim {10}^{-9}$, $|\mathrm{\Delta }{r}_d/r_d|\le 3.82\times {10}^{-3}$, and ${\max }_{0<z<3}|E/E_{\mathrm{\Lambda }\mathrm{CDM}}-1|\le 0.380\%$, while the quasistatic growth indicator ranges from 0.725% to 5.94%. LTIT therefore demonstrates how late activation can preserve the early calibration sector while producing a perturbation-sensitive signature, providing a concrete target for Boltzmann-level tests of interacting dark energy.

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• Cosmology