We investigate the observational constraints on $\alpha$-attractor inflationary models and their postinflationary reheating dynamics in light of the latest cosmic microwave background (CMB) data from ACT DR6 combined with Planck18, $\mathrm{BICEP}/Keck$ 2018, and DESI (collectively denoted P-ACT-LB-BK18). Focusing on both E- and T-type attractor potentials, we analyze how inflationary observables, namely the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$, are indirectly influenced by reheating parameters such as the reheating temperature $T_{\mathrm{RH}}$, the inflaton equation of state $w_{\varphi }$, and consequently the inflaton’s couplings to Standard Model (SM) particles. We incorporate constraints from primordial gravitational wave (PGW) overproduction via $\mathrm{\Delta }{N}_{\mathrm{eff}}$ bounds, which are particularly relevant for stiff postinflationary dynamics. Additionally, we include theoretical constraints from Coleman-Weinberg (CW) one-loop radiative corrections, which impose an upper bound on the couplings, as well as inflaton self-resonance, a nonperturbative effect that can occur even in the absence of inflaton-SM interactions and imposes a strong lower limit, particularly significant for $1/3\lesssim w_{\varphi }\lesssim 0.7$. Our analysis shows that E-models allow a broad range of reheating scenarios, including matter-like reheating ($w_{\varphi }=0$), whereas the T-model yields more restrictive results and remains viable only for $w_{\varphi }>1/2$ for all three nongravitational interaction channels, $\varphi \to \overline{f}f$, $\varphi \to bb$, and $\varphi \varphi \to bb$ are considered. For stiffer equations of state, an intermediate coupling window survives, bounded from above by CW corrections or $2\sigma$ observational limits, and from below by the combined PGW and self-resonance constraints. We derive updated bounds on inflaton couplings for both decay and scattering channels and identify parameter regions consistent with successful perturbative reheating. These results establish a robust connection between inflationary dynamics, reheating physics, and particle interactions, and provide concrete targets for upcoming precision CMB observations.

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• Cosmology
• Evolution of the Universe
• Gravitational waves
• Inflation