We analyze Planck 2018 TT+TE+EE+lowE, Pantheon+, and DESI baryon acoustic oscillation data in an eight-parameter Chevallier–Polarski–Linder (CPL) extension of ΛCDM using Cobaya/CAMB with flat priors on w₀ and wₐ. Four independent Markov chains converge at R̂ − 1 < 0.01 after 53,120 accepted steps (per chain, 30% burn-in discarded), yielding w₀ = −0.967 +0.010/−0.032 and wₐ = 0.055 +0.052/−0.097 (68% CL, marginalized posteriors). The ΛCDM point (w₀ = −1, wₐ = 0) lies near the boundary of the 68% joint contour in the w₀–wₐ plane, corresponding to a mild ~1σ preference for dynamical dark energy. The Bayesian Information Criterion yields ΔBIC ≈ +0.7 relative to ΛCDM, essentially neutral on the Jeffreys scale, while the Akaike Information Criterion gives ΔAIC = −12, mildly favoring the CPL extension. We discuss how these CPL constraints may be interpreted within a chameleon scalar–tensor effective field theory (EFT) with conformal coupling F(φ) = e^(kφ/M_Pl), adopting k = 1.92 as a benchmark calibration that illustrates a region of EFT parameter space consistent with the observed w₀. The EFT parameters are not sampled directly; the present analysis is a phenomenological CPL fit with a chameleon interpretation. The chameleon screening mechanism renders the scalar fifth force undetectable in solar system and laboratory environments, preserving consistency with precision gravitational tests.