Transit spectra of the best-fit non-equilibrium 1D model at resolution 200 (black solid line). This is the approximate best fit on the simulated grid of metallicity, C/O, and K_zz; the values are respectively 280.7, 90.0, and 10⁶ cm² s⁻¹. They are compared to a K2–18 b observation (Madhusudhan et al. 2023) low-resolution representation with an offset of −41 ppm between NIRISS and NIRSpec data. The contributions of the considered molecules are represented as shaded colors.