Model transmission spectra of HD 219666 b (top panel). The dots and error bars denote simulated single-transit JWST transmission observations with NIRISS SOSS and NIRSpec G395H (wavelength coverage at bottom) for two underlying models (solid curves) at temperatures of 600 and 1000 K (see legend). CH₄ shows strongabsorption bands at 1.7, 2.3, and 3.3 μm in the 600 Kmodel; whereas CO and CO₂ show their strongest absorption features at wavelengths beyond 4 μm in the 1000 K model. Bottom panels: composition of the main species that shape the transmission spectrum. Depending on the atmospheric temperature, carbon favours either higher CH₄ (temperatures lower than ~900 K) or CO/CO₂ abundances (otherwise).