ABSTRACT The JWST Early Release Observations (ERO) included a NIRISS/SOSS (0.6–2.8 μm) transit of the ∼ 850 K Saturn-mass exoplanet HAT-P-18 b. Initial analysis of these data reported detections of water, escaping helium and haze. However, active K dwarfs like HAT-P-18 possess surface heterogeneities – star-spots and faculae – that can complicate the interpretation of transmission spectra, and indeed, a spot-crossing event is present in HAT-P-18 b’s NIRISS/SOSS light curves. Here, we present an extensive reanalysis and interpretation of the JWST ERO transmission spectrum of HAT-P-18 b, as well as HST/WFC3 and Spitzer/IRAC transit observations. We detect H2O (12.5σ), CO2 (7.3σ), a cloud deck (7.4σ), and unocculted star-spots (5.8σ), alongside hints of Na (2.7σ). We do not detect the previously reported CH4 (log CH4 < −6 to 2σ). We obtain excellent agreement between three independent retrieval codes, which find a sub-solar H2O abundance (log H2O ≈ −4.4 ± 0.3). However, the inferred CO2 abundance (log CO2 ≈ −4.8 ± 0.4) is significantly super-solar and requires further investigation into its origin. We also introduce new stellar heterogeneity considerations by fitting for the active regions’ surface gravities – a proxy for the effects of magnetic pressure. Finally, we compare our JWST inferences to those from HST/WFC3 and Spitzer/IRAC. Our results highlight the exceptional promise of simultaneous planetary atmosphere and stellar heterogeneity constraints in the era of JWST and demonstrate that JWST transmission spectra may warrant more complex treatments of the transit light source effect.