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The JWST Early Release Observations (ERO) included a NIRISS/SOSS(0.6-2.8$\,\mu$m) transit of the $\sim\,$850$\,$K Saturn-mass exoplanetHAT-P-18$\,$b. Initial analysis of these data reported detections of water,escaping helium, and haze. However, active K dwarfs like HAT-P-18 possesssurface heterogeneities $-$ starspots and faculae $-$ that can complicate theinterpretation of transmission spectra, and indeed, a spot-crossing event ispresent in HAT-P-18$\,$b's NIRISS/SOSS light curves. Here, we present anextensive reanalysis and interpretation of the JWST ERO transmission spectrumof HAT-P-18$\,$b, as well as HST/WFC3 and $\textit{Spitzer}$/IRAC transitobservations. We detect H$_2$O (12.5$\,\sigma$), CO$_2$ (7.3$\,\sigma$), acloud deck (7.4$\,\sigma$), and unocculted starspots (5.8$\,\sigma$), alongsidehints of Na (2.7$\,\sigma$). We do not detect the previously reported CH$_4$($\log$ CH$_4$ $<$ -6 to 2$\,\sigma$). We obtain excellent agreement betweenthree independent retrieval codes, which find a sub-solar H$_2$O abundance($\log$ H$_2$O $\approx -4.4 \pm 0.3$). However, the inferred CO$_2$ abundance($\log$ CO$_2$ $\approx -4.8 \pm 0.4$) is significantly super-solar andrequires further investigation into its origin. We also introduce new stellarheterogeneity considerations by fitting for the active regions' surfacegravities $-$ a proxy for the effects of magnetic pressure. Finally, we compareour JWST inferences to those from HST/WFC3 and $\textit{Spitzer}$/IRAC. Ourresults highlight the exceptional promise of simultaneous planetary atmosphereand stellar heterogeneity constraints in the era of JWST and demonstrate thatJWST transmission spectra may warrant more complex treatments of the transitlight source effect.

Near-Infrared Transmission Spectroscopy of HAT-P-18$\,$b with NIRISS: Disentangling Planetary and Stellar Features in the Era of JWST