Comparison between the equilibrium chemical profiles and free chemistry VMRs inferred via our POSEIDON retrieval analysis. We plot as the colored lines the equilibrium chemistry VMRs as a function of pressure, and as colored points the corresponding VMRs from our free retrieval. We plot error bars to represent the 1σ upper and lower bounds for H₂O, CO₂, and H₂S, whereas we plot left-pointing arrows to represent the 2σ upper limits for CO and CH₄. While our equilibrium chemistry retrieval prefers a low C/O ratio that approaches the lower end of the prior range in conjunction with cloud-muted features, our free retrieval instead prefers a low computed C/O ratio of 0.006 with patchy cloud coverage. This value is calculated from the stoichiometric sum of the freely retrieved molecular VMRs and is not the equilibrium-retrieval C/O posterior shown in Figure 4; because CO is not constrained, the free-retrieval value should not be interpreted as a robust elemental abundance ratio. The discrepancy between our equilibrium and free chemistry retrievals indicates that disequilibrium chemistry is affecting HAT-P-1 b’s atmosphere, rendering equilibrium chemical retrievals inadequate for the purpose of estimating the planet’s atmospheric properties.