Synthetic spectra with grey clouds based on VULCAN chemistry models for HAT-P-12 b. From top to bottom, the metallicity is 1, 10, and 60 times solar. Models with carbon-to-oxygen ratios of 0.25 (orange), 0.50 (black), and 0.75 (green) with photochemistry (solid lines) and without photochemistry (dotted lines) are shown. An offset has been applied to the reference radius-pressure points of the model spectra to match the observed NIRSpec transit depths, and an offset of 0.09% has been added for visual purposes. The JWST NIRSpec G395M data (TEATRO reduction) and the HST WFC3 G141 data are shown in light grey. The coloured regions correspond to the contribution of individual species to the model spectrum with C/O of 0.50 and photochemistry, which is also superimposed on the data.