Sensitivity of K2-18 b’s atmospheric composition to different data treatments and retrieval configurations. Top: comparison between POSEIDON retrievals of data reduced by the FIREFLy and exoTEDRF pipelines. Middle: comparison between POSEIDON retrievals of low-resolution (NIRISS SOSS, R ≈ 25; NIRSpec G395H, R ≈ 100) and high-resolution (pixel-level) exoTEDRF data. Bottom: comparison between the retrieval codes POSEIDON and BeAR on FIREFLy data. The retrieved model transmission spectra, plotted at R ≈ 100 for clarity, show the median (solid lines) and ± 1σ and ± 2σ credible regions (contours) corresponding to the data set(s) shown in each panel. The median retrieved offsets have been applied to the NIRSpec G395H NRS1 and NRS2 data sets. Right: the retrieved CH₄, CO₂, and DMS abundances corresponding to each retrieval. Our retrievals find broadly consistent CH₄ abundances, indicating a robust detection (the median and ±1σ abundance constraints are represented by solid and dashed lines, respectively). However, any suggestions of CO₂ and DMS depend on the specific data reduction and data resolution, indicating unreliable inferences (hence the 95% upper limits, represented by arrows).