Comparison between ECLIPSE-Xλ and the R-TLSE approximation for the JWST/NIRISS SOSS transmission spectrum of LHS 1140 b. Top: combined R ~ 100 NIRISS spectrum (gray points with error bars), together with the best-fitting stellar-contamination models. The magenta curve shows the self-consistent ECLIPSE-Xλ fit that includes wavelength-dependent limb darkening, the cyan curve shows the corresponding fit with limb darkening set to zero (LDCs = 0), and the black curve reproduces the TLS-only stellar-contamination model from the R-TLSE reference retrieval (see Cadieux et al. 2024). The small-scale structure along the model curves follows atomic and molecular absorption bands in the underlying PHOENIX spectra: TiO/VO and metal lines dominate in the optical, while broad features in the NIR are mainly shaped by H₂O bands and CO absorption complexes longward of ~2.2 μm, i.e., precisely the wavelength ranges where exoplanet transmission spectra are usually interpreted in terms of H₂O, CH₄, and CO absorption. Bottom: 1D marginal distributions for the stellar-contamination parameters in the ECLIPSE-Xλ fits, for the cases with limb darkening (top row, magenta) and with LDCs= 0 (bottom row, cyan). From left to right, panels show the filling factor and temperature of spots (f_spot, T_spot) and faculae (f_fac, T_fac). Vertical lines indicate the median (solid) and 16th–84th percentile (dashed) intervals reported in Table 2.