Mass-radius relationships for TOI-1438 b (left panel) and c (right panel). The GASTLI interior structure models are flexible enough to incorporate an inner deep layer of equal parts rock and water (50% each), along with a H/He envelope containing variable metal content. We adopt an intrinsic temperature T_int = 50 K (see text) across all models. The observed masses and radii of TOI-1438 b and c support multiple possible interior configurations, from planets with Fe-rich cores and isolated irradiated hydrospheres to those with mixed rock-water interiors covered by envelopes of varying H/He and metal proportions.