Reduced χ² between our model posteriors and the observed surface brightness (upper panel) and spectral index (bottom panel) radial profiles for different porosity fractions, colour-coded by the outermost disc radius the χ² was averaged over. Dots and diamonds are used for the ‘Ricci’ and ‘DSHARP’ mixtures, respectively. The shaded grey regions indicate where the inferred dust surface density is high enough for the disc to be in the marginally gravitational unstable regime under the assumption of a standard dust-to-gas ratio Z = 10⁻². The ‘Ricci’ mixture provides better fits than the ‘DSHARP’ one for every porosity level and suggests that dust in CI Tau can be up to 50% porous. The ‘DSHARP’ mixture, instead, can explain the data only for compact aggregates.