Relative error distributions for the best-fit parameters in the Euclid+CL (purple) and Euclid+CL+LoS (orange) mocks. We show the relative error distribution for mass (left panel); concentration (right panel: top-left) under the assumption of the sphNFW model; and mass (right panel: bottom-left); concentration (right panel: top-right); and axis ratio (right panel: bottom-right) under the assumption of the eNFW₄ model. We report the difference of the means (Δμ) and it’s bootstrapped uncertainty on the top right of each panel. For Euclid-like data, LoS structure positively biases mass estimates with the sphNFW model on the level of +5.3 ± 1.4%, which is significant at 3.5σ.