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tokens: profiles are extrapolated beyond the edge of the grey area, since no hard strong-lensing constraints are found at these distances. Here, a clear separation between models appears: Bradac (v2), CATS (v3.1), Zitrin-NFW (v3) and our fiducial model profiles tend to keep a high density at large radii, while glafic (v3), Sharon (v3) and our external-shear model generate a lower mass profile. At very large distances, the profile from Williams (v3) drops considerably lower than all others. These effects are seen in both panels of Fig. 12 . Figure 12. The integrated (upper panel), differential (middle panel) and ratio (lower panel) mass profiles of the cluster from different studies. The ratio is computed over our fiducial model, the one with clumps in the outskirts and only gold sample. The black line surrounded by the blue-shaded region represents the mass profile for the external shear and 3 statistical uncertainties. The black line surrounded by the shaded orange region represents mass profile for the fiducial model and the 3 statistical uncertainties. The grey area represents the region where multiple images used as constraints are located. The discrepancies in the inner core could be due to the new hard constraints we add to our mass models, while the discrepancies outside the multi-imaged region may be related to different aspects of the modelling technique and their sensitivity to environmental effects. By probing the overall discrepancies in the mass profiles between different analyses, we can begin to understand the magnitude of systematic uncertainties and their overall effects on the modelling. Compared to other parametric models (v3 from CATS, glafic and Sharon), our fiducial model reaches a similar, or slightly higher rms (0.6arcsec). The main difference is a higher ratio between the number of constraints ( k ) and the number of free parameters ( n ), which can be used as a metric on the level of constraints available. Thanks to the large number of spectroscopic redshifts available for the gold systems, we obtain k / n 134/304.5 compared to 70/302.33 for CATS (v3), and 146/1001.46 for glafic (v3) (Kawamata etal. 2016 ). The ratio between constraints and free parameters in our model is comparable to the Sharon (v3) model ( k / n 108/274, Sharon private communication), but they use multiple emission knots in each galaxy as constraints, making them not strictly independent. We note that our current model is able to accommodate a large number of constraints with only a small number of free parameters, which means that we are reaching a limit (in rms) with this method. 6.4 Estimating the level of systematics in mass models Our fiducial model, containing mass clumps in the outskirts, constrained with the gold set of multiple images and released as CATS (v4) as part of the FFs model challenge, shows a statistical error of stat 1percent on the mass density profile in the cluster core (see Fig. 12 and Section 6.3 ). This error is comparable to