Joint marginal error contours at 1 σ and 2 σ on a subset of the cosmological parameters for a flat f(R) model with |f_R0| = 5 × 10⁻⁶ (HS6). In solid orange the combination of GC_sp and WL and in red contour lines the 3×2 pt combination of photometric probes. It can be seen that it is the spectroscopic probes driving the constraints on Ω_b, 0 and h, which in combination with WL helps break degeneracies.