Structural constraints on interacting dark sector in ΛCDM-mimicking f(Q) gravity

We investigate the consistency of linear dark sector interactions within coincident f ( Q ) gravity restricting to a ΛCDM-mimicking background. We focus on a linear model supplemented by a geometrically motivated background-inert Q correction ensuring expansion history identical to ΛCDM, with all the gravitationally motivated effects arising exclusively at the perturbation level. We show that background consistency enforces a nontrivial structural restriction - any interaction proportional to the matter density is mathematically incompatible leading to a unique dark energy proportional coupling. This unique no-go result reflects the geometric origin of the effective dark energy sector in f ( Q ) gravity. The parameter space of this residual interacting sector is quantified using Markov Chain Monte Carlo analysis using late-time cosmological data including Redshift-Space Distortions, updated Cosmic Chronometers, DESI DR2 BAO and Type Ia Supernovae. The Q term modifies growth structure through a gravitational coupling, introducing perturbation level sensitivity in fσ 8 . We find that all the parameters are tightly constrained and are compatible with zero, indicating the strong observational limits on perturbation level departures from ΛCDM. The model remains statistically compatible with ΛCDM according to information criteria, while preserving a viable growth history. Our results therefore establish sharp structural and observational bounds on interacting dark energy scenarios in Symmetric Teleparallel Gravity, providing a benchmark consistency test for future high precision surveys.