I summarise constraints on new forces in the dark sector, originating from interactions with a dark energy scalar field. For standard constant couplings, cosmological observations constrain any new forces between dark matter particles to be very small, which is a challenge for quintessence model building in string theory. Instead, I propose to consider coupling functions with possess a minimum at finite field values. The effective gravitational constant between dark matter particles grows with time and are consistent with observations of CMB anisotropies and large scale structures. Such couplings might also alleviate the tension between the swampland-de Sitter conjecture and the properties of the quintessence potential. Observational signatures of violations of the equivalence principle in the dark sector are expected in the non-linear regime on intermediate or small scales.