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We consider scalar-vector-tensor (SVT) theories with second-order equationsof motion and tensor propagation speed equivalent to the speed of light. Underthe sub-horizon and the quasi-static approximations we find analytical formulaefor an effective dark energy fluid, i.e., sound speed, anisotropic stress aswell as energy density and pressure. We took advantage of our general,analytical fluid description and showed that it is possible to design SVTcosmological models which are degenerate with $\Lambda$CDM at the backgroundlevel while having gravity strength $G_{\rm eff}<G_{\rm N}$ at late-times aswell as non-vanishing dark energy perturbations. We implemented SVT designermodels in the widely used Boltzmann solver CLASS thus making it possible totest SVT models against astrophysical observations. Our effective fluidapproach to SVT models reveals non trivial behaviour in the sound speed and theanisotropic stress well worth an investigation in light of currentdiscrepancies in cosmological parameters such as $H_0$ and $\sigma_8$.

An effective fluid description of scalar-vector-tensor theories under the sub-horizon and quasi-static approximations