  We compare Einstein-Boltzmann solvers that include modifications to General
Relativity and find that, for a wide range of models and parameters, they agree
to a high level of precision. We look at three general purpose codes that
primarily model general scalar-tensor theories, three codes that model
Jordan-Brans-Dicke (JBD) gravity, a code that models f(R) gravity, a code that
models covariant Galileons, a code that models Ho\v{r}ava-Lifschitz gravity and
two codes that model non-local models of gravity. Comparing predictions of the
angular power spectrum of the cosmic microwave background and the power
spectrum of dark matter for a suite of different models, we find agreement at
the sub-percent level. This means that this suite of Einstein-Boltzmann solvers
is now sufficiently accurate for precision constraints on cosmological and
gravitational parameters.
