  By confronting the two independent Boltzmann codes CLASS and CAMB, we
establish that for concordance cosmology and for a given recombination history,
lensed CMB and matter power spectra can be computed by current codes with an
accuracy of 0.01%. We list a few tiny changes in CAMB which are necessary in
order to reach such a level. Using the common limit of the two codes as a set
of reference spectra, we derive precision settings corresponding to fixed
levels of error in the computation of a CMB likelihood. We find that for a
given precision level, CLASS is about 2.5 times faster than CAMB for computing
the lensed CMB spectra of a LambdaCDM model. The nature of the main
improvements in CLASS (which may each contribute to these performances) is
discussed in companion papers.
