The redshift dependence of the cosmic microwave background temperature is one of the key cosmological observables. In the standard cosmological model one has $T(z)=T_0(1+z)$, where $T_0$ is the present-day temperature. Deviations from this behavior would imply the presence of new physics. Here we discuss how the combination of all currently available direct and indirect measurements of $T(z)$ constrains the common phenomenological parametrization $T(z)=T_0(1+z)^{1-beta}$, and obtain the first sub-percent constraint on the $beta$ parameter, specifically $beta=(7.6pm8.0)times10^{-3}$ at the $68.3%$ confidence level.
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