  We show that the prediction for the primordial tensor power spectrum cannot
be modified at leading order in derivatives. Indeed, one can always set to
unity the speed of propagation of gravitational waves during inflation by a
suitable disformal transformation of the metric, while a conformal one can make
the Planck mass time-independent. Therefore, the tensor amplitude unambiguously
fixes the energy scale of inflation. Using the Effective Field Theory of
Inflation, we check that predictions are independent of the choice of frame, as
expected. The first corrections to the standard prediction come from two parity
violating operators with three derivatives. Also the correlator
$<\gamma\gamma\gamma>$ is standard and only receives higher derivative
corrections. These results hold also in multifield models of inflation and in
alternatives to inflation and make the connection between a (quasi)
scale-invariant tensor spectrum and inflation completely robust.
