  There is solid theoretical and observational motivation behind the idea of
scale-invariance as a fundamental symmetry of Nature. We consider a recently
proposed classically scale-invariant inflationary model, quadratic in curvature
and featuring a scalar field non-minimally coupled to gravity. We go beyond
earlier analytical studies, which showed that the model predicts inflationary
observables in qualitative agreement with data, by solving the full two-field
dynamics of the system -- this allows us to corroborate previous analytical
findings and set robust constraints on the model's parameters using the latest
Cosmic Microwave Background (CMB) data from Planck and BICEP/Keck. We
demonstrate that scale-invariance constrains the two-field trajectory such that
the effective dynamics are that of a single field, resulting in vanishing
entropy perturbations and protecting the model from destabilization effects. We
derive tight upper limits on the non-minimal coupling strength, excluding
conformal coupling at high significance. By explicitly sampling over them, we
demonstrate an overall insensitivity to initial conditions. We argue that the
model \textit{predicts} a minimal level of primordial tensor modes set by $r
\gtrsim 0.003$, well within the reach of next-generation CMB experiments. These
will therefore provide a litmus test of scale-invariant inflation, and we
comment on the possibility of distinguishing the model from Starobinsky and
$\alpha$-attractor inflation. Overall, we argue that scale-invariant inflation
is in excellent health, and possesses features which make it an interesting
benchmark for tests of inflation from future CMB data.
