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We present new constraints on spectator axion-${\rm U}(1)$ gauge fieldinteractions during inflation using the latest $Planck$ (PR4) and BICEP/Keck2018 data releases. This model can source tensor perturbations from amplifiedgauge field fluctuations, driven by an axion rolling for a few e-folds duringinflation. The gravitational waves sourced in this way have a stronglyscale-dependent (and chiral) spectrum, with potentially visible contributionsto large/intermediate scale $B$-modes of the CMB. We first derive theoreticalbounds on the model imposing validity of the perturbative regime and negligiblebackreaction of the gauge field on the background dynamics. Then, we determinebounds from current CMB observations, adopting a frequentist profile likelihoodapproach. We study the behaviour of constraints for typical choices of themodel's parameters, analyzing the impact of different dataset combinations. Wefind that observational bounds are competitive with theoretical ones andtogether they exclude a significant portion of the model's parameter space. Weargue that the parameter space still remains large and interesting for futureCMB experiments targeting large/intermediate scales $B$-modes.

New constraints on axion-gauge field dynamics during inflation from $Planck$ and BICEP/Keck data sets