One of the primary targets of current cosmological observations are light thermal relics of the hot big bang. Within the standard model of particle physics, cosmic neutrinos constitute an important thermal relic but more generally many well-motivated extensions of the standard models predict the existence of other light thermal relics, even more weakly coupled to ordinary matter. Recent advancements in the cosmic microwave background (CMB) measurements have enabled indirect constraints on the cosmic neutrino background. This talk explores the current status and future prospects for enhancing these constraints, with a specific focus on the phase shift in acoustic peaks of the CMB induced by neutrino fluctuations, and more generally by any free-streaming species present at recombination.