Carbon dioxide is a key gas to monitor at volcanoes because its fluctuation relative to other gases can be detected prior to eruptions, yet carbon isotopic fluctuations at volcanic summits are not well constrained. Here, we present carbon isotopes measured from plume samples collected at Stromboli volcano, Italy, by Unoccupied Aerial System (UAS). We found contrasting volcanic source δ13C in 2018 during quiescence (-0.36 ± 0.59 ‰) versus 10 days before the July 3rd 2019 paroxysm (-5.01 ± 0.56 ‰). During the buildup to the eruption, an influx of CO2-rich magma began degassing at deep levels (~100 MPa) in an open system fashion, causing strong isotopic fractionation and maintaining high CO2/St ratios in the gas. This influx occurred between 10 days prior to the event and up to several months beforehand, meaning that isotopic changes in the gas could be detected weeks to months before unrest.