Abstract The discovery of neutrino oscillations and masses provides strong motivation to extend the Standard Model by including right-handed neutrinos, which lead to heavy neutrino states that could exist at the electroweak scale. These states may also be influenced by new high-scale, weakly interacting physics. Incorporating right-handed neutrinos into an effective field theory framework -the νSMEFT- offers a systematic approach to study the phenomenology of heavy neutrinos in current and upcoming experiments. In this work, we present the first prospective 95% exclusion plots achievable at a future lepton collider operating at a center-of-mass energy of s = 0.5 TeV for what we term the agnostic νSMEFT scenario. This study focuses on the high-mass regime where the heavy neutrino N decays promptly into leptons and jets. Specifically, we analyse the processes e + e − → νN → νμ − μ + ν and e + e − → νN → νμ −jj, deriving the exclusion regions in the α Λ 2 versus m N parameter space. When compared to prospective limits for the LHeC, we find that the semi-leptonic process with final jets in a lepton collider offers the greatest sensitivity, even with a straightforward cut-based analysis. The expected bounds are as stringent as those considered in recent studies for the low-mass regime where the N may be long-lived and detectable via displaced decay searches, both at the LHC and future colliders.