Six Birds for Neutrino Mass: Lens-Swap Stability and Packaging Audits

Abstract We investigate a lens-swap instability in cosmological neutrino-mass inference highlighted by arXiv:2601.16277. Replacing the SPT-3G 2018 TT/TE/EE likelihood with SPT-3G D1, while keeping the rest of the setup matched, can tighten constraints on the summed neutrino mass (sum m_nu). We treat this as a packaging-stability problem and run directional cross-lens audits, localization by spectrum and multipole, and a common-staging support control. The mismatch is large and asymmetric, remains after profiling, and is concentrated in high-l TT structure within overlap. Staging changes are small, so support differences alone do not explain the effect. Under DESI DR2 BAO, the D1 swap tightens the 95 percent upper bound on sum m_nu in both tested baselines, with a larger shift in SPT-only plus DESI and a smaller shift in the Planck-subset baseline plus DESI. We present an audit-first, reproducible diagnosis of likelihood disagreement, not a claim of pipeline root cause or a definitive sum m_nu value.