How robust is the preference for dynamical dark energy? A pre-registered specification-curve analysis of DESI DR2 BAO, CMB and Type Ia supernovae

DESI DR2 BAO combined with CMB and Type Ia supernova data prefers a time-evolving dark-energy equation of state (w0wa) over a cosmological constant, at 2.8-4.2 sigma depending on the supernova sample. We pre-register a specification-curve analysis that computes this preference on a single public pipeline (Cobaya + CAMB) over the full set of reasonable analysis choices: supernova sample (Pantheon+, Union3, DES-SN5YR, DES-Dovekie), low-redshift calibration offsets, redshift cuts, removal of single BAO tracers, CMB lensing choices, dark-energy parametrisation (CPL, non-phantom, JBP, binned w), spatial curvature, neutrino mass, lensing amplitude and priors. The note fixes the specification grid, the metrics (profile Delta chi2 and equivalent sigma, Savage-Dickey Bayes factors, AIC/BIC, parameter shifts, Omega_m tension) and explicit decision rules ("robust", "fragile", "inconclusive") before the grid is run. At deposit time only the baseline reproduction had been computed: CMB+DESI 2.8 sigma, +Pantheon+ 2.5, +Union3 3.6, +DES-SN5YR 4.0, +DES-Dovekie 3.1 sigma (first-pass minimisation, 0.2-0.3 sigma below the published DESI values; a refinement stage is specified). The record includes the code, data checksums and the compressed CMB prior derived from our Planck PR4 + ACT DR6 lensing chain. The analysis was carried out with the assistance of an AI model (Claude, Anthropic); see the section "Author contributions and use of AI" in the note.