Abstract Atmospheric biosignature searches with JWST and the next generation of facilities (HWO, LIFE, Ariel) face a steep prior-volume problem: the parameter space of plausible rocky-planet atmospheres far exceeds the regime in which Earth-style carbon-based biospheres can plausibly operate. We propose a pre-spectroscopic filter — the Carbon-Cycle Window (CCW) — defined by the phase chemistry of CO₂ itself. Inside the CCW (216.6 K ≤ T ≤ 304.1 K, with the upper pressure bound set by the Span-Wagner CO₂ vapor curve and a lower limit set by the kinetic floor for atmospheric CO₂), CO₂ exists as a single mobile gaseous phase, allowing the carbonate-silicate cycle that has stabilized Earth’s climate over Gyr timescales to operate. Outside the CCW, CO₂ is condensed (solid or liquid) or supercritical, and the long-term carbon thermostat that supports Earth-style biology cannot function in its established form. We position the CCW relative to existing carbonate-silicate kinetic frameworks (Walker et al., 1981; Krissansen-Totton and Catling, 2017; Hakim et al., 2021; Lehmer et al., 2020) as an upstream thermodynamic envelope nested with the Arrhenius kinetic envelope; ≈84% of the CCW is also kinetically active on a 1 Gyr timescale. We provide three quantitative outputs: (i) a sensitivity analysis of the CCW priormultiplier α = log₁₀(V_total/V_CCW) across physically motivated integration domains, yielding α = 0.42–1.05 (factor 2.6–11.3 prior boost), reported honestly as a domain-dependent quantity rather than a measured posterior; (ii) a CO₂ Excess Index (CEI) defined as log₁₀(P_obs/P_eq,abiotic) computed from a decoupled Monte Carlo where biotic burial is independent of weathering, yielding 0.21 dex separation between biotic and abiotic populations with characterized variance attribution; (iii) an O₂ false-negative analysis showing 67–85% miss rates across JWST/HWO/LIFE for an active CCW biosphere, with 96% of missed detections attributable to anoxic biospheres rather than detector limits. We close with four falsifiable predictions tied to upcoming JWST/HWO/LIFE measurements.