The first 18 months of routine James Webb Space Telescope (JWST) science operations (July 2022–December 2023) generated the highest-quality exoplanet atmospheric spectra in the history of the field and, with them, the first JWST-era biosignature claims requiring formal evidentiary evaluation. The September 2023 Madhusudhan et al. report of methane (CH₄) at 5σ and carbon dioxide (CO₂) at 3σ in the atmosphere of the habitable-zone sub-Neptune K2- 18b, together with a tentative 1–2σ detection of dimethyl sulfide (DMS) — a molecule whose terrestrial atmospheric production is dominated by marine microbial activity — placed the Madhusudhan–Piette–Constantinou (2021) Hycean-world hypothesis at the centre of an emerging biosignature-evaluation debate (Madhusudhan et al., 2023, 2021). The K2-18b case is, on the 2023 evidence, the first JWST-era exoplanet for which a biosignature claim has been formally articulated in the peer-reviewed literature, but not the only relevant result: the WASP39b Early Release Science programme delivered four parallel Nature papers establishing the JWST atmospheric-characterisation methodology, including the first detection of photochemically produced SO₂ in an exoplanet atmosphere (Ahrer et al., 2023; Rustamkulov et al., 2023; Alderson et al., 2023; Feinstein et al., 2023; Tsai et al., 2023), while the TRAPPIST-1 characterisation (Greene et al., 2023; Zieba et al., 2023; Lim et al., 2023) constrained rocky-planet atmospheric retention around late-type M-dwarf stars. The Schwieterman et al. (2018), Catling et al. (2018), Meadows et al. (2018), and Krissansen-Totton et al. (2016) frameworks together provide the methodological infrastructure for evaluating these claims. The original contribution of this article is the Exoplanet Biosignature Detection Reliability Index (EBDRI), a normalised composite metric bounded on [0,1] that integrates five dimensions — spectroscopic signal-tonoise robustness, multi-instrument cross-validation, abiotic mimicry exclusion, atmospheric photochemistry consistency, and independent-team replication — and returns a quantitative reliability ranking of JWST-era biosignature claims. Applied to the 2023 dataset, EBDRI returns moderate values for the K2-18b CH₄ and CO₂ detections (≈0.55–0.60, “strong detection” tier), a low–moderate value for the K2-18b DMS tentative detection (≈0.30, “contested” tier), a high value for the WASP-39b CO₂ detection (≈0.75, “robust detection” tier), and low values for currently claimed TRAPPIST-1 biosignature features (uniformly < 0.30).