JWST-RCA (JWST Residual-Covariance Advisor) is an open scientific software framework for post-calibration residual-covariance diagnostics in processed spectroscopic products from the James Webb Space Telescope (JWST). The framework is designed as a statistical quality-control and interpretation-support layer between processed JWST data products and downstream scientific analysis. It does not make astrophysical claims or detection decisions. Instead, it quantifies residual channel-to-channel covariance, estimates the effective number of independent spectral channels (N_eff), evaluates stacking-efficiency degradation, applies empirical null tests, and generates wavelength-specific diagnostic summaries. JWST-RCA helps researchers assess whether a given spectral region can reasonably be treated as independent statistical evidence, or whether covariance-aware interpretation, conservative uncertainty treatment, or additional caution is required. Key features include: • Residual-covariance diagnostics Robust quantification and visualization of residual channel-to-channel covariance in processed JWST spectroscopic products. • Effective-channel and stacking diagnostics Estimation of N_eff and conservative stacking-efficiency summaries to evaluate whether nominal √N stacking assumptions may overstate statistical support. • Empirical null tests Null-test workflows to assess whether observed residual-covariance statistics exceed selected empirical null expectations. • Reproducible reporting Generation of machine-readable diagnostic outputs, figures, advisor summaries, comparison tables, and shareable Markdown, HTML, and PDF reports suitable for documentation, review, supplementary material, and community testing. • Comparative wavelength-region support A comparison workflow that evaluates supplied processed products over user-defined wavelength intervals and identifies the preferred statistical-support option for the requested region, without implying an absolute instrument ranking. • Non-intrusive integration JWST-RCA complements, rather than replaces, the JWST Calibration Pipeline, MAST, reduction pipelines, atmospheric retrieval tools, or official validation workflows. It adds a lightweight diagnostic layer focused on residual covariance. • Guided and reproducible workflows The package provides both a guided terminal menu and direct command-line workflows, supporting exploratory use, exact reruns, collaborator reproducibility, and release documentation. Validated examples: The current workflows are demonstrated using public JWST/WASP-39 processed products across MIRI/SLITLESS, NIRCam/GRISM, and NIRISS/SOSS examples, including wavelength-specific cross-product comparison. RCA ecosystem note: JWST-RCA is part of a broader Residual-Covariance Advisor (RCA) diagnostic ecosystem under development. The same post-pipeline philosophy is being extended to Rubin/LSST-style tabular products through RUBIN-RCA, with future branches planned for SDSS/BOSS-style spectroscopic products and Euclid-style morphological/catalog products. The long-term goal is not to replace any official pipeline, but to provide a common post-pipeline diagnostic layer for comparing residual-covariance structure, effective independent evidence, empirical null-test behavior, and machine-readable diagnostic summaries across multiple astronomical instruments, surveys, and data-product families. Related RCA branch: RUBIN-RCA: Operational Software for Residual-Covariance Diagnostics in Rubin/LSST-style Tabular Products. https://doi.org/10.5281/zenodo.20406707 Disclaimer: JWST-RCA provides diagnostic evidence, not astrophysical verdicts. Its outputs are conditional on the supplied processed products, wavelength interval, residual-construction choices, masking, and diagnostic configuration. The software is intended to support cautious, covariance-aware interpretation and community testing, not to confirm or reject astrophysical claims.