The Madden–Julian Oscillation (MJO) remains difficult to simulate realistically in atmospheric and coupled models, with model performance being strongly dependent on physical parameterizations and model configurations. A standardized and reproducible diagnostic framework of the MJO is therefore essential for systematic evaluation and model development. A widely used set of MJO diagnostics was developed by the US CLIVAR MJO Working Group in 2009 (MJO-WG09), but its original implementation depends on software and workflows that are no longer readily accessible in modern computing environments.</p> <p>In this study, we reimplement the MJO-WG09 diagnostics using Python-based analysis libraries. The diagnostics are reconstructed following the published methodology and, where necessary, by examining the original source code to resolve ambiguities. As a result of methodological reassessment, the new implementation differs from the original in aspects such as temporal filtering, statistical testing, and spectral analysis; these differences are documented and discussed. In addition, we introduce a new diagnostic to evaluate the Walker circulation associated with the MJO. The open-source package provides a reproducible foundation for consistent evaluation of MJO simulations across models.