For AI agents: the complete documentation index is available at https://mfeflow.github.io/mfeflow/llms.txt, the full documentation bundle is available at https://mfeflow.github.io/mfeflow/llms-full.txt, and this page is available as Markdown at https://mfeflow.github.io/mfeflow/errors/E402.md.
mfeflow

    MFEFLOW_E402

    The registry returned something that is not a feature.

    Fix

    Ask the registry owner to check the entry. It must be a feature.json document.