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/E307.md.
mfeflow

    MFEFLOW_E307

    The shell to check against could not be read.

    Fix

    Pass --against a registry index, a deployed shell mf-manifest.json or a local file of either. A URL must be https (http only for localhost) and under 1 MB.