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/E306.md.
mfeflow

    MFEFLOW_E306

    A dev server could not start.

    Fix

    Free the port or change it in mfeflow.json and the app's rsbuild config, then run mfeflow dev again.