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An extension adds what Mill can do — a board object, a workflow step, a command, a secret source, a view — as a folder of two files you install, and it runs only with your say-so.
Extensions is its own page (⇧⌘X). Installed lists what you have and what each one adds. Browse lists what your marketplaces offer that you have not installed, starting with the examples Mill ships. Updates shows what has a newer version.
Installing shows what the extension can do — the hosts it reaches, whether it writes to your boards, what it adds — and installs only after you confirm. A newly installed extension waits under Installed until you allow it to run.
A marketplace is any repository or folder with a .mill/marketplace.json
file, listing the extensions it offers. Add one under Sources;
Mill reads it only when you add it, refresh it, install from it, or
check for updates — never on its own. You can also install straight
from a repository, a .zip address, or a folder on this Mac.
Every installed extension wears one badge, and it says exactly what was checked:
- Verified — its files match the hash the marketplace published, and a key this Mill trusts signed them.
- Hash-pinned — its files match the hash the marketplace published.
- Unverified — nothing checked these files; Mill asks you to acknowledge that before installing.
- Dev — you installed it from a folder on this Mac.
An extension that changes on disk after you allowed it loses its badge until you allow it again.
An extension declares up front what it contributes and what it may ask for; the list you confirm at install is the list it gets. It renders in its own frame with the theme Mill hands it, reaches the network only through hosts it declared, writes to boards only through the same guarded path an agent's write takes, and never receives another extension's secrets. It cannot register hotkeys, update itself, or phone home.
To install one, follow Install a plugin. To write one, start at Extending the canvas.