Seelen UI — Plugin Guidelines

Reference guide for creating plugins. A plugin is a declaration file that extends the functionality of a specific widget. The plugin itself only declares who it targets and provides a data payload — the target widget is entirely responsible for loading, parsing, and deciding what to do with that data.

Read resource guidelines first for concepts shared across all resource kinds.

Table of Contents

  1. What is a Plugin?
  2. Plugin Structure — metadata.yml
  3. Plugin Data

1. What is a Plugin?

A plugin targets a widget by ID and provides it with arbitrary data. The widget discovers installed plugins at runtime, reads the ones that target it, and uses the data however it sees fit. A plugin has no logic of its own — it is a pure declaration.

Plugins are merely plain, flat files — nothing more. The Plugin resource kind does not define a runtime, a sandbox, or an execution model of any kind. It is only an envelope: id, target, and a free-form plugin payload. There is no such thing as "the plugin system" in the singular — there is one plugin envelope shared by every resource, and as many plugin behaviors as there are widgets willing to consume one. Seelen UI itself does not parse, validate, or execute the contents of plugin in any generic way; it only routes the resource to whichever widget's target matches.

Think of plugins as per-widget extensions: each target widget defines its own tiny "extension API" — its own schema for what plugin must contain, and its own logic for parsing and executing that data (JS eval in a sandbox, a declarative tree it walks, a lookup table it renders from, anything the widget author wants). A toolbar plugin might define a new button with JS callbacks; a tiling window manager plugin might define a static layout tree with no code at all; a calendar widget's plugin might add a new event source. The schema, the parsing, and the execution all live in the target widget — not in the plugin resource, and not in Seelen UI's core.

For Seelen UI's own built-in widgets that accept plugins, see the dedicated guides:

If you are building your own widget and want to support plugins, you are free to design any plugin schema you like — document it for your users the same way the three guides above document Seelen UI's built-in widgets.

2. Plugin Structure — metadata.yml

id: "@yourname/my-plugin"

metadata:
  displayName: My Plugin
  description: A short description of what this plugin adds.
  tags:
    - toolbar

# Icon shown in the Seelen UI settings panel.
# Must be a valid react-icons name (https://react-icons.github.io/react-icons/).
# Defaults to PiPuzzlePieceDuotone if omitted.
icon: PiPuzzlePieceDuotone

# The widget this plugin is for
target: "@someuser/some-widget"

# The plugin data — structure depends entirely on the target widget
plugin:
  someField: someValue

The only required fields are id, target, and plugin.

3. Plugin Data

The plugin field is free-form — it can be any valid YAML value (a map, a list, a string, anything). There is no schema enforced at the resource level. The target widget receives it as-is and is responsible for validating and using it.

Read the documentation of the widget you are targeting to know what structure it expects.

You can use !include in the plugin block just like anywhere else in a resource file:

plugin:
  template: !include plugin/template.js
  tooltip: !include plugin/tooltip.js
  scopes:
    - Power