core-add-module

Development

Add a new module to the core package (src/packages/core/). Use when adding shared UI framework infrastructure that all packages can use — e.g., a new extension system feature, a new utility, a new shared component pattern, a new picker, or a new workspace primitive. Core modules are imported as @umbraco-cms/backoffice/{module-name}. Also use when the user says things like "add a module to core", "create a new core feature", or "add shared infrastructure".

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Source SKILL.md: https://github.com/umbraco/Umbraco-CMS/blob/HEAD/src/Umbraco.Web.UI.Client/.claude/skills/core-add-module/SKILL.md

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Add Core Module

Add a new module to the core package (src/packages/core/).

Naming conventions

Module names are singular (e.g., picker, sorter, entity-flag, modal, section)

What you need from the user

Module name — What to call it in singular form (kebab-case, e.g., picker, sorter, entity-flag)
Purpose — What shared infrastructure this provides

When to add to core vs. create a new package

Core module: Shared UI framework infrastructure used by multiple packages — extension primitives, pickers, validation, repository base classes, workspace utilities.
New package: Domain-specific CMS features (documents, media, webhook). Use the create-package skill instead.

Core modules should not implement CMS-specific features. They provide the building blocks that CMS feature packages use.

Files to create

src/packages/core/{module-name}/
├── index.ts                   # Public API exports
├── manifests.ts               # Extension registrations (if any)
├── constants.ts               # Module constants
└── types.ts                   # Module types

Step 1: Create the module directory and files

{module-name}/index.ts

The public API. Only export what other packages need:

export * from './constants.js';
export type * from './types.js';

Use export type * for type-only exports.

{module-name}/manifests.ts

Extension registrations for this module:

export const manifests: Array<UmbExtensionManifest> = [
	// Extension registrations
];

If the module has no extensions (pure utility), export an empty array.

{module-name}/constants.ts

// Aliases, context tokens, entity types

{module-name}/types.ts

// Type definitions, interfaces, meta types

Step 2: Wire into core's manifests.ts

Add the module's manifests to src/packages/core/manifests.ts:

import { manifests as {moduleName}Manifests } from './{module-name}/manifests.js';

// In the manifests array:
...{moduleName}Manifests,

The imports are in alphabetical order — insert in the right position.

Step 3: Add entry point to core's vite.config.ts

Add the module to the entry object in src/packages/core/vite.config.ts:

'{module-name}/index': './{module-name}/index.ts',

The entries are in alphabetical order — insert in the right position. This tells Vite to build the module as a separate entry point.

Step 4: Register the subpath export

Add to the exports field in the root package.json (src/Umbraco.Web.UI.Client/package.json):

"./{module-name}": "./dist-cms/packages/core/{module-name}/index.js"

This enables imports like @umbraco-cms/backoffice/{module-name}.

Step 5: Regenerate TypeScript config

npm run generate:tsconfig

This updates tsconfig.json paths so the new module resolves correctly in TypeScript.

How consumers import

After registration, other packages import from the module like:

import { MyThing } from '@umbraco-cms/backoffice/{module-name}';
import type { MyType } from '@umbraco-cms/backoffice/{module-name}';

Reference: existing core modules to study

Simple utility: src/packages/core/culture/ — repository + components, minimal surface
Extension primitive: src/packages/core/entity-action/ — kinds, base classes, shared element
Infrastructure: src/packages/core/repository/ — base classes for the data access pattern

Checklist

Module directory created under src/packages/core/
index.ts exports only the public API
manifests.ts exports extension registrations (or empty array)
constants.ts and types.ts created
Module manifests imported in src/packages/core/manifests.ts (alphabetical order)
Entry point added to src/packages/core/vite.config.ts (alphabetical order)
Subpath export added to root package.json
npm run generate:tsconfig executed
Compiles: npm run compile