MCP Server

Model Context Protocol server for custom elements. See MCP Integration for setup.

MCP Resources

URI	Description
`cem://schema`	JSON schema for custom elements manifests
`cem://packages`	Package discovery and overview of available manifest packages
`cem://elements`	Summaries of all available elements with capabilities and metadata
`cem://element/{tagName}`	Detailed element information including attributes, slots, events, CSS properties, parts, and states
`cem://element/{tagName}/attributes`	Attribute documentation with type constraints, valid values, and usage patterns
`cem://element/{tagName}/slots`	Content guidelines and accessibility considerations for slots
`cem://element/{tagName}/events`	Event triggers, data payloads, and JavaScript integration patterns
`cem://element/{tagName}/css/parts`	CSS parts styling guidance
`cem://element/{tagName}/css/custom-properties`	CSS custom properties documentation
`cem://element/{tagName}/css/states`	CSS custom states documentation
`cem://guidelines`	Design system guidelines and best practices
`cem://accessibility`	Accessibility patterns and validation rules
`cem://config`	Current resolved configuration (merged from file + workspace cascade)
`cem://config/schema`	Overview of all config schema sections with links to per-section detail
`cem://config/schema/{section}`	JSON schema for a specific config section (generate, serve, health, mcp, export, warnings)

MCP Tools

`generate_html`

Generate correct HTML structure with proper slots and attributes using manifest data.

Parameter	Type	Required	Description
`tagName`	string	✅	Element to generate HTML for
`attributes`	object		Attribute values to include
`content`	object		Slot content mapping

`validate_html`

Validates custom element usage based on manifest guidelines.

Parameter	Type	Required	Description
`html`	string	✅	HTML content to validate
`tagName`	string		Focus validation on specific element
`context`	string		Validation context for custom elements

Validation Types:

Slot content guidelines
Attribute conflicts (e.g., loading="eager" + lazy="true")
Content/attribute redundancy
Manifest compliance

`generate_config`

Generate or update CEM configuration for a project. Returns the current config, config file path, full JSON schema, and guidance for each config section. Use with an optional focus parameter to narrow to a specific section.

Parameter	Type	Required	Description
`focus`	string		Config section to focus on: `generate`, `serve`, `health`, `mcp`, `export`, or `warnings`. Omit for all sections.

`validate_config`

Validate the current CEM configuration file. Runs schema validation and semantic checks (glob reachability, output path existence, URL rewrite validation). Returns structured JSON with errors and warnings, each with field path and source position.

No parameters required.

Configuration

Claude Desktop

{
  "mcpServers": {
    "cem": {
      "command": "cem",
      "args": ["mcp"]
    }
  }
}

Other Clients

Configure stdio transport with command cem mcp. See MCP specification for details.

Server Options

cem mcp [flags]

Flags:

--package <path> - Specify project directory or package specifier (npm:, jsr:, or URL)
--additional-packages <specs> - Load additional packages alongside local project (repeatable)
--max-description-length <num> - Override 2000 character description limit
--verbose, -v - Increase verbosity (-v info, -vv debug, -vvv trace)
--quiet, -q - Quiet output (warnings and errors only)

Loading Additional Packages

Load elements from external packages that aren’t in your local project:

# Single additional package
cem mcp --additional-packages npm:@rhds/elements@2.0.0

# Multiple packages
cem mcp --additional-packages npm:@vaadin/button@24.3.5 \
        --additional-packages https://cdn.jsdelivr.net/npm/@example/components/

Or configure in .config/cem.yaml:

additionalPackages:
  - npm:@rhds/elements@2.0.0
  - https://cdn.jsdelivr.net/npm/@shortfuse/materialdesignweb/