Corex MCP

Package Hexdocs: corex_mcp.

Introduction

You expose Corex component and design metadata to AI tools (Cursor, Claude Desktop, VS Code) from your running app. The MCP server is self-hosted: no external SaaS.

Do not enable MCP in production. The tools are read-only, but the endpoint still widens your attack surface. Use it only while developing locally (or in :test when generated apps include it for CI). plug Corex.MCP raises at boot in :prod unless you pass force: true (discouraged).

Before you start

Mount the endpoint

Phoenix

Add plug Corex.MCP in lib/my_app_web/endpoint.ex after Plug.Static and before the code reloader block:

if Mix.env() in [:dev, :test] do
  plug Corex.MCP
end

Start the app. MCP is available at http://localhost:4000/corex/mcp (adjust host and port).

Tableau Bandit

Tableau has no Phoenix endpoint. Run MCP on a separate Bandit port; see the Corex Tableau guide (MCP section).

Default URL: http://localhost:4004.

Connect your editor

Point any client that supports streamable HTTP MCP at the running URL above.

Phoenix serves MCP on the app port (http://localhost:4000/corex/mcp). Tableau serves MCP on a dedicated Bandit (http://localhost:4004/corex/mcp by default). Path is always /corex/mcp.

Scaffolds write a project .cursor/mcp.json with the single URL for that app (mix corex.new → 4000, mix corex.tableau.new → 4004). If you use a user-level / shared client config and work on both Phoenix and Tableau apps, register both servers (examples below). Prefer --no-mcp for locked-down scaffolds. Never set allow_remote_access: true casually.

Cursor

Project file (written by the installer when --mcp is on), or user-level ~/.cursor/mcp.json with both entries:

{
  "mcpServers": {
    "corex": {
      "url": "http://localhost:4000/corex/mcp"
    },
    "corex-tableau": {
      "url": "http://localhost:4004/corex/mcp"
    }
  }
}

Claude Desktop

Add both servers to claude_desktop_config.json when you use Phoenix and Tableau:

{
  "mcpServers": {
    "corex": {
      "transport": {
        "type": "http",
        "url": "http://localhost:4000/corex/mcp"
      }
    },
    "corex-tableau": {
      "transport": {
        "type": "http",
        "url": "http://localhost:4004/corex/mcp"
      }
    }
  }
}

VS Code and other HTTP MCP clients

Configure streamable HTTP MCP twice if needed — Phoenix at http://localhost:4000/corex/mcp, Tableau at http://localhost:4004/corex/mcp (or your configured :mcp_port). Omit an Origin header on requests (see Security).

Tools

All tools are read-only.

Prompts: corex_form, corex_controlled, corex_style (via prompts/list / prompts/get).

Call list_components before get_component when you need a valid id. Invalid tool arguments return an MCP error instead of being silently ignored.

Security

• Loopback by default. Only requests from localhost are accepted unless you set allow_remote_access: true on the plug (discouraged outside trusted networks).
• Origin header. POST /corex/mcp and GET /corex/config reject requests that include an Origin header. Clients such as Cursor typically omit it.
• Read-only tools. No code evaluation, SQL, or log access. Component ids are allowlisted before lookup.
• Never production. Mount in :dev / :test only. The plug refuses to initialize in :prod unless force: true is passed.
• Corex MCP does not modify your app's Content-Security-Policy or X-Frame-Options headers.

Configuration

plug Corex.MCP,
  allow_remote_access: false,
  force: false

Optional application config:

config :corex_mcp, mcp_root: "/path/to/project"

mcp_root is the project root used when resolving paths for tooling (defaults to File.cwd!()).

Verbose MCP logging:

config :corex_mcp, debug: true

Related

• Corex installation — mix corex.new enables MCP in dev by default
• Corex Design — tokens and ui-* modifiers