prefab-mcp

prefab-mcp

An MCP server for Prefab, so an agent can see your templates and deploy one for you.

"Set up a new client folder for Northwind, autumn campaign, on my Desktop"

What it exposes

Tool	Does
list_templates	Every template, what it creates, what it asks for
describe_template	One template in full: structure, placeholders, actions
preview_deploy	What a deploy would create, and what values are still missing. Creates nothing
deploy_template	Creates the folders, filling in the values given
list_watched_folders	Folders Prefab is watching, and the rules on them
list_globals	Library-wide placeholders, actions and value presets

Setting it up

Works with any client that speaks MCP over stdio.

As a bundle - download prefab.mcpb from the releases page and double-click it. Clients that install MCP bundles, Claude Desktop among them, bring their own Node, so there is nothing else to install.

From the command line (Claude Code, and anything with the same idea):

claude mcp add prefab -- npx -y prefab-mcp

In a config file - Cursor, Zed, and most others:

{
  "mcpServers": {
    "prefab": { "command": "npx", "args": ["-y", "prefab-mcp"] }
  }
}

Then switch on Let agents and scripts control Prefab in Prefab's Settings ▸ Permissions. It is off by default; reading your templates works without it, creating and deploying does not.

That switch arrives in Prefab 1.1.6. On an earlier version the reading tools work and the rest say so.

Requires macOS, Prefab installed and opened once, and Node 18+ (except for the .mcpb route).

How it works

Prefab is sandboxed and has no network access, so it cannot host anything. This server runs beside it instead: it reads the library straight out of the app group (~/Library/Group Containers/group.VFDG327T66.prefab) and asks the app to deploy through the same prefab:// URL the Finder extension uses. Two consequences worth knowing:

• Prefab has to be running for a deploy. Reading works either way.
• The destination has to be a folder Prefab has been granted. macOS keeps a sandboxed app out of your folders until you point it at one. preview_deploy and deploy_template both check first and say so, rather than leaving a permission dialog on screen that an agent cannot answer. Grant folders in Prefab, under Settings ▸ Permissions.

Nothing here writes to Prefab's library. The app owns those files, and editing them behind its back would be overwritten the next time it saves.

Development

npm test    # checks placeholder resolution matches the app's Swift, case for case
npm run try '[["list_templates",{}]]'    # call a tool the way a client does

src/resolve.js is a deliberate duplicate of RenamePattern.swift. If you change one, change the other - npm test compiles the Swift and compares the two.