/build-mcpb

name: build-mcpb
description: This skill should be used when the user wants to "package an MCP server", "bundle an MCP", "make an MCPB", "ship a local MCP server", "distribute a local MCP", discusses ".mcpb files", mentions bundling a Node or Python runtime with their MCP server, or needs an MCP server that interacts with the local filesystem, desktop apps, or OS and must be installable without the user having Node/Python set up.
version: 0.1.0

Build an MCPB (Bundled Local MCP Server)

MCPB is a local MCP server **packaged with its runtime**. The user installs one file; it runs without needing Node, Python, or any toolchain on their machine. It's the sanctioned way to distribute local MCP servers.

**Use MCPB when the server must run on the user's machine** — reading local files, driving a desktop app, talking to localhost services, OS-level APIs. If your server only hits cloud APIs, you almost certainly want a remote HTTP server instead (see `build-mcp-server`). Don't pay the MCPB packaging tax for something that could be a URL.

---

What an MCPB bundle contains

my-server.mcpb              (zip archive)
├── manifest.json           ← identity, entry point, config schema, compatibility
├── server/                 ← your MCP server code
│   ├── index.js
│   └── node_modules/       ← bundled dependencies (or vendored)
└── icon.png

The host reads `manifest.json`, launches `server.mcp_config.command` as a **stdio** MCP server, and pipes messages. From your code's perspective it's identical to a local stdio server — the only difference is packaging.

---

Manifest

{
  "$schema": "https://raw.githubusercontent.com/anthropics/mcpb/main/schemas/mcpb-manifest-v0.4.schema.json",
  "manifest_version": "0.4",
  "name": "local-files",
  "version": "0.1.0",
  "description": "Read, search, and watch files on the local filesystem.",
  "author": { "name": "Your Name" },
  "server": {
    "type": "node",
    "entry_point": "server/index.js",
    "mcp_config": {
      "command": "node",
      "args": ["${__dirname}/server/index.js"],
      "env": {
        "ROOT_DIR": "${user_config.rootDir}"
      }
    }
  },
  "user_config": {
    "rootDir": {
      "type": "directory",
      "title": "Root directory",
      "description": "Directory to expose. Defaults to ~/Documents.",
      "default": "${HOME}/Documents",
      "required": true
    }
  },
  "compatibility": {
    "claude_desktop": ">=1.0.0",
    "platforms": ["darwin", "win32", "linux"]
  }
}

**`server.type`** — `node`, `python`, or `binary`. Informational; the actual launch comes from `mcp_config`.

**`server.mcp_config`** — the literal command/args/env to spawn. Use `${__dirname}` for bundle-relative paths and `${user_config.<key>}` to substitute install-time config. **There's no auto-prefix** — the env var names your server reads are exactly what you put in `env`.

**`user_config`** — install-time settings surfaced in the host's UI. `type: "directory"` renders a native folder picker. `sensitive: true` stores in OS keychain. See `references/manifest-schema.md` for all fields.

---

Server code: same as local stdio

The server itself is a standard stdio MCP server. Nothing MCPB-specific in the tool logic.

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";
import { readFile, readdir } from "node:fs/promises";
import { join } from "node:path";
import { homedir } from "node:os";

// ROOT_DIR comes from what you put in manifest's server.mcp_config.env — no auto-prefix
const ROOT = (process.env.ROOT_DIR ?? join(homedir(), "Documents"));

const server = new McpServer({ name: "local-files", version: "0.1.0" });

server.registerTool(
  "list_files",
  {
    description: "List files in a directory under the configured root.",
    inputSchema: { path: z.string().default(".") },
    annotations: { readOnlyHint: true },
  },
  async ({ path }) => {
    const entries = await readdir(join(ROOT, path), { withFileTypes: true });
    const list = entries.map(e => ({ name: e.name, dir: e.isDirectory() }));
    return { content: [{ type: "text", text: JSON.stringify(list, null, 2) }] };
  },
);

server.registerTool(
  "read_file",
  {
    description: "Read a file's contents. Path is relative to the configured root.",
    inputSchema: { path: z.string() },
    annotations: { readOnlyHint: true },
  },
  async ({ path }) => {
    const text = await readFile(join(ROOT, path), "utf8");
    return { content: [{ type: "text", text }] };
  },
);

const transport = new StdioServerTransport();
await server.connect(transport);

**Sandboxing is entirely your job.** There is no manifest-level sandbox — the process runs with full user privileges. Validate paths, refuse to escape `ROOT`, allowlist spawns. See `references/local-security.md`.

Before hardcoding `ROOT` from a config env var, check if the host supports `roots/list` — the spec-native way to get user-approved directories. See `references/local-security.md` for the pattern.

---

Build pipeline

Node

npm install
npx esbuild src/index.ts --bundle --platform=node --outfile=server/index.js
# or: copy node_modules wholesale if native deps resist bundling
npx @anthropic-ai/mcpb pack

`mcpb pack` zips the directory and validates `manifest.json` against the schema.

Python

pip install -t server/vendor -r requirements.txt
npx @anthropic-ai/mcpb pack

Vendor dependencies into a subdirectory and prepend it to `sys.path` in your entry script. Native extensions (numpy, etc.) must be built for each target platform — avoid native deps if you can.

---

MCPB has no sandbox — security is on you

Unlike mobile app stores, MCPB does NOT enforce permissions. The manifest has no `permissions` block — the server runs with full user privileges. `references/local-security.md` is mandatory reading, not optional. Every pa