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.

MCPB is the secondary distribution path. Anthropic recommends remote MCP servers for directory listing — see https://claude.com/docs/connectors/building/what-to-build.

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 path must be validated, every spawn must be allowlisted, because nothing stops you at the platform level.

If you came here expecting filesystem/network scoping from the manifest: it doesn't exist. Build it yourself in tool handlers.

If your server's only job is hitting a cloud API, stop — that's a remote server wearing an MCPB costume. The user gains nothing from running it locally, and you're taking on local-security burden for no reason.

---

MCPB + UI widgets

MCPB servers can serve UI resources exactly like remote MCP apps — the widget mechanism is transport-agnostic. A local file picker that browses the actual disk, a dialog that controls a native app, etc.

Widget authoring is covered in the

build-mcp-app

skill; it works the same here. The only difference is where the server runs.

---

Testing

# Interactive manifest creation (first time)
npx @anthropic-ai/mcpb init

# Run the server directly over stdio, poke it with the inspector
npx @modelcontextprotocol/inspector node server/index.js

# Validate manifest against schema, then pack
npx @anthropic-ai/mcpb validate
npx @anthropic-ai/mcpb pack

# Sign for distribution
npx @anthropic-ai/mcpb sign dist/local-files.mcpb

# Install: drag the .mcpb file onto Claude Desktop

Test on a machine without your dev toolchain before shipping. "Works on my machine" failures in MCPB almost always trace to a dependency that wasn't actually bundled.

---

Reference files

• references/manifest-schema.md

  — full

  manifest.json

  field reference

• references/local-security.md

  — path traversal, sandboxing, least privilege