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) }] };
  },
);

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