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";
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
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
npx @anthropic-ai/mcpb init
