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.
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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 โ 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.
The host reads manifest.json, launches server.mcp_config.command as a MCP server, and pipes messages. From your code's perspective it's identical to a local stdio server โ the only difference is packaging.
stdio
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 โ node, python, or binary. Informational; the actual launch comes from mcp_config.
server.mcp_config โ 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 โ the env var names your server reads are exactly what you put in env.
user_config โ 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 โ no auto-prefixconstROOT = (process.env.ROOT_DIR ?? join(homedir(), "Documents"));
const server = newMcpServer({ 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 = awaitreaddir(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 = awaitreadFile(join(ROOT, path), "utf8");
return { content: [{ type: "text", text }] };
},
);
const transport = newStdioServerTransport();
await server.connect(transport);
Sandboxing is entirely your job. There is no manifest-level sandbox โ 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 โ the spec-native way to get user-approved directories. See references/local-security.md for the pattern.
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 โ avoid native deps if you can.
MCPB has no sandbox โ security is on you
Unlike mobile app stores, MCPB does NOT enforce permissions. The manifest has no permissions block โ 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 โ 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 โ 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 โ full manifest.json field reference
references/local-security.md โ path traversal, sandboxing, least privilege
