npx ldm-inbox-check-mcp — zero-install deliverability tool

The npm ecosystem solved one thing beautifully: running a command-line tool without installing it. npx downloads the package, runs it, and gets out of the way. For MCP servers this is the perfect distribution model — you want the latest version every time you launch Claude Desktop, and you do not want to manage a global install. This article shows how to use it and what to know about the quirks.

What npx gets you vs a global install

You could do npm install -g ldm-inbox-check-mcp and point your MCP client at the resulting binary. It works. It also gets stale immediately.

• npx: always runs the latest version published to npm. Zero maintenance. Tiny cold-start cost on the first run after an update (a few seconds).
• Global install: fast every run. Pinned to whichever version you last installed. You have to remember tonpm update -g.

For a developer laptop, npx wins on every axis that matters. For production (containers, shared servers), pin the version — see the self-hosting article.

The one-line config for Claude Desktop

Claude Desktop reads claude_desktop_config.json at launch. Paths:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json
• Windows: %APPDATA%\Claude\claude_desktop_config.json
• Linux: ~/.config/Claude/claude_desktop_config.json

Minimal config:

{
  "mcpServers": {
    "inbox-check": {
      "command": "npx",
      "args": ["-y", "ldm-inbox-check-mcp"],
      "env": { "INBOX_CHECK_API_KEY": "ic_live_..." }
    }
  }
}

The -y flag tells npx to auto-accept the prompt that asks you if you want to install the package. Without it, Claude Desktop would hang forever on first launch waiting for a Y/N that never comes.

Fully quit Claude Desktop (Cmd+Q on macOS, not just close the window) and relaunch. The hammer icon in the composer should list the Inbox Check tools.

The one-line config for Cursor

Cursor's MCP settings live under Settings → MCP → Add new MCP server. Fill in:

Name:    inbox-check
Command: npx
Args:    -y ldm-inbox-check-mcp
Env:     INBOX_CHECK_API_KEY=ic_live_...

Restart Cursor. In a composer window, @inbox-check should appear as a tool source.

Zed and other clients

The pattern is identical. Zed reads its config at ~/.config/zed/settings.json under the context_servers key. Continue.dev and other MCP-compatible clients accept the same command/args/env tuple.

Environment variable conventions

Only one variable is required:

• INBOX_CHECK_API_KEY — your key, starting with ic_live_. Get one from account settings on the dashboard.

Optional:

• INBOX_CHECK_API_BASE — override the default endpoint. Only needed if you are on a private deployment.
• INBOX_CHECK_DEFAULT_DOMAIN — a default sender domain to use when the agent does not specify one. Saves one round-trip on every test.
• MCP_VERBOSE — set to 1 to log every tool call to stderr. Useful for debugging wonky agent behaviour.

Versioning: using a tag

By default npx -y ldm-inbox-check-mcp resolves to the latest published version on npm. If you want to pin to a specific version or a release channel:

# Pin to exact version.
npx -y ldm-inbox-check-mcp@0.7.2

# Follow a channel (e.g. beta).
npx -y ldm-inbox-check-mcp@beta

# Stay on a major line.
npx -y ldm-inbox-check-mcp@^0.7

A team that values reproducibility over freshness would pin to an exact version and bump it deliberately. A solo developer who wants the latest features should just run @latest (the default).

Caching behaviour of npx

npx does not re-download the package on every run. It caches under ~/.npm/_npx (macOS/Linux) or %LOCALAPPDATA%\npm-cache\_npx (Windows). On subsequent runs:

• Same version requested → uses cached install. Sub-second start-up.
• Newer version available → downloads and replaces cache. First-run penalty, usually 2–5 seconds.
• Exact version pinned → never re-downloads.

That is why npx -y ldm-inbox-check-mcp feels slow once every few weeks (when we publish an update) and fast the rest of the time.

Forcing a refresh

# Nuke the npx cache for this package.
rm -rf ~/.npm/_npx

# Or force a specific package download.
npm cache clean --force
npx -y ldm-inbox-check-mcp@latest

Offline fallback

If you are on a plane, or your corporate proxy is blocking registry.npmjs.org, npx will fail on a cache miss. Three ways to survive:

1. Keep the cache warm. Run npx -y ldm-inbox-check-mcp --version before you leave wifi.
2. Install globally as a backup. npm install -g ldm-inbox-check-mcp. Even if npx fails, the binary stays on PATH.
3. Configure your corp proxy. Set npm_config_registry or use a local npm mirror like Verdaccio.

Debugging output

When things go wrong, check:

Is the key being passed?

Run the server manually from a terminal to confirm:

INBOX_CHECK_API_KEY=ic_live_... \
  MCP_VERBOSE=1 \
  npx -y ldm-inbox-check-mcp

# Expect to see: "server listening on stdio, api key loaded"
# If you see: "INBOX_CHECK_API_KEY missing" -> env var not being passed.

Is the client finding the binary?

On macOS, Claude Desktop may not inherit your shell's PATH. If npx isn't found, use the absolute path:

which npx
# -> /Users/you/.nvm/versions/node/v22.0.0/bin/npx

# Then in config:
{
  "mcpServers": {
    "inbox-check": {
      "command": "/Users/you/.nvm/versions/node/v22.0.0/bin/npx",
      "args": ["-y", "ldm-inbox-check-mcp"],
      "env": { "INBOX_CHECK_API_KEY": "ic_live_..." }
    }
  }
}

Is the server actually running?

With MCP_VERBOSE=1 the server logs every tool call to stderr. Claude Desktop captures MCP server stderr in ~/Library/Logs/Claude/mcp*.log. Tail that file while you trigger a call.

Frequently asked questions