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Exposing an MCP server to the internet — tunneling guide for Claude Desktop and Cursor

#mcp #ai #webdev #tutorial

MCP servers usually run on your localhost — which means hosted
Claude, Cursor on another machine, or a teammate can't reach them.
The fix is a tunnel, but the details matter: which URL survives
restarts, how the config files actually look, and how to avoid
handing your AI tooling a credential it can burn down your account
with.

 This is the full walkthrough.

 ## Why your MCP server is unreachable

 The Model Context Protocol server you just wrote listens on a
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local port. That works beautifully when the client is on the same
machine. The moment the client is hosted Claude, a colleague's
Cursor, or anything off-box, localhost becomes a wall: NATs,
firewalls, and the simple fact that localhost:8080 means
something different on every machine.

 A tunnel solves it by running an agent on your machine that
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holds a persistent outbound connection to a public edge. Traffic
to your public URL flows down that connection to your local port.

Step 1: Pick a URL that survives restarts

 This is the detail most guides skip, and the one that bites
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first. MCP client configs hardcode the server URL. If your tunnel
hands you a new random address on every restart, your config
breaks silently every morning.

 Use a named subdomain so the address is permanent:
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 ```bash
 mytunnel http 8080 --subdomain my-mcp
 # ✓ Tunnel active: https://my-mcp.21tunnel.com → 127.0.0.1:8080
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That URL is yours across restarts and reboots — paste it into
configs once and stop thinking about it.



   Step 2: Wire up Claude Desktop

   Edit claude_desktop_config.json:



   ```json
     {
       "mcpServers": {
         "my-server": {
           "url": "https://my-mcp.21tunnel.com/sse"
         }
       }
     }
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Restart Claude Desktop and the hosted client now talks to the MCP
server on your machine, through the tunnel.

Step 3: Wire up Cursor

Same shape in .cursor/mcp.json:

     {
       "mcpServers": {
         "my-server": {
           "url": "https://my-mcp.21tunnel.com/sse"
         }
       }
     }
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Now Cursor on any machine — yours, a teammate's, a CI box — can
reach the same server.

Step 4: Don't hand the agent your master token

If an AI agent is the one opening tunnels, pause here. The default
is exporting your tunnel service's account token into the agent's
environment. That token can typically open unlimited tunnels,
never expires, and shows up in transcripts and logs.

The safer pattern: a master key that cannot open tunnels itself,
which the agent uses to mint its own scoped, short-lived child key
per project:

     eval "$(mytunnel eval mint --project mcp --ttl 4h --output-env)"
     mytunnel http 8080 --subdomain my-mcp
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The agent works autonomously, the credential dies on schedule, and
one click cascade-revokes everything it ever created.

Step 5: Gate it if it matters

A public MCP endpoint is a public door. For anything beyond a
quick test, enable edge auth so the URL only responds to signed-in
teammates or an allowlisted domain. One toggle in the dashboard,
no code changes.

That's the whole setup: permanent URL, two config files, scoped
credentials, access control. The full guide with more config
variants: exposing an MCP server to the internet

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