How to Connect MercadoLibre MCP in Antigravity/Cursor: The Missing Guide

Source: Dev.to

Introduction

If you are using Antigravity (or Cursor) as your AI‑native IDE, you may have noticed that the official MercadoLibre developers documentation hasn’t caught up with these tools yet. Setting up the Model Context Protocol (MCP) for MercadoLibre can be tricky because of specific port requirements and OAuth flows. After a long troubleshooting session, here’s a definitive guide for Antigravity users.

The most common issue

When you authorize in the browser, the page often shows:

“La aplicación no está preparada para conectarse a Mercado Libre”

The official MercadoLibre MCP proxy app strictly expects a redirect to http://localhost:18999. However, the mcp-remote client used by IDEs like Antigravity often picks a random port (e.g., 17135) if it isn’t told otherwise.

Update mcp_config.json

In your mcp_config.json (usually located at %APPDATA%\.gemini\antigravity\), add 18999 as a positional argument:

{
  "mercadolibre": {
    "command": "npx",
    "args": [
      "-y",
      "mcp-remote",
      "https://mcp.mercadolibre.com/mcp",
      "18999"
    ]
  }
}

Warning
Remove the following lines from mcp_config.json:

"--header",
"Authorization: Bearer APP_USR-..."

Keeping these while trying to log in via the browser will cause the authentication exchange to fail with “Existing OAuth client information is required”.

Resolve “address already in use 127.0.0.1:18999”

If you see this error, a previous (failed) attempt left a “zombie” process running.

Kill Node.js zombie processes

taskkill /F /IM node.exe

Clear the corrupt auth cache

rmdir /S /Q %USERPROFILE%\.mcp-auth

Browser specifics

If Firefox is your default browser and the “Authorization Successful” page doesn’t seem to talk back to your IDE, copy and paste the callback link into Chrome. Chrome often handles the local callback more reliably for development tools.

Beat the timeout

Antigravity has a built‑in timeout. Once the browser opens, complete the authorization quickly. If it fails, you’ll see a “context deadline exceeded” error.

Ngrok is not required

You don’t need Ngrok for this setup. The MercadoLibre MCP Proxy is an official app that already has permission to talk to your localhost:18999.

Summary checklist

• Force port 18999 in your mcp_config.json.
• Remove any manual Authorization headers.
• Clear zombie Node.js processes.
• Delete the corrupted auth cache (.mcp-auth).
• Use Chrome for the OAuth callback if the default browser fails.

Happy AI‑coding! 🚀