I've built a handful of MCP servers. Here's what separates a good one from a demo.

This year I've built more MCP servers than I meant to, across three pretty different shapes:

vellum: a self-hosted server over a folder of markdown, so my agent has a memory that's just files I own. (Open source, MIT.)

Metrifyr: a marketing-data server that puts GA4, Search Console, AdSense and Tag Manager behind one connection.

A read-only gateway over a company's internal user-data stack, federating half a dozen internal services behind one audited MCP surface.

Personal, product, enterprise. Totally different data, totally different stakes. And yet the things that made each one good, versus a flashy demo that falls over the moment a real agent uses it in anger, were the same every time. Here are the seven that keep recurring.