API (application programming interface)

The socket one piece of software plugs into another. It's how a program asks another program to do something — look this up, book that in, send this — with nobody clicking anything.

A service counter with a fixed set of forms. You can't wander into the back office and rummage; you fill in one of the forms on the counter and get an answer back. What's on the counter is exactly what you're allowed to ask for.

AI companies sell access to their models through an API. And an agent (an AI that works towards a goal on its own, step after step) reaches your calendar, your accounting software or the system holding your customer records the same way.

Why it matters: "does it have an API?" is the first question to ask about any system you want AI to work with. No API means the AI is locked out of it — or reduced to clicking round the website like a person, which is slow and breaks the moment the website changes.

MCP is the newer version of this same idea, built for AI specifically. Underneath, MCP connectors are still talking to APIs. Where a system has no MCP connector yet, its API is how you reach it.

Related: MCP (Model Context Protocol) · Agent