API.

A programmatic doorway that lets you plug a model or service into an app, or automate a workflow. It's the same thing happening when an agent makes a Tool calls to reach another app: the API is the door it walks through.

Here's why it matters when you're building. A model with no API is a dead end. You can chat with it, but you can't wire it into something you're making or run it on a schedule. As Remy put it about one tool, "there was no API. You couldn't integrate it into an app you were building or automate a video workflow."

Two things to know going in. Usage costs money, usually per call. And an API key is the password that proves a request is really coming from you.