OAuth.

OAuth is the "log in with Google" pattern, but for your agent's tools. Instead of hunting down an API and pasting it in, you log in to a service you already pay for and authorize the agent to use it.

If you have a Grok subscription, it plugs straight into your agent over OAuth. No separate key, no extra billing. You click authorize and it's connected.

For building, this is the difference between a tool you wire up in ten seconds and one that sends you on a developer detour. When an integration offers OAuth, take it.