If you've built an MCP server or any API that costs you money to run (an LLM call, a paid data source, compute), you've probably hit the same wall I did:
How do you get paid per call — when the caller is an AI agent, not a human with a credit card form?
A human can sign up, enter a card, get an API key. An autonomous agent can't fill out a Stripe checkout form mid-task. And you don't want to hand an agent a raw API key with no spending limit — one runaway loop and your bill explodes.
This post walks through the design I landed on. It's not the only way, but the pieces are reusable even if you build your own.
The core idea: HTTP 402
