Conclusion: What You Now Know, and Where to Go Next

The course began with a simple claim:

An API design interview is not a test of how many endpoints you can name. It is a test of whether you can turn caller jobs into a stable contract and defend its costs.

Eight chapters later, the method has survived URL shortening, Twitter, live notifications, payments, LLM inference, chat, booking, feature flags, checkout, calendars, collaborative documents, news-feed screens, offline messaging, ride sharing, file storage, and API critique.

The products changed from chapter to chapter, and the small set of ideas underneath them never did.