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.

Across eight chapters, you have applied the same method to URL shortening, social feeds, notifications, payments, AI inference, messaging, booking, feature flags, checkout, calendars, documents, ride sharing, file storage, and API reviews.

The products had different requirements, but the main design questions stayed the same.