Backend and full-stack developers who ship endpoints and want to design them deliberately rather than by imitation, students preparing for backend interviews where API design is a standard round, and frontend engineers who consume APIs and want to argue precisely for better ones. It is language-agnostic and assumes you have built at least one HTTP endpoint in some framework — Express, Spring, Django, Rails, ASP.NET, it does not matter which. No code is written during the lessons; every question is about the contract rather than the implementation.

An API is the hardest thing in a codebase to change, because every consumer you cannot see depends on the shape you shipped. Design mistakes therefore compound rather than decay, and most of them are well known in advance:

• a POST used for an operation that should be idempotent, so retries double-charge
• 400 returned for a semantic validation failure that callers cannot distinguish from a malformed body
• offset pagination over a table that keeps changing, silently skipping or repeating rows
• internal exception text leaked in an error response
• a new required field added to a request, breaking every existing client at once
• a GraphQL resolver issuing one query per item, turning a single request into hundreds

Each of these is a lesson, and each is a question a senior interviewer is likely to ask in some form.

Each lesson opens with a short reading card, then puts one multiple-choice question in front of you at a time. Every answer — right or wrong — returns an explanation immediately, so a wrong pick becomes the moment you learn the rule rather than a score you move past. The AI guide tracks which lessons you keep missing and steers later practice back toward them. Nothing to install, and you can stop mid-lesson and resume where you left off. Progress saves continuously, so a ten-minute session on a commute counts as much as an hour at a desk, and the course picks up exactly where you stopped. The lessons are generated as you advance rather than handed over as one fixed quiz, so the practice pool stays ahead of you instead of running out halfway through a topic. Accuracy is recorded per lesson and carried into the completion certificate, which means working carefully through a hard topic is worth more than clicking quickly through an easy one.

Udemy and Coursera cover APIs mostly through framework-specific video courses, and the Postman Learning Center is a good free reference for the request-and-collection side of the work. Both leave the design decisions implicit — you absorb whatever the instructor happened to do. This course makes them the subject: which status code fits, whether that operation is safely retryable, what a cursor buys you over an offset, when a change counts as breaking. Those are the questions in a design review and in the interview round that mirrors one.

Learn HTTP properly first. Most API disagreements turn out to be disagreements about method semantics and status codes, and they dissolve once both people share the same model. Then work through error design and idempotency before the advanced topics, because those two decide whether a client can trust your endpoint when the network misbehaves — which it will. Read your own API's responses as a consumer would, without the source in front of you, and see whether they are actionable. Pair this with the framework you actually use: Node.js or Spring Boot Fundamentals. Then read a public API you admire end to end; seeing consistency applied at scale teaches more than any single rule.