api design

A staple at OpenAI and Anthropic in both coding and design rounds. Most candidates can name the algorithms; few can say which one lets 2x your limit through, and that's the part that gets scored.

You control the sender; thousands of customer endpoints you've never seen control your fate. The rubric hides in three places: what 'ordering' really costs, why retries need the receiver's cooperation, and the debugging surface most designs forget.

Rate limits protect your servers; quotas protect your business model, and most candidates design one and think they've designed both. The strong answer also covers the part customers actually feel: what hitting the limit looks like.

The customer's team has to live with this API for years, on a schedule you don't control. The interview is about how you ship changes without a 2 a.m. page on their side.

A reported Anthropic system-design prompt. The skeleton is standard API platform work; the AI-specific 20%, token-denominated limits, safety classifiers in the path, abuse identity, is what the round actually grades.