1. Introduction

If you've worked with legacy backends, you'll agree: when they fail, it's not always obvious.

Routing logic duplicates across endpoints. One route checks a header; another forgets entirely. Data transformations? Same story. Price is a number in one route, a string in another. Error responses are a guessing game.

Over time, every "just ship it" decision compounds into a codebase that's easier to break than it is to read.

Imagine adding region support across endpoints with code longer than the API's age 😊. AI coding agents can help, but this is exactly where they make mistakes that become even harder to debug.

You don't have a scaling problem. You have a separation-of-concerns problem.

You don't need a rewrite or more abstraction. You need to extract those "dress codes" (routing and transformation logic) to an API gateway.…