The Agent Loop: How 20 Lines of Swift Turn an API Client into a Coding Agent

A language model can reason about code — it can plan how to fix a bug, suggest a refactoring, or design a feature. But it can't touch the real world. It can't read files, run tests, or check whether its suggestion actually compiles. Without some kind of bridge, every interaction is a dead end: the model suggests something, we copy-paste it into a terminal, paste the result back, the model adjusts, and we do it all over again. We are the loop. The entire point of a coding agent is to close that loop automatically. Give the model a way to execute commands, feed the results back, and let it keep going until it's done. That's what we'll build in this guide — and it turns out the core mechanism is surprisingly small. The complete source code for this stage is available at the 01-agent-loop tag on GitHub. Code blocks below show key excerpts. The problem: we are the middleware Let's say we ask the model to create a file. Without an agent loop, the interaction looks like this: we send a prompt