Everyone obsesses over which model to use. Almost nobody engineers the fifty lines of code wrapped around it — the loop that decides what the model sees, when it gets to act, when it's allowed to stop, and what happens when it does something dumb. That loop is the agent. The model is just an expensive function you call from inside it.

This is a build-along field guide to loop engineering: the discipline of designing, hardening, and shipping the control loop that turns a language model into an agent you can trust to run unattended. We start with the naive loop — the one that works in a demo and falls over in production — and harden it chapter by chapter into a small TypeScript runtime, with an on-call triage agent as the running example. Along the way: context budgeting, stopping conditions, retries and idempotency, the approval boundary, self-verification (the DOER/CHECKER split), observability, cost control, resumability, and composing loops out of sub-loops. The model is the easy part. The loop is the craft — and it's where reliability is won or lost.