the four pillars of loop engineering. the loop itself is six lines, and nobody competes on it. every serious agent framework lands on the same tiny while-loop. model reads context, calls a tool, you feed the result back, repeat until it stops asking. so if that part is solved, what is everyone actually engineering? the answer is everything around the model. Boris Cherny, who built Claude Code, put it plainly. he doesn't prompt Claude anymore, he writes loops and lets them run. that shift has a name now, and it rests on four pillars that are harder than the six lines make them look. these are the parts that actually break: → knowing when to stop. a terminal message ends the turn, not the task. an agent will write failing code, glance around, and declare victory. "done" has to mean the tests pass, not the agent feeling good about its work. → keeping the context clean. long loops rot from the inside as old outputs and dead ends pile up. a worse context produces a worse decision, which adds more noise, and the agent gets dumber the longer it runs. you fight it by treating context as a budget, not a bucket. → tools the agent can actually use. pile on a hundred tools and it loses track of which one to reach for. writes have to be safe to repeat, because loops retry, and a retried "create customer" call leaves you with duplicate records. → something that can say no. left alone, an agent agrees with itself. the fix is to separate the maker from the checker so the worker never grades its own homework. put those four together and your job changes. you stop steering the agent move by move and start designing the system that steers it. Karpathy runs research loops overnight that tweak a script, test it, keep what works, and throw away what doesn't, with himself nowhere in the loop. he arranges it once and hits go. the model is becoming a commodity. the loop around it is where the real engineering lives now. the best builders stopped asking what they should tell the agent to do. they started asking what system would do this without them. I wrote the full breakdown. the article is quoted below. stay tuned for more on this!