The Harness (everything around the loop that the model never touches)

Building with LLMs. The board that steps back from The Loop (an agent is a while loop around one model call)‘s twenty lines and draws the two thousand wrapped around them: one turn of the returns desk traced through every box it passes, the latency each box costs, the four things the model is never handed, and five named ways a turn breaks. The claim in its subtitle is the whole lesson: this part is a normal service, and it deserves to be built like one.

Before you start. The Loop (an agent is a while loop around one model call) built the twenty lines that assemble a window, call the model, run the tool it asks for, and check the answer - call that the loop, and remember its window is just the messages sent so far. This lesson is everything a company builds around that loop before it lets it talk to a paying customer.