Picture your first day at a fine-dining restaurant. The manager says tonight is the busiest night. Very important guests are coming, and every table will be full. You do not know the restaurant, the customers, the food or the menu.
Manager number one hands you everything. The suppliers and the logistics. The regulations of the restaurant industry. Every person in the kitchen, their names, how old they are, how many kids they have, who washes the dishes. How many plates, how many spoons. And at the very end, the menu of the day. You have no choice but to read every page. Can you serve those tables well tonight?
Manager number two says: five waiters tonight, you take this corner, four tables. This one page is today's special menu. Forget last summer's menus. Here are the soups, the appetizers, the main dishes. Fish of the day, swordfish, wild caught. That's it.
You, the same new waiter on the same busy night, do far better with manager number two, because we did not dump a trash can of the entire history of the restaurant on you. Give an MIT graduate eighty thousand pages and the day's job does not get done; give the same graduate one clean sheet of paper, read this, execute that, put the result here, and it gets done well.
That is context engineering, the first part of the harness. In 2023 everybody was doing prompt engineering instead: you are a senior QA analyst with twenty years of experience, make no mistakes, find all the bugs. People sold courses on that. Telling the agent it is a senior analyst does not give it one fact about your project. Choosing what it reads does.
Your test-writing agent keeps missing an edge case on the login page. The case is written down in one of nine hundred old bug reports, and you do not know which one. The agent could read all nine hundred in a few minutes. Do you load them all, or none?
Neither. You give it a second place to look: if you cannot find something, search here, in the bug database, through its tool. It opens that place only when it needs it, and pulls out the one report that matters. Load all nine hundred and you are manager number one again, and your waiter is reading about the spoons while the guests wait.