Prompts as Code: Templates That a Customer Cannot Break

Think of a printed form at a bank counter. Most of the page is printed once, for everybody: the bank's name, the questions, the small print at the bottom. There is one empty box that says "Write your name here". A thousand customers fill in the same form, and each one writes something different in the box. The bank never reprints the page for each customer. It only reads what is in the box.

This works because the printed part and the box stay separate. Whatever a customer writes in the box, it cannot change the questions printed above it, or the small print below it. Now imagine a strange form where the words you write in the box could reach out and change the printed text: write the right word, and a line from the bank's private instructions appears on your copy. Nobody would design a paper form like that. But it is surprisingly easy to build a prompt like that in code.

In every lesson of this chapter, my code built each prompt the same way: fixed words written once, and a gap where the customer's message goes. This lesson looks at that code. I tested four common ways of filling the gap against the kinds of text real customers type, and then measured what happens to the model's answers when the fixed words change by only a space or a line break.