ChatGPT system instructions

ChatGPT does best when you can name the shape of the answer up front: JSON, a table, a function signature, a chain of tool calls. Custom instructions and project files carry your rules from one conversation to the next, so whatever you paste into every message should live there. It handles long inputs well. It also obeys the latest instruction harder than the first, which is why the binding rules go at the end of a long prompt.