One of the few rules where a single breach can do real harm. Everything else is guidance.
You recognise a hard rule by one question: what breaks if the rule is broken just once? You write hard rules briefly, with the harm as the reason. If a breach is only unsightly or costs time, the rule is guidance: it comes with a reason and an allowed exception, so that Claude can weigh the individual case. Every hard rule has a guardrail or the note “Guardrail missing”.
Example from Module 8
Planning office [example]: “Anything that goes to clients or subcontractors is approved by a person first.” is a hard rule, because a letter that has been sent cannot be called back. Politeness towards clients is guidance.
Module 8: “Storing knowledge for good”. In the Werkbank course room, with sign-in.