A rule should change behavior
‘Write clean code’ is impossible to verify. ‘Use the shared request validator for every public endpoint’ tells an agent where to look and what choice to make. Good rules reduce ambiguity at the moment of editing.
Scope beats volume
Repository-wide instructions should cover universal safety and workflow. Package-level rules should explain the local architecture, tests, and forbidden patterns. This prevents a long global document from burying the one constraint that matters.
Rules need maintenance
Outdated guidance is worse than missing guidance because an agent will follow it confidently. Review instructions when build tools, directory ownership, or release processes change, and pair important rules with checks when possible.