Instruction files store standing guidance for coding assistants. They can describe repository layout, build and test commands, coding conventions, safety limits, and the expected definition of done. Tools load them automatically at the relevant project or directory scope.
They solve the problem of repeating stable information in every prompt. Without them, each session may guess how to run tests or which helper to use. Keep the guidance short, direct, and specific. Link to longer documentation instead of copying it all. Never put secrets in these files. Review changes like code because stale or conflicting instructions can make an agent less reliable than having no rule at all.
Rewriting in plainer words…
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