If you've used Claude Code, Cursor, or Copilot on a Kotlin codebase — Android, Spring Boot, Ktor, or KMP — you've watched the model regress your stack by five years in three turns. LiveData instead of StateFlow . kapt instead of KSP . runBlocking in tests. Gson reflection on data classes that already have @Serializable . String IDs that should be inline value classes. lateinit on every Compose ViewModel . The suggestions compile. They also rot. The fix is not "write better prompts every time." It is a CLAUDE.md checked into the root of your repo — a file the model reads on every turn. Twelve rules below, tuned for Kotlin 2.0+ , Coroutines 1.9+ , Compose 1.7+ , and JDK 21 . They cover Android, JVM backend, and KMP (Kotlin Multiplatform), with explicit notes when a rule reshapes for one of them. Tired of writing this file from scratch for every stack?…