MUST USE when fixing a bug, diagnosing an error, stack trace, exception, crash, failing or flaky test, or any 'why is X not working / returning the wrong value' investigation in any language. Enforces root-cause-before-patching: reproduce, trace the failure backward to its origin, test one hypothesis, then fix once — instead of guarding the symptom. Also covers the 3-attempts-then-question-the-design rule.
MUST USE when writing or reviewing Drizzle ORM schemas, migrations, relational queries, or drizzle-kit configuration. Enforces identity columns over serial, proper relation definitions, migration safety, type inference, and query patterns.
MUST USE when writing or editing React components, hooks, state management, effects, performance, loading/empty states, Zustand stores, or styling. Covers React 19.2 (use, Actions, useActionState, useOptimistic, useFormStatus, ref as prop, Context provider, Activity), the Rules of Hooks, React Compiler v1.0 memoization, component splitting and custom hooks, composition over boolean props, useEffect avoidance, re-render performance, list keys, loading skeletons and empty states, Zustand client-state stores, and Tailwind v4 design tokens.
MUST USE when writing or editing TypeScript types, interfaces, generics, type guards, error handling patterns, or tsconfig configuration. Enforces TypeScript 7.x strict-mode best practices and type safety patterns.
MUST USE when writing or editing TanStack Router routes, file-based routing, navigation, loaders, search params, beforeLoad auth guards, pendingComponent / loading UI, or route configuration. Enforces type-safe file-based routing, pending UI that prevents 'frozen on slow internet' navigation, auth checked once per subtree (not per page), loader caching with staleTime, and TanStack Query integration.
MUST USE when writing or reviewing code that handles user input, authentication, authorization, API endpoints, database queries, secrets, or any security-sensitive functionality. Enforces OWASP Top 10 prevention, secure defaults, and defense-in-depth patterns.
MUST USE when writing, reviewing, or modifying tests. Enforces Arrange-Act-Assert, factory-based test data, test isolation, mocking boundaries, and pyramid-balanced coverage.
MUST USE when creating or editing any code file in any language — deciding how to split files, functions, classes, and modules, or whether to extract a shared helper, base class, or utility from duplicated code. Enforces the Single Responsibility Principle (one reason to change, file/function size limits, separation of concerns) and Avoid-Hasty-Abstractions (AHA / Rule of Three — prefer duplication over the wrong abstraction).