React 19.x development patterns and APIs. Use when building React applications with React 19+ features including Actions (useTransition with async), useActionState, useFormStatus, useOptimistic, use() API, Form Actions, ref as prop (no forwardRef), Context as provider, document metadata/stylesheets/scripts, Server Components, Server Actions, prerender(), the Activity component, useEffectEvent, cacheSignal(), and Partial Pre-rendering. Triggers on React 19 questions, form handling, server components, or when using new React 19 APIs.