What problem does it solve? Making changes to a Next.js application requires knowing which framework boundary owns the behavior: page, layout, route handler, cache, middleware, or build configuration. This Skill classifies the change, enforces MDE-specific invariants like narrow use client boundaries and authenticated route handlers, and prevents workarounds that mask unrelated bugs. ## Core Features & Use Cases - Change Classification: Categorizes work as page/layout, route handler, rendering boundary, cache, build, or deployment behavior before editing code. - MDE Invariant Enforcement: Keeps server-only secrets out of client components, requires authentication before privileged writes in route handlers, and treats caching as observable behavior. - Version-Sensitive Verification: Checks the installed Next.js version (16.2.6) and official documentation before relying on framework APIs. - Use Case: When adding a new API endpoint to the MDE app, use this Skill to confirm the route handler authenticates before privileged writes, matches existing src/app patterns, and passes targeted type, lint, and build checks. ## Quick Start Ask the assistant to implement a Next.js App Router change in the MDE app, such as adding a route handler or adjusting a server/client component boundary, and have it verify the result with targeted type and lint checks.