The most common migration breakpoints are component boundaries, routing hooks, data-fetching and metadata conventions, and version-specific caching or navigation behavior. They are worth auditing, not assumed to have broken in every project: the documented framework changes identify likely trouble spots, but do not establish what happened in any particular client project.
Should you migrate all routes at once?
Usually, you do not need to. The Pages Router and App Router can coexist, which allows you to move routes incrementally while leaving routes that still depend on the Pages Router in place. The Next.js migration guide recommends keeping _app and _document until no remaining Pages Router routes need them.
| Approach | What it means | Main trade-off |
|---|---|---|
| Incremental | Move selected routes to app while routes in pages continue to run. |
Limits the scope of each change and makes regressions easier to associate with a route or behavior. While both trees coexist, shared setup may need attention in both. |
| All at once | Move the project’s routes and related conventions in one migration. | A wider change can make it harder to isolate which route, component boundary, or configuration caused a regression. |
During an incremental migration, do not assume the new root layout replaces the setup for routes still served from pages. Check global styles, providers, and scripts in both contexts. A React Context provider that needs client-side behavior belongs in a Client Component.
What breaks at Server and Client Component boundaries?
Browser APIs, hooks, and event handlers in server code
App Router pages and layouts are Server Components by default. That changes the environment in which their code runs: browser-only APIs such as window and localStorage, interactive state, effects, and event handlers belong in Client Components. If a migrated component fails to build or behaves differently, inspect whether it assumes it runs in a browser when it is now rendered on the server.
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Client boundaries that are broader than necessary
Adding 'use client' can be a useful transition when existing page UI depends on client-side behavior. The migration guide describes moving that UI into a Client Component while the new server page fetches data and passes it in as props. Treat this as a boundary to define deliberately: converting an entire route to client-side code may preserve old assumptions, but it also changes the component architecture and bundle.
Which routing hooks need to change?
In Client Components under app, import routing hooks from next/navigation. The old next/router hook remains valid in pages but is not supported in app. The new useRouter does not expose the old pathname and query fields; read those concerns separately:
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Audit migrated code for assumptions about router.pathname, router.query, asPath, locale fields, isReady, and router events. If a component must temporarily be shared by both route trees, the migration guide describes next/compat/router as a bridge. Verify that shared component in both contexts, and remove the bridge when it is no longer needed.
What needs translating besides the route file?
Data fetching and route conventions
Pages Router data-fetching functions are not conventions to copy unchanged into an App Router page. The migration guide maps getServerSideProps and getStaticProps to data fetching in Server Components and related APIs, and getStaticPaths to generateStaticParams. App Router routes also use special files such as page, layout, error, and not-found; API endpoints can be implemented as Route Handlers.
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When a route renders but returns stale or unexpectedly changing data, check both what the UI displays and the underlying request behavior. Rendering successfully does not prove that the migrated route has the freshness or caching semantics you intended. Also check what data crosses from the server page into any Client Component props.
Metadata
Replace the Pages Router use of next/head with the built-in Metadata API for App Router routes. Check the rendered metadata as well as the visible page; a route can load while its metadata migration remains incomplete.
Why do caching and navigation behave differently?
There is no single caching rule that safely describes every App Router release and configuration. First identify the installed Next.js version and whether Cache Components are enabled; then check the upgrade documentation for that exact release before changing behavior.
| Reference | Documented behavior to check |
|---|---|
| Next.js 15 upgrade guide | Route Handler GET functions are no longer cached by default. During ordinary <Link> or useRouter navigation, page segments are not reused in the client router cache, while layouts and loading states remain reused. |
| Next.js 16 upgrade guide | Documents further changes, including async request APIs and routing/navigation changes. Use the guide for the installed target release rather than assuming Next.js 15 behavior applies unchanged. |
| Cache Components migration guidance | When Cache Components are enabled, route segment configuration changes; the guide describes replacing certain configuration with use cache and cacheLife. Cache Components require the Node.js runtime. |
For a stale-data, unexpectedly dynamic-rendering, or navigation-state issue, record the Next.js version, relevant configuration, whether Cache Components are enabled, and how the route was reached: direct load, client transition, or browser back/forward. Compare like with like before applying a fix from another major version.
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- Pin down the route and reproduction. Record the affected URL, the action that triggers the issue, and whether it happens on direct load, client navigation, or browser back/forward.
- Identify which router owns the route. Check whether it is still served from
pagesor has moved toapp; a shared component may run under both. - Check the component environment. Look for hooks, event handlers, effects, or browser APIs in Server Components. Confirm that any
'use client'boundary is in the component that needs client behavior. - Translate old conventions. Review routing hooks, data fetching, route files, API endpoints, and metadata rather than carrying Pages Router APIs over unchanged.
- Record version and configuration before debugging freshness or navigation. Check the release-specific upgrade guidance and whether Cache Components are enabled.
- Check shared setup while both routers are active. Verify styles, providers, and scripts in each route tree, and keep
_appand_documentwhile Pages Router routes still depend on them.
This checklist separates documented App Router differences from project-specific failures. A particular error, regression, or outcome should be described as an observed project experience only when it has actually been reproduced or documented in that project.
Which documentation should you check?
- Next.js, “Migrating: App Router” — version 15 Pages Router migration guidance, last updated April 15, 2025; covers coexistence, component boundaries, routing, data fetching, and metadata.
- Next.js Server and Client Components documentation — current page carries a March 2026 update date.
- Next.js Cache Components migration documentation — current page carries a March 2026 update date.
- Next.js 15 and Next.js 16 upgrade guides — release-specific references for changes affecting caching, requests, and navigation.
These are official Next.js documentation references from Vercel. Confirm their guidance against the exact version and configuration being migrated.
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