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Choose how much to migrate at once
Next.js allows the app and pages directories to coexist. An incremental migration can limit the scope of each change, but it also means maintaining some setup in both route trees until the Pages Router is no longer needed. An all-at-once migration avoids that overlap sooner, but moves more routes and assumptions together.
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| Approach | What it means | Trade-off to plan for |
|---|---|---|
| Incremental | Move selected routes to app while routes in pages continue to use the Pages Router. The Next.js migration guide supports coexistence. |
Contains changes to a smaller part of the application at a time, but shared setup may need attention in both route trees. |
| All at once | Move the application’s routes and conventions to the App Router in one migration. | Can remove the period of maintaining two route trees sooner, but makes it harder to isolate which change caused a regression. |
For either approach, record the Next.js version and configuration before changing behavior. The migration guide cited here is the version 15 Pages Router guide, last updated April 15, 2025; version-specific upgrade guidance may differ for a newer target.
Check whether code still assumes it runs in the browser
App Router pages and layouts are Server Components by default. As the Next.js migration guide puts it, “Pages in the app directory are Server Components by default.” A component moved from pages may therefore execute in a different environment than it did before.
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Move interactive behavior behind a Client Component boundary
Use a Client Component for interactive state, event handlers, effects, and browser APIs such as window or localStorage. When one of these appears in a component that remains a Server Component, check the component boundary before changing the route’s data-fetching behavior.
Add 'use client' at the boundary that needs client behavior rather than reflexively making an entire route client-rendered. The migration guide describes moving existing page UI into a Client Component as a transitional option: the new server page can fetch data and pass it to that component as props. This preserves a path for reusing UI while keeping server-side work in the page.
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Replace Pages Router routing assumptions
Code in the App Router should use routing hooks from next/navigation, not the Pages Router’s next/router. The new useRouter does not expose the old pathname and query fields. Read those concerns separately with usePathname, useSearchParams, and useParams.
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- Check for
router.pathname,router.query, andasPath; identify whether each use needs the pathname, search parameters, or route parameters. - Audit uses of locale fields,
isReady, and router events rather than assuming the old router object is available inapp. - If one component must temporarily serve routes in both trees, the migration guide describes
next/compat/routeras a compatibility option. Treat it as a bridge and verify the component in both route trees.
Translate data fetching, route files, and metadata
Pages Router conventions do not transfer unchanged. In the App Router, data fetching belongs in Server Components and related APIs; getServerSideProps and getStaticProps are not copied over as page functions. The Pages Router’s getStaticPaths maps to generateStaticParams.
Audit the route structure and each data flow
- Check that route files use App Router conventions such as
page,layout,error, andnot-found. - Where an API route is being migrated, check whether it belongs in an App Router Route Handler.
- Follow fetched data across the server/client boundary. Confirm what is fetched in the server page and what is passed as props to a Client Component.
- Replace metadata handled through
next/headwith the built-in Metadata API.
A page rendering successfully does not by itself confirm that the migrated route has the intended freshness or caching behavior. Check request behavior as well as the rendered result.
Keep shared setup working while both routers exist
The root App Router layout does not automatically replace setup for routes still served from pages. The migration guide recommends keeping _app and _document while Pages Router routes still depend on them. During coexistence, check whether global styles, providers, and scripts are correctly included in both contexts.
React Context providers that need client behavior should live in Client Components. When moving a provider, check which routes consume it in each router; moving it into the App Router layout alone does not cover routes that still render through pages.
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Diagnose caching and navigation against the exact version
App Router caching behavior is not one unchanging rule. The Next.js 15 upgrade guide documents that Route Handler GET functions are no longer cached by default. It also documents that page segments are not reused in the client router cache during ordinary <Link> or useRouter navigation, while layouts and loading states remain reused.
The Next.js 16 upgrade guide documents further changes, including async request APIs and routing and navigation changes. If Cache Components are enabled, route segment configuration changes as well: the Cache Components migration guidance describes replacing certain configuration with use cache and cacheLife, and requires the Node.js runtime. Do not apply a fix described for a different major version or configuration without checking its applicable upgrade guidance.
Capture the conditions for a stale-data or navigation bug
- Record the installed Next.js version and relevant configuration.
- Record whether Cache Components are enabled.
- Note whether the behavior occurs on a direct load, a client-side transition, or browser back/forward navigation.
- For stale data or unexpected dynamic rendering, inspect the specific data-fetching and caching path for that version rather than assuming a general App Router default.
A practical migration audit
- Inventory routes still served from
pagesand routes already moved toapp; keep the Pages Router setup that those remaining routes depend on. - For each migrated route, identify Server Components and Client Component boundaries. Check browser APIs, effects, event handlers, and interactive state.
- Search migrated code for imports from
next/routerand old router fields. Replace them with the appropriatenext/navigationhooks, or use the documented compatibility bridge only where a component is shared across routers. - Translate data fetching, static route generation, route files, API handlers, and metadata to their App Router equivalents.
- Check global styles, providers, and scripts in both route trees while they coexist.
- Reproduce freshness and navigation issues under the target version and configuration, noting the navigation path used for each case.
The official references for these behaviors are Next.js documentation: “Migrating: App Router” (version 15 Pages Router guide, last updated April 15, 2025), “Server and Client Components” and “Cache Components” (pages carrying March 2026 update dates), and the version 15 and 16 upgrade guides. Use the guide for the project’s actual target release when behavior is version-sensitive.
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