@sentry/nextjs bundles the whole Pages Router runtime (next/router, ~87 KB raw / 36 KB gzip) into every App Router app

[AlbertoMihai98]

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Sentry Saas (sentry.io)

Which SDK are you using?

@sentry/nextjs

SDK Version

10.72.0 (measured); the import chain is unchanged in 10.73.0 and on develop

Framework Version

Next 16.3.3, App Router only (no pages/ directory), Turbopack

Link to Sentry event

No response

Reproduction Example/SDK Setup

// instrumentation-client.ts
import * as Sentry from '@sentry/nextjs';

Sentry.init({ dsn: process.env.NEXT_PUBLIC_SENTRY_DSN });

export const onRouterTransitionStart = Sentry.captureRouterTransitionStart;

Nothing else is needed: any App Router app that calls init() reproduces it. The app has no pages/ directory and no import of next/router anywhere.

Steps to Reproduce

1. npx create-next-app@latest (App Router), add @sentry/nextjs and the instrumentation-client.ts above.
2. next build --experimental-analyze (or any other bundle analysis).
3. Open the chunk that contains the SDK client (the one with the [@sentry/nextjs] init warnings).

Expected Result

An App Router app downloads no Pages Router code. The SDK already decides at runtime which router it is on (routing/nextRoutingInstrumentation.ts: !WINDOW.document.getElementById('__NEXT_DATA__')), so the Pages Router branch is never taken on App Router.

Actual Result

The SDK client chunk carries the whole Pages Router client runtime: next/dist/shared/lib/router/router.js, next/dist/compiled/path-to-regexp, next/dist/client/route-loader.js, next/dist/client/script.js, sorted-routes.js, route-regex.js, prepare-destination.js, ... 52 modules from next, 87.2 KB raw / 35.6 KB gzip in our app (Turbopack analyzer, Next 16.3.3).

Sentry client chunk (App Router, __SENTRY_TRACING__ compiled to false)	raw	of which next (Pages Router runtime)
@sentry/nextjs 10.72.0 as published	168.4 KB	87.2 KB (52 modules)
with next/router imported on demand (see below)	82 KB	0 - the runtime lands in an async chunk that an App Router app never requests

It is there with tracing enabled and after compiling tracing out: the import is static, so no bundler can drop it.

Root cause - packages/nextjs/src/client/routing/pagesRouterRoutingInstrumentation.ts:

import RouterImport from 'next/router';

const Router: typeof RouterImport = RouterImport.events
  ? RouterImport
  : (RouterImport as unknown as { default: typeof RouterImport }).default;

reached statically through client/index.ts -> browserTracingIntegration.ts -> routing/nextRoutingInstrumentation.ts -> routing/pagesRouterRoutingInstrumentation.ts. Router is used in exactly one place, pagesRouterInstrumentNavigation (Router.events.on('routeChangeStart', ...)); the pageload half only reads __NEXT_DATA__ and __BUILD_MANIFEST. next declares no sideEffects, and the app/pages decision is made at runtime, so the module and everything next/router pulls in stay in every client bundle.

Why the SDK's own size gate cannot see it: .size-limit.js measures @sentry/nextjs (client) with ignore: ['next/router', 'next/constants'].

Additional Context

What I measured before proposing a fix (same app, same analyzer, patching the installed build):

• Moving the module-scope interop (RouterImport.events ? ...) into a function changes nothing: the cost is the static import at module-graph level, not the getter access.
• Replacing the static import with import('next/router') inside pagesRouterInstrumentNavigation moves the Pages Router runtime into an async chunk: Sentry chunk 168.4 -> 82 KB raw on our App Router app. On a Pages Router app the routeChangeStart listener is registered once that import settles; the router module is already part of the framework runtime there. A minimal Pages Router app built with both Turbopack and webpack, pageload and mount-time router.replace() navigation spans included, is in the PR that follows.
• window.next.router.events (no next/router at all) was considered and rejected: Next marks instance.events as "never documented ... remove the following major version".

Priority

React with 👍 to help prioritize this issue.

[linear-code]

JS-3570

[chargome]

@AlbertoMihai98 thanks for flagging I'll take a look!