Best place to add side effects (analytics/logging) during URL rewrites?

[glekner]

I'd like to capture an analytics event (Sentry breadcrumb) whenever a legacy redirect fires.

Two questions:

1. Is the rewrite input function expected to be pure (no side effects beyond URL mutation)?
   The docs show it mutating the URL object, but it's unclear whether side effects like logging or analytics calls are safe here .

2. What's the recommended pattern for observing rewrites? Should I:
   Fire the side effect directly inside input (simplest, but couples concerns)
   Use a router event/hook like router.subscribe('onResolved', ...) and detect that a rewrite occurred
   Something else entirely?

Appreciate any guidance on the intended contract for rewrite functions.

[schiller-manuel]

Is the rewrite input function expected to be pure (no side effects beyond URL mutation)?

yes, must be pure. rewrites can happen a lot (link building for example)

please share more details what you want to log here using a complete minimal reproducer project

[glekner]

We have a concept of "Legacy Redirects". static array of old paths that should be redirected to a new one.
We want to log to Sentry whenever that happens. If the rewrites must be pure I guess the only path forward is something like

import { useEffect } from 'react'
import { useRouter } from '@tanstack/react-router'

export function useNavigationSentry() {
  const router = useRouter()

  useEffect(() => {
    const unsub = router.subscribe('onResolved', ({ fromLocation, toLocation }) => {
       // ... check if from is a legacy redirect.
      Sentry.captureMessage(`Navigated to ${toLocation.pathname}`)
    })
    return () => unsub()
  }, [router])
}

any thoughts @schiller-manuel ?