next-intl middleware locale detection redirect loop error

Fix next-intl middleware locale detection redirect loop

TL;DR

The middleware keeps redirecting because the locale it detects never sticks, so stop the loop by excluding the redirect target from detection. Usually the matcher re-runs on the already-prefixed URL, or the cookie and the Accept-Language header disagree forever. Make detection idempotent: if the URL already has a valid prefix, do nothing.

The error

ERR_TOO_MANY_REDIRECTS on YOUR_HOST/
/ to /en to /en to /en ... (network tab shows the same redirect repeating)

Fix it

Step 1: Watch the redirect chain to see the loop

curl -s -o /dev/null -w "%{redirect_url}" YOUR_HOST/ | head -c 200

Expected: You see the URL redirecting to itself or bouncing between two URLs.

Step 2: Check the middleware matcher for self-matching

grep -n "matcher" -A 3 src/middleware.js

Expected: The matcher must not re-trigger on URLs that already carry a locale prefix.

Step 3: Make detection skip already-prefixed paths

node -e "console.log('pattern: if the pathname starts with a known locale, return NextResponse.next() with no redirect')"

Expected: One request, one redirect at most, then the prefixed page serves.

Step 4: Clear the locale cookie and retry

curl -s -c /tmp/cj -b /tmp/cj -o /dev/null -w "%{http_code} %{redirect_url}" YOUR_HOST/

Expected: You get a single 307 to /en and then a 200.

When to use this

• The browser reports too many redirects on a next-intl site
• The network tab shows / bouncing to /en repeatedly

When NOT to use this

• A single redirect to the wrong locale, that is a detection config issue not a loop
• Redirects involving auth pages, check your auth middleware order

Tool and version compatibility

• next-intl v3/v4 middleware, Next.js 12/13/14
• Locale detection via Accept-Language header, cookie, or both

Variant phrasings

loop between /en and /en-US

Your locales list has both but detection canonicalizes differently than the prefix. Pick one form and stick to it.

loop only for logged-in users

Something sets the locale cookie after the middleware runs. Set it before, or let the middleware own it.

Why it happens

The middleware redirects to add the locale prefix, but on the next request it runs again and redirects again, because the already-prefixed URL still looks undetected to it, or the cookie it sets gets overwritten. Each pass looks like a fresh visitor, so it never stops.

Edge cases

• Multiple middleware in a chain can each redirect, merge them into one response
• Domain-based routing plus prefix routing double up, use one strategy
• Bots without Accept-Language can loop forever, give them the default locale with no redirect

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_emoBG81uYRSnQ6uy8QAvfg