Problem

When using Next.js 15 App Router with next-intl and localePrefix: "as-needed", Web Vitals and transaction names were inconsistent across locales:

• /foo (default locale, no prefix) → Transaction: /:locale ❌
• /ar/foo (non-default locale, with prefix) → Transaction: /:locale/foo ✅

This caused all default locale pages to collapse into a single /:locale transaction, making Web Vitals data unusable for apps with i18n routing.

After investigation it seems like the route parameterization logic couldn't match /foo (1 segment) to the /:locale/foo pattern (expects 2 segments) because the locale prefix is omitted in default locale URLs.

Solution

Implemented enhanced route matching with automatic i18n prefix detection:

1. Route Manifest Metadata - Added hasOptionalPrefix flag to route info to identify routes with common i18n parameter names (locale, lang, language)
2. Smart Fallback Matching - When a route doesn't match directly, the matcher now tries prepending a placeholder segment for routes flagged with hasOptionalPrefix
   • Example: /foo → tries matching as /PLACEHOLDER/foo → matches /:locale/foo ✓
3. Updated Specificity Scoring - changed route specificity calculation to prefer longer routes when dynamic segment counts are equal
   • Example: /:locale/foo (2 segments) now beats /:locale (1 segment)

Result

After fix:

URL: /foo         → Transaction: /:locale/foo    ✅
URL: /ar/foo      → Transaction: /:locale/foo    ✅
URL: /products    → Transaction: /:locale/products ✅
URL: /ar/products → Transaction: /:locale/products ✅

All routes now consistently use the same parameterized transaction name regardless of locale, making Web Vitals properly grouped and usable.

Backwards Compatibility

• No breaking changes - only applies when direct matching would fail
• Only affects routes with first param named locale/lang/language
• Non-i18n apps completely unaffected
• Direct matches always take precedence over optional prefix matching

Fixes #17775

Maybe we should make certain aspects of this configurable, like the ['locale', 'lang', 'language'] collection