Redirects in next.config.ts are case-insensitive and lead to ERR_TOO_MANY_REDIRECTS in Next.js 15

[hdodov]

Link to the code that reproduces this issue

https://github.com/hdodov/test-nextjs/tree/bug-redirects-case-insensitive

To Reproduce

1. pnpm dev
2. Open http://localhost:3000/foo-Bar
3. You'll get ERR_TOO_MANY_REDIRECTS in the browser

Current vs. Expected behavior

Currently: If you have a page /foo-bar, /foo-Bar will be a 404. But when you add the following redirect:

{
  source: "/foo-Bar",
  destination: "/foo-bar",
  permanent: true,
}

…you get an infinite redirect chain, because /foo-Bar redirects to /foo-bar and then /foo-bar redirects to /foo-bar again.

Expected: The redirect should be case-sensitive and only /foo-Bar (with a capital B) should lead to a redirect.

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 23.6.0: Thu Apr 24 20:26:36 PDT 2025; root:xnu-10063.141.1.705.2~1/RELEASE_X86_64
  Available memory (MB): 32768
  Available CPU cores: 16
Binaries:
  Node: 20.17.0
  npm: 10.8.2
  Yarn: N/A
  pnpm: 10.4.1
Relevant Packages:
  next: 15.3.3 // Latest available version is detected (15.3.3).
  eslint-config-next: 15.3.3
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Redirects

Which stage(s) are affected? (Select all that apply)

next start (local)

Additional context

I need this redirect for SEO purposes, as having 404 pages not your site is a bad SEO signal. However, "fixing" this with a redirect causes an infinite redirect chain, which is worse…

_NEXT-4665

[icyJoseph]

I know there's an experimental flag for this, experimental.caseSensitiveRoutes

import type { NextConfig } from 'next';

const nextConfig: NextConfig = {
  experimental: {
    caseSensitiveRoutes: true,
  },
  redirects: async () => {
    return [
      {
        source: '/foo-Bar',
        destination: '/foo-bar',
        permanent: false,
      },
    ];
  },
};

export default nextConfig;

Let me search more context to it

[devjiwonchoi]

Confirmed experimental.caseSensitiveRoutes flag solves this issue.