Infinite redirect loop when proxying on Vercel to external WordPress server due to trailing‑slash normalization

I have a Next.js application deployed on Vercel and a WordPress blog running on an external server (nginx/Apache). I’d like all requests to /blog/* on my Next.js app to be proxied or redirected to the external server.

However, Vercel automatically strips trailing slashes from every URL. My WordPress setup enforces trailing slashes, so it issues a 301 Moved Permanently to the URL with the slash. Vercel then removes the slash again, triggering another 301 from WordPress—and so on, ad infinitum.

What I’ve Tried:

• vercel.json rewrite/redirect rules
• next.config.js with trailingSlash: true / false
• middleware.ts (Next.js Edge Middleware) to normalize slashes and proxy headers

None of these approaches have stopped the slash‑removal ↔ slash‑addition ping‑pong.

// vercel.json

{
  
  "redirects": [
    {
      "source": "/blog/", // Match requests specifically ending with /blog/
      "destination": "https://my-wp-blog.com/blog/", // Ensure destination has the needed slash
      "permanent": true // Or false (use 301/302 if temporary, 307/308 if permanent - true often defaults to 308)
    },
    
  ]
}

// next.config.js

module.exports = {
  trailingSlash: true,
}

// middleware.ts

import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export function middleware(request: NextRequest) {
  const url = request.nextUrl.clone()
  
  // Force trailing slash for all blog paths
  if (url.pathname.startsWith('/blog') && !url.pathname.endsWith('/')) {
    const redirectUrl = new URL(request.url)
    redirectUrl.pathname = `${url.pathname}/`
    return NextResponse.redirect(redirectUrl, 308) // 308 Permanent Redirect preserves method
  }
  
  // Continue with proxying for requests that already have trailing slashes
  const requestHeaders = new Headers(request.headers)
  requestHeaders.set('host', 'my-wp-blog.com')
  
  const clientIP = request.ip || request.headers.get('x-forwarded-for') || ''
  requestHeaders.set('x-real-ip', clientIP)
  requestHeaders.set('x-forwarded-for', clientIP)
  
  requestHeaders.set('x-forwarded-proto', request.nextUrl.protocol.replace(':', ''))
  requestHeaders.set('x-forwarded-host', 'my-wp-blog.com')
  
  // Also explicitly ensure the proxied URL has a trailing slash
  const proxyUrl = new URL(request.url)
  proxyUrl.protocol = 'https:'
  proxyUrl.hostname = 'my-blog-server-ip'
  
  // Make sure the pathname still has the trailing slash when proxying
  if (!proxyUrl.pathname.endsWith('/')) {
    proxyUrl.pathname = `${proxyUrl.pathname}/`
  }
  
  return NextResponse.rewrite(proxyUrl, {
    request: {
      headers: requestHeaders,
    },
  })
}

export const config = {
  matcher: '/blog/:path*',
}

Example of the Loop (via curl -vIL):

> HEAD /blog/my-post/ HTTP/2
< HTTP/2 308  # Vercel removes the slash
< location: /blog/my-post

> HEAD /blog/my-post HTTP/2
< HTTP/2 301  # WordPress adds the slash
< location: https://example.com/blog/my-post/

…repeat indefinitely…

So my questions are:

1. How can I disable Vercel/Next.js’s automatic trailing‑slash redirects entirely so that my middleware can fully control URL normalization?

2. Is it possible to use both vercel.json and Edge Middleware together to achieve this, or must I choose one approach?

3. Are there any additional headers or settings I need to tweak on Vercel or my external server to break out of this redirect loop?

Any guidance on a reliable pattern for proxying /blog to an external WordPress host—without triggering infinite redirects—would be greatly appreciated