Creating middleware in Next.js

// proxy.ts (Next.js 16 - was middleware.ts; in root or src/)
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export function proxy(request: NextRequest) {
  // Clone the request headers
  const requestHeaders = new Headers(request.headers)
  requestHeaders.set('x-custom-header', 'my-value')
  
  // Create response with modified headers
  const response = NextResponse.next({
    request: {
      headers: requestHeaders,
    },
  })
  
  // Set response headers
  response.headers.set('x-response-header', 'value')
  
  return response
}

// Configure which paths proxy runs on
export const config = {
  matcher: [
    // Match all paths except static files and api
    '/((?!api|_next/static|_next/image|favicon.ico).*)',
    // Or specific paths
    '/dashboard/:path*',
    '/api/:path*',
  ],
}