Next.js request.url Returns Internal Proxy URL on Containerized Platforms

Problem

When deploying Next.js to Railway, Vercel, or similar containerized platforms, request.url in Edge runtime or API routes can return the internal proxy URL (e.g., http://localhost:8080) instead of the public domain (e.g., https://example.com).

This causes NextResponse.redirect(new URL(path, request.url)) to redirect users to the internal URL, which fails with ERR_CONNECTION_REFUSED.

Context / Trigger Conditions

• Symptom: Browser shows ERR_CONNECTION_REFUSED to localhost:8080 or similar
• Console errors: Failed to fetch RSC payload followed by connection refused
• Network tab: Shows 307 redirect to https://localhost:8080/...
• Environment: Next.js deployed to Railway, Vercel, Docker, or any reverse proxy setup
• Code pattern: Using new URL('/path', request.url) for redirects