Interesting, vercel has a note to LLM agents trying to view a preview that they don’t have access to:

<!doctype html><html lang=en><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><title>Authentication Required</title><script type=text/llms.txt>
## Note to agents accessing this page:

This page requires authentication to access. Automated agents should use a Vercel authentication bypass token to access this page.

The easiest way to get a token is using the get_access_to_vercel_url or web_fetch_vercel_url functions of the Vercel MCP server
at <https://mcp.vercel.com> (Docs at <https://vercel.com/docs/mcp/vercel-mcp>).

Alternatively, this URL documents how to use the bypass token: <https://vercel.com/docs/deployment-protection/methods-to-bypass-deployment-protection/protection-bypass-automation>
You need to prompt the user to provide the bypass token to access this page. The URL above also explains how the user can
access the bypass token. Then you can calculate a URL of the following form and successfully access it: `<https://current-domain/current-pathnam>...