export function validateChatGptShareUrl(raw: string): string {
  let url: URL;
  try {
    url = new URL(raw);
  } catch {
    throw new Error("Enter a valid public ChatGPT share URL.");
  }

  if (url.protocol !== "https:") {
    throw new Error("Only HTTPS ChatGPT share URLs are accepted.");
  }

  const host = url.hostname.toLowerCase();
  const allowedHost = host === "chatgpt.com" || host === "chat.openai.com";
  if (!allowedHost || !url.pathname.startsWith("/share/")) {
    throw new Error("Only public ChatGPT share URLs under /share/ are accepted.");
  }

  url.hash = "";
  return url.toString();
}