Error: ENOENT: no such file or directory when generating sitemap in Next.js on Vercel

I’m building a Next.js app deployed on Vercel, and I’ve implemented a dynamic sitemap generator in /src/app/sitemap.ts. The sitemap is generated by scanning markdown files in a _posts directory located in the root of my project. Here's my code:

import fs from 'fs';
import { join } from 'path';
import { MetadataRoute } from 'next';

export default async function sitemap(): Promise<MetadataRoute.Sitemap> {
  const blogPosts = fs.readdirSync(join(process.cwd(), '_posts'));
  const blogPostPages: MetadataRoute.Sitemap = blogPosts.map((post) => ({
    url: `${url}${post}`,
    lastModified: getLastModified(`_posts/${post}.md`),
    changeFrequency: 'daily',
    priority: 1,
  }));
  return blogPostPages;
}

This works perfectly in my local environment. However, when I deploy the app to Vercel, I get the following error:

Error: ENOENT: no such file or directory, scandir '/var/task/_posts'

It seems like the _posts directory isn’t accessible or doesn’t exist in the Vercel deployment environment.

Additional Context

• _posts contains markdown files (post1.md, post2.md, etc.).
• The app is deployed on Vercel using their default build pipeline.

Questions

• Why does this error occur on Vercel but not locally?
• How can I ensure the _posts directory is available in the deployed environment?
• Is there a better way to dynamically generate a sitemap in Next.js that works seamlessly on Vercel?