Deploying Next JS on Vercel

This is a classic environment variable issue when deploying Next.js + Sanity to Vercel! The error you're seeing happens because getStaticPaths is returning undefined instead of an array, which means your Sanity client isn't successfully fetching data during the Vercel build.

Based on this exact issue from the Sanity community, the problem is almost always incorrect environment variables in Vercel. Here's what's happening:

The Root Cause

Your code works locally because you're using local environment variables (likely in a .env.local file), but when Vercel builds your site, it's using the environment variables configured in your Vercel project settings. If these are incorrect or missing, the Sanity client can't connect to your dataset, returns undefined, and Next.js throws that error.

The Solution

1. Check your Vercel environment variables: Go to your Vercel project settings → Environment Variables and verify:

   • NEXT_PUBLIC_SANITY_PROJECT_ID matches your actual Sanity project ID
   • NEXT_PUBLIC_SANITY_DATASET is set (usually production)
   • Any other Sanity-related variables are correct

2. Verify your dataset is public: In your Sanity project settings, make sure your production dataset has public read access enabled (if you're not using authentication).

3. Add debugging: Temporarily add console.log() statements to see what's happening:

export async function getStaticPaths() {
  const client = getClient()
  console.log('Project ID:', client.config().projectId) // Check this matches
  
  const routes = await client.fetch(`*[_type == "route" && defined(slug.current)]{
    "params": {"slug": slug.current}
  }`)
  
  console.log('Routes fetched:', routes) // This will show in Vercel build logs
  
  return {
    paths: routes || [],  // Use empty array instead of null
    fallback: true
  }
}

4. Use an empty array as fallback: Change paths: routes || null to paths: routes || [] - Next.js expects an array, and an empty array with fallback: true is valid.

The community member who posted this exact issue solved it by discovering their project ID was incorrect and their Vercel environment variables weren't set up properly. Check those Vercel environment variables first - that's almost certainly where your issue is!