Why isn't my .env.local file working in Next.js with Sanity?

I understand your frustration! Environment variable issues in Next.js projects with Sanity can be tricky. Here are the most common reasons why your .env.local file might not be working, even though the values are correct:

1. Variable Naming for Browser vs Server

In Next.js, where you use the environment variable matters:

• Server-side code (API routes, getServerSideProps, getStaticProps): Can access variables without any prefix
• Browser/client-side code: Requires the NEXT_PUBLIC_ prefix

If you're trying to use Sanity client in a component that runs in the browser, your variables need to look like this:

NEXT_PUBLIC_SANITY_PROJECT_ID=your-project-id
NEXT_PUBLIC_SANITY_DATASET=production

Without the NEXT_PUBLIC_ prefix, Next.js won't expose them to the browser, even though they work fine in server-side code. This is the most common issue!

2. Server Restart Required

Did you restart your Next.js dev server after creating or modifying .env.local? Changes to environment variables require a full restart:

# Stop your dev server (Ctrl+C) and restart
npm run dev

Hot reload doesn't pick up environment variable changes.

3. File Location

Make sure .env.local is in the root of your Next.js project (same directory as package.json and next.config.js), not in a subdirectory.

4. Sanity Studio vs Next.js App

If you have Sanity Studio embedded in your Next.js project, remember:

• Sanity Studio requires SANITY_STUDIO_ prefix for its own configuration
• Next.js app requires NEXT_PUBLIC_ prefix for client-side code

You might need both:

# For Next.js client components
NEXT_PUBLIC_SANITY_PROJECT_ID=your-project-id
NEXT_PUBLIC_SANITY_DATASET=production

# For embedded Sanity Studio
SANITY_STUDIO_API_PROJECT_ID=your-project-id
SANITY_STUDIO_API_DATASET=production

5. Vercel/Deployment Environment

If this is happening on Vercel or another host (not locally), you need to add the environment variables in your hosting platform's dashboard. The .env.local file is gitignored and won't be deployed.

Quick Debug

Add this to a component to see what's actually available:

console.log('Project ID:', process.env.NEXT_PUBLIC_SANITY_PROJECT_ID)
console.log('All public vars:', Object.keys(process.env).filter(k => k.startsWith('NEXT_PUBLIC_')))

Based on the Sanity environment variables documentation, the prefix requirements are critical for proper configuration. The most likely culprit is missing the NEXT_PUBLIC_ prefix if you're using these variables in client-side React components.