Sanity Studio missing projectId despite .env.development configuration

The issue is with your environment variable naming! Sanity Studio v3 uses a specific naming convention for environment variables, and you've got a slight mismatch.

Your environment variables should be named SANITY_STUDIO_PROJECT_ID and SANITY_STUDIO_DATASET (not SANITY_STUDIO_API_PROJECT_ID and SANITY_STUDIO_API_DATASET).

Here's how your .env.development file should look:

# For Next.js
NEXT_PUBLIC_SANITY_PROJECT_ID="#######"
NEXT_PUBLIC_SANITY_DATASET="production"

# For Studio
SANITY_STUDIO_PROJECT_ID="#######"
SANITY_STUDIO_DATASET="production"

Then in your sanity.config.js or sanity.config.ts, you can reference them in your defineConfig like this:

import { defineConfig } from 'sanity'

export default defineConfig({
  projectId: process.env.SANITY_STUDIO_PROJECT_ID,
  dataset: process.env.SANITY_STUDIO_DATASET,
  // ... rest of your config
})

Important notes:

1. All Sanity Studio environment variables must be prefixed with SANITY_STUDIO_ to be accessible in your Studio code
2. The environment file is determined automatically: .env.development loads when running sanity start, and .env.production loads during sanity build or sanity deploy
3. After changing your .env file, restart your dev server for the changes to take effect
4. Project IDs and dataset names aren't secret - they'll be exposed in your built application, so it's safe to use them in environment variables

If you want to use different variable names (like SANITY_STUDIO_API_PROJECT_ID), you'll need to explicitly map them in your config file, but the standard convention as documented in the Sanity environment variables docs is SANITY_STUDIO_PROJECT_ID and SANITY_STUDIO_DATASET.