How to set initialValue for a field in Sanity schema?

Based on the Sanity documentation, you're experiencing a common issue with initialValue in Sanity Studio. Here's what's likely happening and how to fix it:

The Most Common Issue: Existing Documents

Field-level initialValue only works when creating NEW documents. If you've already created the document (even as a draft), the initial value won't populate. This is the most frequent reason developers think initialValue isn't working.

Solution: Delete any existing drafts/documents and create a completely new document to see the initial value.

Your Code Looks Correct

The example you're using is valid for Sanity Studio v3:

export default {
  name: 'project',
  type: 'document',
  title: 'Project',
  fields: [
    {
      name: 'title',
      title: 'Title',
      type: 'string',
      initialValue: 'This string'
    }
  ]
}

Better Approach: Initial Value Templates

For Sanity Studio v3, the recommended approach is using Initial Value Templates instead of field-level initialValue. These are more powerful and reliable:

// sanity.config.ts
export default defineConfig({
  // ... other config
  document: {
    newDocumentOptions: (prev, context) => {
      return [
        ...prev,
        {
          id: 'project-with-title',
          title: 'Project',
          schemaType: 'project',
          value: {
            title: 'This string'
          }
        }
      ]
    }
  }
})

Additional Troubleshooting

1. Use defineField helper for better type safety:

import {defineField, defineType} from 'sanity'

export default defineType({
  name: 'project',
  type: 'document',
  fields: [
    defineField({
      name: 'title',
      type: 'string',
      initialValue: 'This string'
    })
  ]
})

2. Clear cache: Clear browser cache/local storage and restart your dev server
3. Verify Studio version: Make sure you're on Studio v3 (check package.json)
4. Check console: Look for any errors in the browser developer console

The key takeaway: initialValue at the field level does work in Sanity v3, but only for brand new documents. For more control and reliability, Initial Value Templates are the modern recommended approach.