How to make a field required or set default values in Sanity Studio

Great question! There are two different things you're asking about - making fields required and setting default/auto-generated values. Let me cover both:

Making Fields Required

To make a field required in Sanity Studio, you use the validation property with the Rule.required() method:

defineField({
  name: 'title',
  type: 'string',
  validation: (rule) => rule.required(),
})

You can also add a custom error message:

validation: (rule) => rule.required().error('Title is required before publishing')

Important note: This validation only works in the Studio interface. API mutations bypass these rules, so if you're creating content via the API, you'll need separate server-side validation.

Auto-Generating Slug from Title

For slug fields specifically, Sanity has a built-in slug field type that provides a "Generate" button and can automatically create URL-friendly slugs from another field:

defineField({
  name: 'slug',
  type: 'slug',
  options: {
    source: 'title',
    maxLength: 96,
  },
  validation: (rule) => rule.required(),
})

The source option tells it which field to use when generating the slug. You can even use multiple fields:

options: {
  source: (doc) => `${doc.title} ${doc.date}`,
}

Setting Default Values with initialValue

For other fields where you want a default value, use the initialValue property on defineField:

defineField({
  name: 'status',
  type: 'string',
  initialValue: 'draft',
})

For more complex defaults, you can use a function:

defineField({
  name: 'publishedAt',
  type: 'datetime',
  initialValue: () => new Date().toISOString(),
})

The initialValue sets the value when a new document is created, while validation rules ensure the field meets requirements before publishing. Together, they give you powerful control over your content structure!