How to limit array items to one instance each in Sanity schema?

Looking at your code, you're trying to ensure that only one header and one footer can be added to the array. The Rule.unique() method you're using is actually for ensuring array items are unique within that array, but what you want is to limit each type to appearing only once.

Here's the correct approach - use custom validation on the array itself:

export default defineType({
  title: 'Component Builder',
  name: 'pageBuilder',
  type: 'array',
  of: [
    {
      type: 'header',
    },
    {
      type: 'footer',
    },
  ],
  validation: Rule => Rule.custom((items) => {
    if (!items) return true;
    
    const headerCount = items.filter(item => item._type === 'header').length;
    const footerCount = items.filter(item => item._type === 'footer').length;
    
    if (headerCount > 1) {
      return 'Only one header is allowed';
    }
    if (footerCount > 1) {
      return 'Only one footer is allowed';
    }
    
    return true;
  })
})

This custom validation function:

1. Filters the array items by _type to count how many of each type exist
2. Returns an error message if more than one header or footer is found
3. Otherwise returns true to indicate the validation passed

The validation will run whenever someone tries to add items to the array and will prevent publishing if they try to add multiple headers or footers. The editor will see a clear error message telling them what the limit is.

Note that Rule.unique() is meant for ensuring values within an array don't duplicate (like ensuring all items in a tags array are different), not for limiting how many times a specific type appears. That's why you need the custom validation approach shown above.