How to set the initialValue of a block field in Sanity.io

Yes, you're encountering the _key warning because block content in Sanity requires unique _key properties for each object in the array structure, including both the block itself and its children.

The solution is to explicitly add _key values using a UUID generator. Here's the working approach:

import {v4 as uuidv4} from 'uuid'

defineField({
  title: "Description",
  name: "description",
  type: "array",
  of: [{ type: "block" }],
  initialValue: [
    {
      _key: uuidv4(),
      _type: "block",
      children: [
        {
          _key: uuidv4(),
          _type: "span",
          marks: [],
          text: "Hello there\n\nThis is a new line!",
        },
      ],
      markDefs: [],
      style: "normal",
    },
  ],
})

Important notes:

• You need to add _key to both the block object and each child span object
• The _key values must be unique identifiers (UUID is perfect for this)
• Unfortunately, there's no way to auto-generate these keys directly in the initialValue - you must set them manually in your schema definition

There is an autoGenerateArrayKeys: true option in the Sanity client config, but this only applies when writing documents via the client API, not for schema-level initialValue definitions. As confirmed in the Sanity community discussion, it's not possible to use this approach with initialValue.

The manual UUID approach is the recommended solution and will eliminate the warning while ensuring your initial block content is properly structured.