How to access parent field in custom input component like conditional hidden?

Great question! You're right that conditional hidden fields have access to parent and document in their callback functions, but custom input components don't get these props directly. The modern approach in Sanity Studio v3 is to use the useFormValue hook instead of the old withDocument pattern.

Here's how to access parent values in your custom input component:

import { useFormValue } from 'sanity'

const MyCustomInput = (props) => {
  // Get the parent object (similar to {parent} in conditional fields)
  const { path } = props
  const parentPath = path.slice(0, -1) // Remove the last segment to get parent path
  const parentValue = useFormValue(parentPath)
  
  // Or get the entire document (similar to {document} in conditional fields)
  const document = useFormValue([])
  
  // Access specific sibling fields
  const siblingField = useFormValue([...parentPath, 'siblingFieldName'])
  
  return (
    // Your component JSX
  )
}

The key is understanding that useFormValue takes a path array:

• [] returns the entire document
• ['fieldName'] returns a top-level field
• ['parent', 'child', 'fieldName'] navigates nested structures

For array items or nested objects, you can derive the parent path from props.path by slicing off the last segment. This gives you access to sibling fields within the same object, which is usually what you need when replicating the {parent} behavior from conditional fields.

Important note: Unlike v2 where props.document and props.parent were available, v3 requires using useFormValue for all form value access. The hook only works within the form builder context (custom input components), so you can't use it in document actions or other contexts outside the form.

This approach gives you the same power as conditional hidden fields but within your custom components!