Custom form components that suspend

Studio puts a fallback boundary around every field and every array item, so one suspending component cannot blank the form. That fallback is a generic skeleton: it does not know the size of what it replaces, so the form shifts when the component loads, and Studio warns about it in the console. The fix always lives in your code: declare a Suspense around the part that suspends, with a fallback that takes the same space as what it stands in for. Any boundary you declare is closer to what suspends than Studio's, so it always wins and nothing warns. That includes a boundary in a custom field around props.renderDefault(props), which also covers the field's input: use it when one fallback should cover both, for example a lazy input that also fetches.

Workflow

1. Find the component. The warning names the custom components declared for that field or item (components.field, .input or .item), the path, the document type and the schema type, and logs the components themselves; click one in the browser console to open its source. Production builds minify the names, so use the path and document type there. When the schema type declares none, the warning starts with "A form component" and the culprit is usually a plugin's form.components. React DevTools' Suspense tab shows which component actually suspended.
2. Find what suspends. Usually one of: React.lazy, use(promise), an App SDK hook (useDocumentProjection, useDocument, useQuery, useDocuments), or a library in suspense mode (useSuspenseQuery).
3. Move it into its own component if it is not already, and wrap only that component in <Suspense> inside the custom component. Keep props.renderDefault(props) outside the boundary, so the built-in input, field or item never unmounts (and never loses focus while someone types) when the async part suspends again.
4. Reserve the space. The fallback must have the size of the loaded UI:
   • Additive UI (warnings, previews, counts): render it after renderDefault so it never pushes the rest down, and give the fallback the height of one loaded row.
   • A whole component that loads lazily: a skeleton with the component's height, not null.
   • Array items: the list is virtualized and measures each row, so an item fallback that is shorter or taller than the loaded row makes the rows below it jump. Match the row height.
5. Do not suspend on a document that does not exist yet. A new document only exists once the first edit creates it, and SDK hooks that read the current document wait for it forever. Gate on useFormValue(['_createdAt']): it is unset until the document exists.
6. Keep promises stable. A promise created during render, in useState or in useMemo is thrown away while the component suspends on mount, so use() gets a new one every retry and never settles. Use the SDK hooks, a module-scope cache keyed by the inputs, or a data library.
7. Verify. Open a new document of the type and an existing one: the warning is gone, the rest of the form is editable right away, and nothing moves when the async part lands.

Example

import {createDocumentHandle, useDocumentProjection} from '@sanity/sdk-react'
import {Card, Skeleton, Stack, Text} from '@sanity/ui'
import {Suspense} from 'react'
import {type ArrayOfObjectsInputProps, getPublishedId, useFormValue} from 'sanity'

export function ModelSizeInput(props: ArrayOfObjectsInputProps) {
  const documentId = useFormValue(['_id']) as string
  const documentExists = Boolean(useFormValue(['_createdAt']))
  const hasModels = (props.value?.length ?? 0) > 0

  return (
    <Stack gap={3}>
      {props.renderDefault(props)}
      {documentExists && hasModels ? (
        <Suspense fallback={<Skeleton radius={2} style={{height: 33}} animated />}>
          <ModelSizeWarnings documentId={getPublishedId(documentId)} />
        </Suspense>
      ) : null}
    </Stack>
  )
}

function ModelSizeWarnings(props: {documentId: string}) {
  const {data} = useDocumentProjection<{missing: number}>({
    ...createDocumentHandle({documentType: 'product', documentId: props.documentId}),
    projection: `{"missing": count(models[!defined(@->size)])}`,
  })
  return (
    <Card padding={2} radius={2} tone={data.missing > 0 ? 'critical' : 'positive'}>
      <Text size={1}>{data.missing} models are missing a size</Text>
    </Card>
  )
}

The skeleton's height matches the one-line card it stands in for. If the loaded UI's height varies, reserve the most common case and keep the variable part below the input.