The bug where the API errored when there is no item for the requested sitelink is fixed now. Can be tried out here: https://doc.wikimedia.org/Wikibase/master/js/graphql-explorer/?query=ewogIGl0ZW1CeVNpdGVsaW5rKHNpdGVJZDogImVud2lraSIsIHRpdGxlOiAiVGhpcyBkb2VzIG5vdCBleGlzdCIpIHsKICAgIGlkCiAgfQp9

This is fixed on beta wikidata now. Can be tried out using the following query:

{
  searchItems(
    query: {
      or: [
        { property: "P443", value: "potato" },
        { property: "P443", value: "does not exist" },
      ]
    }
  ) {
    edges {
      node { id }
    }
  }
}

But when adding a single statement to the "or"-block that doesn't return a result, the whole result is empty.

Here's my suggestion for the schema of this new field:

Can be tested on https://test.wikidata.org/w/api.php?action=wbgraphql&format=json e.g. with the following query:

{
  itemBySitelink(siteId: "enwiki", title: "Test") {
    id
  }
}

We won't do this as a subtask of T413657 but rather as a separate story.

In T419571#11726878, @Ifrahkhanyaree_WMDE wrote:

@ECohen_WMDE and I think a slightly different version of option 3 is best - so return all results with a warning saying "The external ID is in use by multiple items"

is that do-able?