All the subtasks of this task are done, but I guess the task isn’t done yet:

link exists…

✔

…and is marked as external link

✘ oops

clicking link leads to external tool

✔

external tool has Schema code pre-filled

✔

AND I can enter a SPARQL query to get the list of entities to check against

not really (you first need to figure out how to configure the SPARQL endpoint, and then where and how to enter the SPARQL query

I’m not sure what to do with this task now… create additional subtasks?

In T216816#5039809, @Lucas_Werkmeister_WMDE wrote:

…and is marked as external link

✘ oops

For me the link already has the class external. However, there don't seem to be any CSS rules active for this class on this element?

In T216816#5039809, @Lucas_Werkmeister_WMDE wrote:

AND I can enter a SPARQL query to get the list of entities to check against

not really (you first need to figure out how to configure the SPARQL endpoint, and then where and how to enter the SPARQL query

Not sure what we need to do to make this work with Wikidata?

I’m not sure what to do with this task now… create additional subtasks?

The first one could have an additional subtask, even though it should hopefully only be a minor patch. But with a subtask it can be properly verified.
The second could use one as well to describe the problem better and to decide what we should still do, if anything?

For me the link already has the class external. However, there don't seem to be any CSS rules active for this class on this element?

MediaWiki only has styles for .mw-parser-output .external. However, I don’t think adding class="mw-parser-output" to our custom output is a great idea, so we should investigate a few other options first.

See also follow-up: T218886: Improve shex-simple tool for WikibaseSchema use