@markkelnar so it looks like the original issue of not showing "something is wrong with the form data" is gone, but it still seems like something is wrong.

If I try and save a document with an invalid document (like just a string saying "invalid document") I get an admin error that states:

Did not save invalid graphql query string "invalid document"

However, even though the error states it did not save, it actually is saved and I can see the document in the list of GraphQL Documents:

perhaps the save_post hook is the wrong hook?

https://wordpress.stackexchange.com/questions/42013/prevent-post-from-being-published-if-custom-fields-not-filled#answer-42178

As m0r7if3r pointed out, there is no way of preventing a post from being published using the save_post hook, since the by the time that hook is fired, the post is already saved.

@markkelnar when testing, we can still "publish" the document without a valid document.

Since the current issue was specifically targeting the error messaging, I'll create another issue to follow-up on tightening up the behavior around publishing.

related: #243