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We have recently become unable to access the WorldCat API which provided the ability to generate citations using ISBN numbers. The Wikimedia Foundation's Editing team is investigating several options to restore the functionality, but will need to disable ISBN citation generation for now.
This affects citations made with the VisualEditor Automatic tab, and the use of the citoid API in gadgets and user scripts, such as the autofill button on refToolbar. Please note that all the other automatic ways of generating citations, including via URL or DOI, are still available.
You can keep updated on the situation via Phabricator, or by reading the next issues of m:Tech News. If you know of any users or groups who rely heavily on this feature (for instance, someone who has an upcoming editathon), I'd appreciate it if you shared this update with them.
Elitre (WMF), on behalf of the Editing team.