"search
currently takes URLs, DOIs, PMCIDs, and PMID, ISBNs, QIDs, and fully formatted citations / free text search. If the unique identifier is unresolvable, you will get back a 404 response and a JSON error message."
From the Wikipedia REST API documentation: "Generates citation data given a URL, DOI, PMID, or PMCID."
And from the Citoid MediaWiki entrie: "To request metadata about a URL, DOI, PMCID, PMID or QID..."
Each one varies from the other. It's confusing. I've been triying to make petitions with ISBNs as query to verify, but I've got 404 responses.
Same with "free text search": @Mvolz (WMF) already clarified that in the current discussion page.