The services it calls

Four, and nothing else leaves the machine.

	What is sent	What comes back
NCBI E-utilities	the query, your email, your API key if set	the records
Unpaywall	a DOI, your email	whether a free copy exists, and where
Semantic Scholar	PMIDs, your API key if set	citation counts
radiopaedia.work/cite	an identifier you cited	the formatted citation

The email is not authentication

Both NCBI and Unpaywall require a contact address in every request. It exists so that a caller whose script misbehaves can be warned rather than silently blocked. Neither sends mail, neither has an account to create. Unpaywall returns 422 without one; NCBI throttles harder.

Rate limits

NCBI allows 3 requests a second, 10 with an API key, and the app paces itself accordingly. Unpaywall publishes 100,000 calls a day and no per-second limit; the app takes ten a second, which is what a polite client does.

What "not found" means

PubMed reports phrases it could not match in errorlist.phrasesnotfound — but only when the search returned nothing at all. It is not a broken query, and the app treats it as information rather than an error.

This matters because the ISSG filters, which are published strings thousands of characters long, contain terms that match nothing in the whole of MEDLINE: chemotreatment* is one. Treating that as fatal made every zero-result search with a guidelines filter fail with a message about syntax, hiding the real reason — there is nothing on that subject.

A field that does not exist is a different matter, and still raises.

Unpaywall over Semantic Scholar

Both can offer a free PDF link. When they disagree, Unpaywall wins: it is the service that does only this.