Coverage, including the gaps

There is no complete denominator for global scholarship. The best current estimate is that the free, redistributable federation can reach 60–70% of worldwide research output. That estimate is not a recall measurement.

Queried live

Crossref: DOI-registered journal articles, proceedings, books, chapters, and reports, Metadata completeness is publisher-supplied and uneven.
Europe PMC: Life-science literature and preprints, Domain-specific; upstream availability can be intermittent.
DataCite: Datasets, software, theses, preprints, and other research outputs, Granular versions and files can resemble duplicate works.
PubMed: Biomedical and life-science literature, Domain-specific and rate-limited.
DOAJ: Open-access journals and articles across disciplines, Only journals admitted to DOAJ are represented.
Zenodo direct API: Datasets, software, posters, reports, and publications, A descriptive User-Agent is required; requests without one return 403. Files stay link-only.
HAL: French and international repository deposits, Repository coverage is strongest in France.
arXiv: Preprints in physics, mathematics, computer science, and adjacent fields, Most records lack a version-of-record DOI.

Local authority snapshots

OpenAlex: Cross-disciplinary venue, institution, publisher, and field authority data, The live API is credit-limited; Allodium uses redistributable snapshots.

Scheduled-harvest horizon

African Journals Online: African-published journals, OAI-PMH supports scheduled harvesting, not live relevance search.
LA Referencia: Latin American repository output in Spanish and Portuguese, OAI-PMH supports scheduled harvesting, not live relevance search.

Named blind spots

DBLP direct API: Computer-science bibliography (publications, proceedings, authors), The adapter is implemented, but DBLP's publication search API now returns an Anubis anti-bot HTML challenge instead of JSON from Cloudflare Worker egress. Computer-science records with DOIs remain reachable through Crossref, DataCite, and arXiv.
CNKI and Wanfang: Chinese-language scholarship, No free, redistributable public search API is available.
Standards organizations: ISO, IEEE, ANSI, DIN, and related standards, Metadata and documents are generally proprietary or paywalled.
RePEc: Economics working papers, No stable public JSON search API.
PhilPapers: Philosophy literature, No open API suitable for federated public search.

16 sources or explicit gaps documented. See The Standard for scoring rules.