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doi.org content-negotiation provider.
Asks the DOI system itself for CSL-JSON (Accept: application/vnd.citationstyles.csl+json); doi.org redirects to the DOI’s
registration agency — Crossref, DataCite, mEDRA, JaLC, … — which answers.
So it covers every registered DOI, not just one agency’s, and its 404 is
the authoritative “this DOI does not exist”.
One DOI per request (no batching), so it sits after the batch-capable providers as a fallback. Observed 2026-11-10: Crossref-registered DOIs answer with Crossref’s own type names and a 10 req/s limit header; DataCite’s answers (e.g. arXiv) carry abstracts.