scholarly-metadata
OpenCitations
An open index of scholarly citation links, with more than two billion DOI-to-DOI connections as of its February 2026 refresh. Citation counts, incoming citations, and reference lists for any DOI are available through a free API; the website's search page sits behind a CAPTCHA, but the API answers cleanly. Use it to see who cites a paper or to build citation trails from fully open data.
Why it’s useful & how it works
The HTML search page carries hCaptcha, but the API answers cleanly both ways (token optional/encouraged). Integrate via API for citation-graph enrichment.
What’s inside
2B+ citation links (Feb 2026 refresh).
API access
https://api.opencitations.net/index/v2/citation-count/doi: <doi> ; citations/references endpoints
Access
Programmatic API access (a key may be required — see the API tag).