OpenCitations

Open citation data (who cites whom) as open lists of citing/cited PIDs. Use it when you need an openly licensed citation edge, a count you can cite, or an Open Citation Identifier (OCI). It is not a paper search and it does not return titles on the Index endpoints.

For a literature search use PubMed, OpenAlex, or Semantic Scholar. For a citation graph with titles and abstracts, start with Semantic Scholar or OpenAlex and treat OpenCitations as the open-data check. Europe PMC /citations is the biomedical alternative when you already have a {source}/{id} pair.

All figures below verified 2026-09-10.

Base URLs

https://api.opencitations.net/index/v2    # citation edges and counts
https://api.opencitations.net/meta/v1     # bibliographic metadata for a PID

Index v2 is current (v2.2.0, 2025-04-15). Meta lives at v1 — meta/v2/... is HTTP 404.

Authentication

Optional. Public calls work without a token. For heavier use, request an access token from OpenCitations and send Authorization: <token>. Do not add a new .env key for this; proceed without one.

Rate Limits

The Index API documents 180 requests/minute per IP. Serialize requests. Call /citation-count before /citations — the list endpoint returns every incoming citation in one body, with no page parameter.

Identifier prefix (required)

Index v2 accepts doi:, pmid:, omid:, issn:, and isbn:; for papers prefer the first three. A bare DOI is HTTP 400:

GET /index/v2/citation-count/10.1038/nature12373
  -> 400  the value '10.1038/nature12373' is not valid for parameter 'id'
          Example: /index/v2/citation-count/doi:10.1108/jd-12-2013-0166

GET /index/v2/citation-count/doi:10.1038/nature12373
  -> 200  [{"count": "1806"}]

Key Endpoints

1. Incoming citation count

GET /index/v2/citation-count/{id}

Always a one-element JSON array. count is a string, not an integer.

A missing work is HTTP 200 with "0", not 404. Do not treat 200 as "this DOI is in the index."

2. Outgoing reference count

GET /index/v2/reference-count/{id}

Same shape as citation-count.

3. Incoming citations / outgoing references

GET /index/v2/citations/{id}
GET /index/v2/references/{id}

Each item:

Verified on doi:10.1038/nature12373 /references: 30 rows. First citing is omid:br/06120344846 doi:10.1038/nature12373 openalex:W2159974629 pmid:23903748. Parse the doi: token out; do not take the whole string as one DOI.

Verified on doi:10.1186/1756-8722-6-59 /citations: 217 rows in one response. For nature12373 the count is 1806 — do not pull that list unless the user asked for the full set.

4. One citation by OCI

GET /index/v2/citation/{oci}

oci is the two-number form without an oci: prefix.

5. Metadata (titles, authors)

GET /meta/v1/metadata/{id}

Same doi: / pmid: / omid: prefix. Returns id (space-separated PIDs), title, author (semicolon-separated, may include ORCID + OMID). Use this when you have an edge from Index and need a human-readable label.

Typical Workflow

1. You have a DOI or PMID.
2. citation-count first. If "0", say OpenCitations has no incoming citations — not that the paper is uncited everywhere.
3. For a short list, /references or /citations. Extract the doi: token from each citing/cited string.
4. Resolve titles with Meta, Crossref, or Semantic Scholar. Do not invent them from the OCI.

Failure Modes

Official sources reviewed 2026-09-30

• https://api.opencitations.net/index/v2
• https://api.opencitations.net/meta/v1