---
title: Free Citation API — Resolve DOIs, PMIDs & ISBNs to Citations
description: A free citation API and DOI resolver API. Resolve a DOI, PMID, PMCID, ISBN, ISSN, arXiv ID, or ADS bibcode to formatted citations (10,000+ styles) and bibliography exports, and check retraction, open-access, and fabrication status. No key required for light use.
last_updated: "2026-06-17"
---

# A free citation API for DOIs, PMIDs, and ISBNs

> Resolve a DOI, PubMed ID, PMCID, ISBN, ISSN, arXiv ID, or ADS bibcode to a formatted citation or a bibliography file with a single HTTP call. Free for light use, no key required, deterministic output — built for scripts, reference managers, and AI agents.
> Last updated: 2026-06-17
> HTML version: https://scholar-sidekick.com/citation-api
> Full reference: https://scholar-sidekick.com/docs

## Quickstart

Send one identifier (or many, one per line) to `/api/format` and get back a formatted citation. No signup, no key for light use:

```bash
curl -sS -X POST "https://scholar-sidekick.com/api/format" \
  -H "Content-Type: application/json" \
  -d '{"text":"10.1038/nphys1170","style":"apa"}'
```

Full endpoint reference, parameters, and response shapes: https://scholar-sidekick.com/docs
Machine-readable spec (OpenAPI): https://scholar-sidekick.com/openapi/openapi.yml

## What the citation API does

- **Resolve an identifier** — DOI, PMID, PMCID, ISBN, ISSN, arXiv ID, ADS bibcode, or WHO IRIS URL → structured CSL-JSON metadata (title, authors, journal, year, identifiers).
- **Format a citation** — Vancouver, APA, AMA, IEEE, CSE, or any of 10,000+ CSL styles (Chicago, Harvard, Nature, BMJ, Lancet…), as text or HTML.
- **Export a bibliography** — BibTeX, RIS, EndNote XML, RefWorks, MEDLINE/NBIB, Zotero RDF, CSL-JSON, or CSV.
- **Check retraction status** — via Retraction Watch and Crossref.
- **Check open access** — OA status and the best legal URL, via Unpaywall.
- **Verify a citation** — cross-check a claimed title against the paper its identifier resolves to, catching the real-DOI / invented-title fabrication pattern a plain lookup misses.

## A DOI resolver API

Point the API at a DOI and it returns the full bibliographic record — who wrote it, where it was published, when, and what other identifiers it carries. The same endpoint resolves PubMed IDs against PubMed, preprints against arXiv, books against OpenLibrary, and astrophysics records against NASA ADS, so you don't have to special-case each registry.

## A citation API for AI agents

Scholar Sidekick is also an open-source MCP (Model Context Protocol) server, so Claude, ChatGPT, Cursor, and custom agents can call it as a tool with no glue code. Whether you call the REST endpoints or the MCP tools, the output is **deterministic and version-pinned**: the same input returns the same bytes, which matters when an agent's citations have to be reproducible and auditable.

- MCP server: https://scholar-sidekick.com/mcp
- Engineering principles (provenance + versioning): https://scholar-sidekick.com/engineering-principles

## Why not just call Crossref or OpenAlex directly?

You can — and for raw metadata, you should. But Crossref and OpenAlex hand you a metadata record, not a citation. To get from a DOI to a finished reference you still have to detect the identifier type, normalise it, run a CSL engine for the style you want, and write each export format by hand — then repeat that for PMIDs, ISBNs, arXiv IDs, and the rest. Scholar Sidekick does all of that behind one endpoint, across eight identifier types, with deterministic output and built-in retraction, open-access, and fabrication checks. It is a thin, transparent layer over the same authoritative sources — not a replacement for them.

## Pricing

Free for light, anonymous use with no key. A free key raises the rate limit; paid tiers add a higher monthly quota and are billed through RapidAPI. **Every tier has the full feature set** — only the request quota and per-IP burst scale with price.

| Plan | Price | Requests / month | Per-IP burst | Best for |
|------|-------|------------------|--------------|----------|
| BASIC | Free | 500 | 1× (base) | Trying the API, hobby scripts, low-volume agents |
| PRO | $9 / mo | 10,000 | 2× | Indie apps, research tooling, steady production use |
| ULTRA | $49 / mo | 100,000 | 4× | High-traffic apps and batch citation pipelines |
| MEGA | $199 / mo | 500,000 | 8× | Platforms and bulk bibliography processing at scale |

All prices are USD per month. Machine-readable pricing: https://scholar-sidekick.com/pricing.md

## FAQ

**Is there a free citation API?**
Yes. The Scholar Sidekick citation API is free for light, anonymous use with no key — you can call it from a script or an AI agent immediately. A free API key raises the rate limit, and paid tiers add a higher monthly quota; every tier has the full feature set.

**What is a DOI resolver API?**
A DOI resolver API takes a DOI (or other scholarly identifier) and returns its bibliographic record — title, authors, journal, year, and identifiers. Scholar Sidekick resolves DOI, PMID, PMCID, ISBN, ISSN, arXiv ID, and ADS bibcode against the appropriate registry and returns clean CSL-JSON or a formatted citation.

**Is there a citation API for AI agents and coding agents?**
Yes. Scholar Sidekick is a REST API and an open-source MCP server, so Claude, ChatGPT, Cursor, and custom agents can call it directly. Output is deterministic and version-pinned, so the same input returns the same bytes.

**Does the citation API require an API key?**
No key is required for light use. For a higher rate limit, create a free first-party key (prefixed `ssk_`) at https://scholar-sidekick.com/account and send it as an `Authorization: Bearer ssk_…` header. Paid/managed tiers are available through the RapidAPI gateway. Scholar Sidekick does not use OAuth.

## Related

- API docs: https://scholar-sidekick.com/docs
- API docs (markdown): https://scholar-sidekick.com/docs.md
- MCP server: https://scholar-sidekick.com/mcp
- Developer resources: https://scholar-sidekick.com/developers
- RapidAPI: https://rapidapi.com/scholar-sidekick-scholar-sidekick-api/api/scholar-sidekick
- API Terms: https://scholar-sidekick.com/legal/api-terms