🧰 Crossref Citation MCP Server

An MCP server over the official Crossref REST API (api.crossref.org) — scholarly work metadata and citation counts across registered publishers. Runs as an Apify Standby MCP server; connect an MCP client to the server URL at path /mcp.

This is a standby MCP server: it stays warm on Apify and answers tool calls from any MCP client (Claude Desktop, Cursor, an agent framework) at https://nexgenwatch--crossref-citation-mcp.apify.actor/mcp. Billing is pay-per-event: $0.05 when the server starts, then $0.05 per tool call (mcp-tool-call). Source: api.crossref.org, doi.org, dx.doi.org.

No login, no source API key and no CAPTCHA solving are involved: the source is read logged-out with an identified contact User-Agent.

📊 Sample Output

A standby server answers tool calls rather than writing a dataset, so the sample is the server's own live health answer (GET https://nexgenwatch--crossref-citation-mcp.apify.actor/, HTTP 200, read on 2026-09-17) followed by its tool table:

{
  "server": "crossref-citation",
  "billing_gate": true,
  "declared_events": [
    "apify-actor-start",
    "mcp-tool-call"
  ],
  "scope": "The official Crossref REST API (api.crossref.org): scholarly work metadata and citation counts (is-referenced-by-count) across registered publishers. Citation counts reflect Crossref's coverage, not a universal count.",
  "rate_limit_per_sec": 5.0,
  "rate_waits": 0,
  "ready": true
}

Tool	Arguments	What it returns
search_works	query: str, rows: int = 20	Search Crossref by free text. Returns matching works with their DOI, title, year, type and citation count (is-referenced-by-count), newest-relevance first.
get_work	doi: str	Full Crossref metadata for one DOI: title, authors, journal, year, publisher, citation count and reference count. A DOI not in Crossref is a sourced 404 answer.
get_citation_count	doi: str	How many works Crossref records as citing this DOI (is-referenced-by-count), plus the work's own reference count. A DOI not in Crossref is a sourced 404 answer.
explain_limits	``	What this server can and cannot tell you about Crossref data.

✅ What you get

4 tools, each returning a JSON object built only from what the source answered, with a scope and note field stating what the source does and does not cover. A lookup the source answers with 'not found' is a sourced answer (charged); malformed input or a source failure returns a notice that is NOT charged; nothing is ever fabricated.

Tools

• search_works(query, rows) — search Crossref by free text; returns DOIs, titles, years and citation counts.
• get_work(doi) — full metadata for one DOI: title, authors, journal, year, publisher, citation and reference counts.
• get_citation_count(doi) — how many works Crossref records as citing this DOI (is-referenced-by-count).
• explain_limits() — scope, the meaning of the citation count, and what the server will not assert.

Billing

Event	Price
Server start (apify-actor-start)	$0.05, charged once per run by the platform
Tool call (mcp-tool-call)	$0.05 per completed tool call

One flat charge per completed tool call. A DOI that Crossref returns 404 for is a sourced answer ("not in Crossref") and is charged; a search matching zero works is charged. A malformed DOI, an empty query, or an upstream Crossref failure is a notice and is not charged.

Source

The Crossref REST API, queried anonymously in the polite pool with a mailto. No API key. is-referenced-by-count reflects Crossref's coverage (a lower bound), not a universal count.

⚙️ Sample inputs

1. Connect an MCP client (Claude Desktop / Cursor style config; your Apify API token goes in the header):

{
  "mcpServers": {
    "crossref-citation-mcp": {
      "url": "https://nexgenwatch--crossref-citation-mcp.apify.actor/mcp",
      "headers": {
        "Authorization": "Bearer <YOUR_APIFY_TOKEN>"
      }
    }
  }
}

2. One tool call over JSON-RPC (what the client sends to /mcp; each call is one mcp-tool-call event):

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "search_works",
    "arguments": {
      "query": "<query>",
      "rows": 20
    }
  }
}

3. Health check from a shell (free — no tool call is charged):

$curl -H "Authorization: Bearer $APIFY_TOKEN" https://nexgenwatch--crossref-citation-mcp.apify.actor/

🧾 JSON sample record

The live health record above is the server's own answer; a tool result carries the fields listed in the tool table plus scope and note.

🔧 How it works

Source. api.crossref.org, doi.org, dx.doi.org — endpoints: http://doi.org/, https://api.crossref.org, https://api.crossref.org/works, https://api.crossref.org/works/10.1038/nature12373. Public, logged-out reads only.

Standby. Apify keeps one server process warm; each MCP session is served by that process and idle servers are stopped by the platform. Nothing is cached between calls except what the source itself caches.

Charging. Every tool call charges one mcp-tool-call at $0.05 through Apify's pay-per-event billing before the answer is returned; the declared events are asserted against the live pricing at startup (billing_gate in the health answer).

What is not done. No login, no CAPTCHA bypass, no private data, no scraping of pages the source does not publish to any visitor.

💰 Pricing example

Event	Free	Bronze	Silver	Gold
Actor Start (apify-actor-start)	$0.05	$0.05	$0.05	$0.05
Tool call (mcp-tool-call)	$0.05	$0.05	$0.05	$0.05

• 10 tool calls in one session: $0.05 start + 10 × $0.05 = $0.55
• 100 tool calls in one session: $0.05 start + 100 × $0.05 = $5.05
• 1000 tool calls in one session: $0.05 start + 1000 × $0.05 = $50.05

A health check or a connection that makes no tool call charges only the $0.05 start fee. A sourced 'not found' answer is a completed tool call and is charged; a notice for malformed input or a source failure is not charged (settle() charges only chargeable outcomes).

⚖️ Legal & ToS

The server reads public data published by api.crossref.org, doi.org, dx.doi.org to any visitor, identifies itself with a contact User-Agent, and does not access accounts or private data. Use the answers in line with the source's terms and your local law; the intended use is B2B research and agent tooling.

❓ FAQ

Q: Do I need a source API key?
A: No. api.crossref.org, doi.org, dx.doi.org is read logged-out. You need only your Apify API token to connect to the server.

Q: Which MCP clients work?
A: Any client that speaks the MCP HTTP transport: Claude Desktop, Cursor, Windsurf, LangChain/LlamaIndex MCP adapters and Apify's own MCP client. Point it at the /mcp URL above.

Q: What happens when the source has no answer?
A: A sourced 'not found' (for example a 404 for an unknown identifier) comes back as an answer with scope and note explaining what was checked (where the server carries those fields), and is charged as a completed call; malformed input or a source outage returns a notice that is not charged. Nothing is invented.

Q: How is a call billed?
A: One mcp-tool-call event per tool call at $0.05, plus $0.05 when the server starts. Idle time is not billed to you.

Q: Is it rate limited?
A: The server paces requests to the source and answers calls in the order they arrive.

Q: Can I call it without an MCP client?
A: Yes — the /mcp endpoint is plain JSON-RPC over HTTP (sample input 2), and GET / is a free health check.

🆘 Troubleshooting

• 401 / 403 from the server → the Authorization header is missing or the token is wrong → use Bearer <your Apify API token>.
• 400 Missing session ID → the client hit /mcp without initialising an MCP session → let the MCP client connect (it performs the handshake); raw JSON-RPC needs the session header the initialise call returns.
• Tool returns a notice instead of data → the input was malformed or empty, or the source failed → the call was not charged; check the note/reason field, fix the input and call again.
• First call is slow → the standby server was cold and Apify started it → later calls in the same session are fast.

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