crossref-mcp-server

<div align="center"> <h1>@cyanheads/crossref-mcp-server</h1> <p><b>Resolve DOIs, search ~155M scholarly works, and fetch references via the Crossref REST API. STDIO or Streamable HTTP.</b> <div>5 Tools</div> </p> </div>

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Tools

Five tools for working with Crossref data — DOI resolution, full-text search across all scholarly works, outgoing reference lists, and journal/funder lookup:

| Tool | Description | |:-----|:------------| | crossref_get_work | Resolve a DOI to its full Crossref metadata record: title, authors, affiliations, abstract (when deposited), journal, publication date, type, license, full-text links, funder acknowledgements, and outgoing reference list | | crossref_search_works | Search the Crossref works index by free text and/or structured filters. Supports sort, field selection, and cursor-based deep paging. | | crossref_get_references | Return the outgoing reference list for a DOI — the works cited by this paper, with raw citation strings and resolved DOIs where available | | crossref_search_journals | Find Crossref journal records by ISSN or title query; optionally retrieve the journal's most recent works | | crossref_search_funders | Find funders registered in the Crossref Funder Registry by name or funder DOI; optionally retrieve funded works |

crossref_get_work

Resolve a DOI to its canonical Crossref record.

• DOI validated against 10.NNNN/suffix regex before the upstream call
• Returns title, authors with affiliations, abstract (when deposited), container/journal, publication date, work type, ISSN, license URLs, full-text link URLs, and funder acknowledgements
• Incoming citation count (is-referenced-by-count) is included; citing works are not — Crossref does not expose that data. Use OpenAlex for citation graphs.

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crossref_search_works

Search across ~155M Crossref-registered works.

• Free-text query plus a structured filter object using Crossref's hyphen-separated key syntax: from-pub-date, until-pub-date, type, funder, issn, member, has-abstract, has-references, has-full-text, directory (use DOAJ to restrict to open-access content)
• Sort by relevance, is-referenced-by-count, published, deposited, or score
• fields parameter narrows response payload — useful for large result sets
• Offset paging up to ~10K results; deep paging requires cursor=* on the first call, then pass the returned next-cursor token. Cursor and offset cannot be combined.

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crossref_get_references

Fetch the outgoing reference list for a DOI.

• Each reference includes its raw citation string and, where Crossref has resolved it, a DOI for follow-up lookup
• Coverage varies by publisher — pre-2000 literature and non-participating publishers may have no reference list
• Single-hop only; agents that need N-hop traversal chain calls explicitly

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crossref_search_journals

Find journal records by ISSN or title.

• include_works: true triggers a second upstream call to fetch the journal's most recent works
• Returns journal title, publisher, ISSN-L, subject areas, and DOI prefix

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crossref_search_funders

Find funders in the Crossref Funder Registry.

• Accepts a name query or a direct funder DOI
• include_works: true retrieves funded works for the matched funder
• Returns funder name, DOI, country, and alternate names

Features

Built on @cyanheads/mcp-ts-core:

• Declarative tool definitions — single file per tool, framework handles registration and validation
• Unified error handling across all tools
• Pluggable auth (none, jwt, oauth)
• Swappable storage backends: in-memory, filesystem, Supabase, Cloudflare KV/R2/D1
• Structured logging with optional OpenTelemetry tracing
• STDIO and Streamable HTTP transports

Crossref-specific:

• Polite-pool User-Agent header injected on every request — priority access granted via CROSSREF_MAILTO email address, no API token required
• withRetry: 3 attempts, exponential backoff, handles both 429 and 503 responses
• Cursor-based deep paging for result sets beyond the ~10K offset cap
• Filter key validation: Crossref uses hyphens (has-abstract, has-references, from-pub-date); the server enforces correct syntax and surfaces API validation errors with actionable recovery hints

Getting started

Add the following to your MCP client configuration file. CROSSREF_MAILTO is optional but recommended — without it the server uses Crossref's anonymous pool with stricter rate limits.

{
  "mcpServers": {
    "crossref-mcp-server": {
      "type": "stdio",
      "command": "bunx",
      "args": ["@cyanheads/crossref-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_LOG_LEVEL": "info",
        "CROSSREF_MAILTO": "your-email@example.com"
      }
    }
  }
}

Or with npx (no Bun required):

{
  "mcpServers": {
    "crossref-mcp-server": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "@cyanheads/crossref-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_LOG_LEVEL": "info",
        "CROSSREF_MAILTO": "your-email@example.com"
      }
    }
  }
}

Or with Docker:

{
  "mcpServers": {
    "crossref-mcp-server": {
      "type": "stdio",
      "command": "docker",
      "args": [
        "run", "-i", "--rm",
        "-e", "MCP_TRANSPORT_TYPE=stdio",
        "-e", "CROSSREF_MAILTO=your-email@example.com",
        "ghcr.io/cyanheads/crossref-mcp-server:latest"
      ]
    }
  }
}

For Streamable HTTP, set the transport and start the server:

MCP_TRANSPORT_TYPE=http MCP_HTTP_PORT=3010 CROSSREF_MAILTO=your-email@example.com bun run start:http
# Server listens at http://localhost:3010/mcp

Prerequisites

• Bun v1.3.11 or higher (or Node.js v24+).
• An email address for CROSSREF_MAILTO is optional but recommended — Crossref's polite pool grants priority access to clients that identify themselves. No account or token is required.

Installation

1. Clone the repository:

git clone https://github.com/cyanheads/crossref-mcp-server.git

1. Navigate into the directory:

cd crossref-mcp-server

1. Install dependencies:

bun install

1. Configure environment:

cp .env.example .env
# edit .env and optionally set CROSSREF_MAILTO for polite-pool access

Configuration

All configuration is validated at startup via Zod schemas in src/config/server-config.ts.

| Variable | Description | Default | |:---------|:------------|:--------| | CROSSREF_MAILTO | Email address embedded in the polite-pool User-Agent header. Optional — server starts without it but logs a warning and uses the anonymous pool with stricter rate limits. | — | | CROSSREF_BASE_URL | Crossref API base URL. Override for testing against a local proxy. | https://api.crossref.org | | CROSSREF_TIMEOUT_MS | Per-request timeout in milliseconds. | 10000 | | MCP_TRANSPORT_TYPE | Transport: stdio or http. | stdio | | MCP_HTTP_PORT | Port for the HTTP server. | 3010 | | MCP_AUTH_MODE | Auth mode: none, jwt, or oauth. | none | | MCP_LOG_LEVEL | Log level (RFC 5424). | info | | LOGS_DIR | Directory for log files (Node.js only). | <project-root>/logs | | OTEL_ENABLED | Enable OpenTelemetry instrumentation. | false |

See .env.example for the full list of optional overrides.

Running the server

Local development

• Build and run:

  # One-time build
  bun run rebuild

  # Run the built server
  bun run start:stdio
  # or
  bun run start:http

• Run checks and tests:

  bun run devcheck   # Lint, format, typecheck, security
  bun run test       # Vitest test suite
  bun run lint:mcp   # Validate MCP definitions against spec

Project structure

| Directory | Purpose | |:----------|:--------| | src/index.ts | createApp() entry point — registers tools and inits services. | | src/config | Server-specific environment variable parsing and validation with Zod. | | src/mcp-server/tools | Tool definitions (*.tool.ts). Five tools for Crossref data access. | | src/services/crossref | CrossrefService — HTTP client, polite-pool header, retry, pagination helpers. | | tests/ | Unit and integration tests mirroring src/. |

Development guide

See CLAUDE.md for development guidelines and architectural rules. The short version:

• Handlers throw, framework catches — no try/catch in tool logic
• Use ctx.log for request-scoped logging, ctx.state for tenant-scoped storage
• Register new tools via the barrel in src/mcp-server/tools/definitions/index.ts
• Wrap external API calls: validate raw → normalize to domain type → return output schema; never fabricate missing fields (abstracts, reference lists, and affiliations are frequently absent in Crossref records)

Contributing

Issues and pull requests are welcome. Run checks and tests before submitting:

bun run devcheck
bun run test

License

Apache-2.0 — see LICENSE for details.