paper.doi-lookup

Resolve a DOI to authoritative bibliographic metadata via Crossref. Accepts bare DOI (10.1038/nature12373) or full DOI URL (https://doi.org/10.1038/nature12373). Returns work type, title, container (journal/conference), publisher, issued/published dates, abstract, authors (with ORCID + affiliations), page range, volume/issue, ISBN + ISSN lists, canonical URL, reference + citation counts, license blocks, and subject classifications. ~160M works in Crossref. Best authoritative DOI → bibliographic resolver. Metadata is CC0.

Overview

The doi lookup API is a pay-per-call paper endpoint built for AI agents and autonomous software. Resolve a DOI to authoritative bibliographic metadata via Crossref.

There is no signup and no API key. An agent (or any HTTP client) hits the endpoint, receives an x402 "402 Payment Required" challenge, signs a sub-cent USDC payment on Base or Solana, and retries — the data comes back on the paid request. That makes it a drop-in doi lookup data source for an agent tool-use loop, an MCP host, or a backend that needs paper data on demand without onboarding to yet another vendor portal.

Use cases

• Give an AI agent live doi lookup lookups as a tool, with no API key to provision or rotate — the agent pays per call from a wallet you fund.
• Enrich records in a data pipeline with paper data, paying only for the calls you actually make instead of a monthly seat.
• Ground LLM answers in authoritative paper data to cut hallucination, then optionally request a signed response for verifiability.
• Prototype against the live doi lookup API in minutes using the free trial call, then flip to per-call x402 payment for production with no code change.

Parameters

Code samples

# 1. Probe with no auth → 402 envelope with PaymentRequirements
curl -sS 'https://2s.io/api/paper/doi-lookup?doi=xxxxxxx'

# 2. Sign + retry with PAYMENT-SIGNATURE:
curl -sS 'https://2s.io/api/paper/doi-lookup?doi=xxxxxxx' \
  -H 'PAYMENT-SIGNATURE: <base64-json-payload>'

# Or use the canonical runner (handles probe → sign → retry):
#   EVM_PRIVATE_KEY=0x... node --env-file=.env.local \
#     --experimental-strip-types scripts/x402-pay.ts \
#     'https://2s.io/api/paper/doi-lookup?doi=xxxxxxx'

import { TwoS } from '@2sio/sdk'

const client = new TwoS({
  privateKey: process.env.EVM_PRIVATE_KEY as `0x${string}`,
})

const result = await client.paper.doiLookup({
  "doi": "xxxxxxx"
})

console.log('endpoint:', result.endpoint)
console.log('cost:', result.costUsd, 'USDC')
console.log('tx:', result.settlement?.txHash)
console.log('data:', result.data)

import os
from twosio import TwoS

client = TwoS(private_key=os.environ["EVM_PRIVATE_KEY"])

result = client.paper.doi_lookup(doi="xxxxxxx")

print("endpoint:", result.endpoint)
print("cost:", result.cost_usd, "USDC")
print("tx:", (result.settlement or {}).get("tx_hash"))
print("data:", result.data)

// 1. Add @2sio/mcp to your MCP host config (Claude Desktop example below).
//    EVM_PRIVATE_KEY funds x402 payments per call.

// claude_desktop_config.json
{
  "mcpServers": {
    "2sio": {
      "command": "npx",
      "args": ["-y", "@2sio/mcp"],
      "env": { "EVM_PRIVATE_KEY": "0x..." }
    }
  }
}

// 2. Once the server is running, agents call this tool via standard MCP:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "paper.doi-lookup",
    "arguments": {
      "doi": "xxxxxxx"
    }
  }
}

Response

{
  "ok": true,
  "items": [
    {
      "doi": "example",
      "type": "example",
      "title": "example",
      "containerTitle": "example",
      "publisher": "example",
      "issued": "example",
      "publishedPrint": "example",
      "publishedOnline": "example",
      "abstract": "example",
      "authors": [
        {
          "given": "example",
          "family": "example",
          "orcid": "example",
          "affiliation": [
            "example"
          ]
        }
      ],
      "pageRange": "example",
      "volume": "example",
      "issue": "example",
      "isbn": [
        "example"
      ],
      "issn": [
        "example"
      ],
      "url": "example",
      "referenceCount": 1,
      "isReferencedByCount": 1,
      "license": [
        {
          "url": "example",
          "contentVersion": "example"
        }
      ],
      "subjects": [
        "example"
      ]
    }
  ],
  "total": 1,
  "source": {
    "provider": "example",
    "url": "example",
    "license": "example"
  }
}

FAQ

Do I need an API key to use the doi lookup API?

No. paper.doi-lookup is x402-native — there is no signup and no API key. Your client makes the call, receives a 402 with payment requirements, signs a USDC payment, and retries. Funds come from a wallet you control.

How much does the doi lookup API cost?

$0.0012 USDC per call, charged per request. There are no monthly fees, seats, or minimums — you pay only for the calls you make.

Can I try the doi lookup API for free first?

Yes. Add ?trial=1 (or the header X-2s-Trial: 1) to get a free real call per endpoint per hour, so you can verify the response shape before wiring payment.

Which networks and tokens are supported?

USDC on Base (via EIP-3009 transferWithAuthorization) or Solana (SPL transfer), using the open x402 payment protocol.

How do I call paper.doi-lookup from an AI agent or MCP host?

Use @2sio/sdk (TypeScript), 2sio (Python), or the @2sio/mcp server for any MCP host — each handles the probe → sign → retry loop for you. See the code samples on this page.

Discovery

• /api/directory — full catalog of every endpoint
• /openapi.json — OpenAPI 3.1 spec (per-op x-payment-info, x402Payment security)
• /.well-known/x402 — machine-readable service descriptor for x402-aware crawlers
• /.well-known/mcp/server-card.json — MCP SEP-1649 server card
• /llms.txt — plain-text manifest for LLM ingestion