/finding-open-access-papers

name: Finding Open Access Papers
description: Use Unpaywall API to find free full-text versions of paywalled papers
when_to_use: When paper behind paywall. When PMC full text not available. When DOI resolution hits paywall. When need free access to paper. Before giving up on full text access.
version: 1.0.0

<!-- ╔══════════════════════════════════════════════════════════════╗ ║ 本文件为开源 Skill 原始文档，收录仅供学习与研究参考 ║ ║ CoPaper.AI 收集整理 | https://copaper.ai ║ ╚══════════════════════════════════════════════════════════════╝

来源仓库: https://github.com/kthorn/research-superpower 项目名称: research-superpower 开源协议: MIT License 收录日期: 2026-04-02

声明: 本文件版权归原作者所有。此处收录旨在为社会科学实证研究者 提供 AI Agent Skills 的集中参考。如有侵权，请联系删除。 -->

Finding Open Access Papers

Overview

Use Unpaywall to find legally available open access versions of papers that appear to be behind paywalls.

**Core principle:** Many paywalled papers have free versions (preprints, author manuscripts, institutional repositories). Unpaywall finds them.

When to Use

Use this skill when:

• DOI resolution hits a paywall
• Paper not available in PubMed Central
• Publisher site requires subscription
• Need full text for highly relevant paper (score ≥7)

**Use BEFORE giving up on full text access**

Unpaywall API

**Simple REST API - no authentication required for reasonable usage**

Basic Request

curl "https://api.unpaywall.org/v2/DOI?email=YOUR_EMAIL"

**Parameters:**

• `DOI` - The paper's DOI (URL-encoded if needed)
• `email` - User's email (required, for courtesy/contact)

**IMPORTANT: Ask user for their email at the start of research session. Do NOT use placeholder emails like claude@anthropic.com or researcher@example.com.**

**Example:**

curl "https://api.unpaywall.org/v2/10.1038/nature12373?email=user@example.com"

Response Format

{
  "doi": "10.1038/nature12373",
  "title": "Paper Title",
  "is_oa": true,
  "best_oa_location": {
    "url": "https://europepmc.org/articles/pmc3858213",
    "url_for_pdf": "https://europepmc.org/articles/pmc3858213?pdf=render",
    "version": "publishedVersion",
    "license": "cc-by",
    "host_type": "repository"
  },
  "oa_locations": [
    {
      "url": "https://europepmc.org/articles/pmc3858213",
      "version": "publishedVersion"
    },
    {
      "url": "https://arxiv.org/abs/1234.5678",
      "version": "submittedVersion"
    }
  ]
}

Key Response Fields

**`is_oa`** (boolean)

• `true` - Open access version available
• `false` - No free version found

**`best_oa_location`** (object or null)

• Unpaywall's recommended best open access source
• Prioritizes published versions over preprints
• Includes PDF URL when available

**`oa_locations`** (array)

• All known open access locations
• Includes repositories, preprint servers, institutional sites
• Ordered by quality/version

**`version`** types:

• `publishedVersion` - Final published version (best)
• `acceptedVersion` - Author's accepted manuscript (good)
• `submittedVersion` - Preprint before peer review (useful)

Implementation Pattern

1. Check Unpaywall After Paywall Hit

# Try DOI first
curl -L "https://doi.org/10.1234/example.2023"

# If paywall detected (403, subscription required, etc):
curl "https://api.unpaywall.org/v2/10.1234/example.2023?email=your@email.com"

2. Extract Best URL

# Parse JSON response
response=$(curl -s "https://api.unpaywall.org/v2/DOI?email=EMAIL")

# Check if OA available
is_oa=$(echo $response | jq -r '.is_oa')

if [ "$is_oa" = "true" ]; then
  # Get best PDF URL
  pdf_url=$(echo $response | jq -r '.best_oa_location.url_for_pdf // .best_oa_location.url')

  # Download
  curl -L -o "papers/paper.pdf" "$pdf_url"
fi

3. Report to User

**When OA found:**

⚠️ Paper behind paywall at publisher
✓ Found open access version via Unpaywall!
   Source: Europe PMC (published version)
   PDF: https://europepmc.org/articles/pmc3858213?pdf=render
   → Downloading...

**When no OA found:**

⚠️ Paper behind paywall at publisher
✗ No open access version found via Unpaywall
   Options:
   - Request via institutional access
   - Contact authors for preprint
   - Continue with abstract only

4. Prioritize by Version

If multiple locations available:

**Priority order:** 1. `publishedVersion` from publisher or PMC 2. `acceptedVersion` from institutional repository 3. `submittedVersion` from preprint server (arXiv, bioRxiv)

Integration with evaluating-paper-relevance

**Add to full text fetching workflow:**

Stage 2: Fetch Full Text

Try in order:
A. PubMed Central (free full text)
B. DOI resolution → If paywall, try Unpaywall
C. Unpaywall direct lookup
D. Preprints (bioRxiv, arXiv)

**Updated workflow:**

# 1. Try PMC
pmc_result=$(curl "https://eutils.ncbi.nlm.nih.gov/...")
if has_pmc_fulltext; then
  fetch_pmc
  exit 0
fi

# 2. Try DOI
doi_result=$(curl -L "https://doi.org/$doi")
if is_paywall; then
  # 3. Try Unpaywall
  unpaywall_result=$(curl "https://api.unpaywall.org/v2/$doi?email=$EMAIL")
  if has_oa; then
    fetch_unpaywall_pdf
    exit 0
  fi
fi

# 4. No full text available
report_no_fulltext

Rate Limiting

**Free tier (with email):**

• 100,000 requests per day
• No hard rate limit, but be respectful
• Include email in requests (required)

**Best practices:**

• Add 100ms delay between requests
• Cache responses (don't re-check same DOI)
• Only check for papers you actually need

Python Helper Example

import requests
import time

def find_open_access(doi, email):
    """
    Find open access version via Unpaywall
    Returns: (pdf_url, version, source) or (None, None, None)
    """
    url = f"https://api.unpaywall.org/v2/{doi}"
    params = {"email": email}

    try:
        response = requests.get(url, params=params, timeout=10)
        response.raise_for_status()
        data = response.json()

        if not data.get('is_oa'):