# Open-Access PDF Locator via DOI (Unpaywall + OpenAlex)

> Open-Access PDF Locator via DOI (Unpaywall + OpenAlex) is a paid API for AI agents from payai.agentstools.dev, paid per call via x402, $0.008/call, status unknown (last checked 2026-09-14).

Given a DOI, checks whether a free open-access PDF exists and returns the best PDF link along with its license, version, and host type.

## Facts

- Endpoint: GET https://payai.agentstools.dev/scholar/oa
- Price: $0.008/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/open-access-pdf-locator-via-doi-unpaywall-openalex-765066b8
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_vY86ugz2ue5l2Ffad5rXs

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability open-access-pdf-locator-via-doi-unpaywall-openalex-765066b8
```

Example prompt: Can you check if there's a free open-access PDF available for the paper with DOI 10.1038/s41586-021-03819-2 — and if so, give me the best PDF link along with the license and version?

## When to prefer this

Use this endpoint when you have a specific DOI and need to quickly determine whether a free legal PDF exists, and where to get it. It is cheaper and more focused than a full paper metadata resolver. Prefer it over general web search when you want structured, license-aware open-access data from authoritative sources (Unpaywall + OpenAlex) rather than scraping or guessing.

## Known failure modes

- DOI not found in Unpaywall or OpenAlex — returns no open-access record
- Invalid or malformed DOI — returns an error or empty result
- Paper exists but has no open-access copy — returns is_oa: false with no PDF link
- Upstream Unpaywall/OpenAlex service unavailable — timeout or 5xx error

## How this service works

Locate a free open-access PDF for a DOI. Returns whether an open-access copy exists and the best PDF link with its license, version and host type, from Unpaywall and OpenAlex. A cheap, focused open-access locator.

## Output

Returns whether an open-access copy exists (boolean), the best available PDF URL, the license (e.g. CC-BY), the version (e.g. publishedVersion, acceptedVersion), and the host type (e.g. repository, publisher). Sourced from Unpaywall and OpenAlex.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET",
      "HEAD",
      "DELETE"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [],
     "properties": {
      "id": {
       "type": "string",
       "description": "A DOI (alias for doi)"
      },
      "doi": {
       "type": "string",
       "description": "DOI, with or without a doi.org prefix"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object"
    }
   }
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/open-access-pdf-locator-via-doi-unpaywall-openalex-765066b8/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from payai.agentstools.dev](https://www.zero.xyz/host/payai.agentstools.dev/llms.txt)