# ISBN Validator (ISBN-10 and ISBN-13)

> ISBN Validator (ISBN-10 and ISBN-13) is a paid API for AI agents from checkdigit.openverbs.com, paid per call via x402, $0.002/call, status unknown (last checked 2026-09-14).

Validates an ISBN-10 or ISBN-13 book number using mod-11 or GTIN mod-10 checksum algorithms respectively, with automatic scheme selection based on digit length.

## Facts

- Endpoint: POST https://checkdigit.openverbs.com/v1/isbn
- Price: $0.002/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/isbn-validator-isbn-10-and-isbn-13-f885cefd
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_p8jqX_OPtKCehWa5QsCXW

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 isbn-validator-isbn-10-and-isbn-13-f885cefd -d '<json body>'
```

Example prompt: Can you check whether this ISBN is valid: 978-0-306-40615-7?

## When to prefer this

Choose this endpoint when you need to validate the check digit of a specific ISBN-10 or ISBN-13 book identifier. It automatically selects the correct algorithm (mod-11 for ISBN-10, GTIN mod-10 for ISBN-13) based on length, and handles hyphens and spaces natively. Prefer this over a generic Luhn or GTIN validator when you know the identifier is specifically a book ISBN and want ISBN-aware validation including the trailing 'X' allowed for ISBN-10.

## Known failure modes

- ISBN string is empty or missing — returns validation error
- Length after stripping hyphens/spaces is not 10 or 13 — scheme cannot be determined, returns error
- Check digit does not match computed checksum — returns invalid result
- ISBN-10 check digit is not a digit or 'X' — returns invalid
- Non-numeric characters other than hyphens, spaces, or trailing X present — returns error
- Network or payment failure may prevent the call from completing

## How this service works

Validate an ISBN-10 (mod-11, trailing X allowed) or ISBN-13 (GTIN mod-10) book number. Length after removing spaces/hyphens selects the scheme.

## Output

Returns whether the supplied ISBN passes its checksum: valid (true/false), the scheme detected (ISBN-10 or ISBN-13), and optionally details about why it failed if the check digit is incorrect or the length is invalid.

## 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",
    "bodyType",
    "body"
   ],
   "properties": {
    "body": {
     "type": "object",
     "required": [
      "isbn"
     ],
     "properties": {
      "isbn": {
       "type": "string",
       "maxLength": 32,
       "minLength": 1,
       "description": "ISBN-10 or ISBN-13 (spaces/hyphens allowed)."
      }
     },
     "additionalProperties": false
    },
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "POST"
     ],
     "type": "string"
    },
    "bodyType": {
     "enum": [
      "json",
      "form-data",
      "text"
     ],
     "type": "string"
    }
   },
   "additionalProperties": false
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/isbn-validator-isbn-10-and-isbn-13-f885cefd/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from checkdigit.openverbs.com](https://www.zero.xyz/host/checkdigit.openverbs.com/llms.txt)