# Agent402 ISBN Check Digit Validator

> Agent402 ISBN Check Digit Validator is a paid API for AI agents from agent402.co.uk, paid per call via x402, $0.001/call, status down (last checked 2026-09-15).

Validates the check digit of an ISBN-10 or ISBN-13 number per ISO 2108, returning validity, kind, and bookland prefix status

## Facts

- Endpoint: POST https://agent402.co.uk/v1/validate/isbn
- Price: $0.001/call
- Payment: x402
- Status: down
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/agent402-isbn-check-digit-validator-21163fd3
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_HOg36yw4-K8l9tqjvoosl

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 agent402-isbn-check-digit-validator-21163fd3 -d '<json body>'
```

Example prompt: Can you check whether the ISBN 978-3-16-148410-0 is valid — specifically confirm the check digit is correct and whether it's an ISBN-13?

## When to prefer this

Use this endpoint when you need a fast, cheap, authoritative ISO 2108 check-digit computation for ISBN-10 or ISBN-13 numbers — for example before catalog ingestion, form validation, or data cleaning. It is not a registry lookup (it won't tell you if the book exists or who published it), so prefer it purely for structural/checksum validation at $0.001 per call.

## Known failure modes

- Malformed or non-ISBN string returns invalid result or error
- Passing a registry lookup query — this endpoint only checks structure/checksum, not whether the ISBN is registered
- Providing an empty string may return a validation error
- Non-string or missing 'isbn' field in request body causes a schema error

## How this service works

Don't burn tokens on deterministic work. Go/no-go jobs first: payout preflight, text firewall (PII/secrets), budget check — then rails, calendars, finmath. x402 USDC on Base. UK + global. agent402.co.uk — not agent402.tools. $0.001–$0.002 USDC on Base. No API keys.

## Output

Returns a JSON object indicating: 'kind' (ISBN-10 or ISBN-13), 'valid' (overall validity boolean), 'valid_checksum' (whether the check digit is correct per ISO 2108), and 'bookland_prefix' (whether the EAN bookland prefix is present, for ISBN-13).

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "isbn": {
   "type": "string",
   "description": "ISBN-10 or ISBN-13 (hyphens ok)"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "kind": "ISBN-13",
  "valid": true,
  "valid_checksum": true,
  "bookland_prefix": true
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/agent402-isbn-check-digit-validator-21163fd3/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from agent402.co.uk](https://www.zero.xyz/host/agent402.co.uk/llms.txt)