# code402 ISBN Check

> code402 ISBN Check is a paid API for AI agents from hcrb.in, paid per call via x402, $0.005/call, status unknown (last checked 2026-10-11).

Validates an ISBN (International Standard Book Number) using deterministic checksum verification, returning validity, reason, and normalized form — paid per call via x402 USDC micropayment.

## Facts

- Endpoint: POST https://hcrb.in/v1/tools/isbn-check/call?utm_source=zero.xyz
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-11
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/code402-isbn-check-2dba060e
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_s0G2GFdvJOV8vSezMBVXv

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 code402-isbn-check-2dba060e -d '<json body>'
```

Example prompt: Can you check whether ISBN 978-3-16-148410-0 is a valid book identifier — I need to know if the checksum is correct and what the normalized form looks like.

## When to prefer this

Choose this endpoint when you need a fast, offline, deterministic ISBN checksum validation without relying on external book databases or ML models. It is ideal for batch data hygiene, catalog ingestion pipelines, or any workflow where you need mathematical certainty that an ISBN is structurally correct before querying a slower or costlier lookup service. The per-call micropayment model ($0.005 USDC) and first-3-free-per-day policy make it economical for occasional or moderate-volume use. Prefer this over web-scraping-based book lookup services when you only need structural validity, not bibliographic metadata.

## Known failure modes

- Invalid or malformed input returns valid:false with a descriptive reason
- Checksum failure returns valid:false with reason explaining the mismatch
- Payment not included or insufficient results in HTTP 402 response before processing
- Unknown or unsupported ISBN format may return format-only scope with limited validation
- Network timeout on the micropayment settlement layer

## How this service works

Offline deterministic checks (no ML, no scraping) for business identifiers and rental due-diligence, paid per call via x402 (gasless EIP-3009 USDC on Base). Checksum-verifiable (IBAN, LEI, VAT, ABA, ISBN, SEDOL…) vs format-only (EIN, IFSC, SWIFT/BIC…) — every response returns a scope field. Non-custodial, signed receipts. First 3 calls/day free.

## Output

A JSON object with: `valid` (boolean indicating whether the ISBN passes checksum), `reason` (human-readable explanation such as 'checksum valid' or specific failure reason), `scope` (indicating whether this was a checksum-verifiable or format-only check), `normalized` (the canonical form of the ISBN), and optionally `country` and `tool` fields.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "isbn"
   ],
   "properties": {
    "isbn": {
     "type": "string",
     "description": "ISBN-10 or ISBN-13; dashes/spaces allowed."
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "schema": {
  "type": "object",
  "required": [
   "valid",
   "reason"
  ],
  "properties": {
   "tool": {
    "type": "string"
   },
   "scope": {
    "type": "string"
   },
   "valid": {
    "type": "boolean"
   },
   "reason": {
    "type": "string"
   },
   "country": {
    "type": "string"
   },
   "normalized": {
    "type": "string"
   }
  }
 },
 "example": {
  "valid": true,
  "reason": "checksum valid"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/code402-isbn-check-2dba060e/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from hcrb.in](https://www.zero.xyz/host/hcrb.in/llms.txt)