# ISSN Checksum Validator

> ISSN Checksum Validator is a paid API for AI agents from agentshelf.syntexa.ch, paid per call via x402, $0.001/call, status unknown (last checked 2026-09-29).

Validates whether a given ISSN (International Standard Serial Number) has a correct checksum

## Facts

- Endpoint: POST https://agentshelf.syntexa.ch/v1/check-issn?utm_source=zero.xyz
- Price: $0.001/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-29
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/issn-checksum-validator-1d4c5c6f
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_DyqLStxTIhNGYuuvlWUcl

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 issn-checksum-validator-1d4c5c6f -d '<json body>'
```

Example prompt: Check whether the ISSN 1234-5679 has a valid checksum — just tell me if it passes or fails the check digit verification.

## When to prefer this

Use this endpoint when you need a lightweight, deterministic checksum validation for an ISSN without any network lookups or LLM involvement. Prefer the free sandbox endpoint POST /v1/sandbox/check-issn first; use this paid endpoint when sandbox access is unavailable or rate-limited. Ideal for pipelines that need cheap, reliable ISSN integrity checks at scale.

## Known failure modes

- Missing or empty value field returns a validation error
- Non-ISSN string (wrong length, non-numeric characters) may return invalid rather than an error
- Payment failure results in 402 response before processing
- Malformed request body returns 400 bad request

## How this service works

Call when an agent needs whether an ISSN checksum is valid. Exact $0.001 USDC. Prefer unpaid POST /v1/sandbox/check-issn first.

## Output

A boolean or status indicating whether the supplied ISSN passes the standard checksum algorithm (mod-11 with weighting), along with any relevant validation detail about why it is or is not valid.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "data": {
   "description": "JSON value for schema-check SKUs. Omit it on string checks such as email or IBAN."
  },
  "value": {
   "type": "string",
   "examples": [
    "1234-5679"
   ],
   "maxLength": 20000,
   "minLength": 1,
   "description": "Value to check. The route description names the format: email, phone, IBAN, postal code, VAT, national id, or similar."
  },
  "schema": {
   "description": "JSON Schema document for schema-check SKUs. Omit it on string checks."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/issn-checksum-validator-1d4c5c6f/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from agentshelf.syntexa.ch](https://www.zero.xyz/host/agentshelf.syntexa.ch/llms.txt)