ISBN validator API: check ISBN-10 and ISBN-13, convert to ISBN-13

$0.01 per call · failed calls are never charged · no account or subscription: pay per call with a card key or USDC

Check an ISBN-10 or ISBN-13 check digit and convert valid ISBN-10s to ISBN-13. Hyphens and spaces are ignored. Use before cataloguing or ordering books.

When to use it

Run it free (25 free calls) Buy credits by card (from $1) Example result (JSON)

Try it with your own inputs

The ISBN

Uses one of your 25 free calls. Nothing is charged, and a call that fails is not counted.

Inputs

NameTypeDescription
isbn (required)stringThe ISBN

Request

Over HTTP, with a card key from /credits:

curl -H "Authorization: Bearer adt_…" "https://tools.funwords.eu/validate/isbn?isbn=0-306-40615-2"

Or one of your 25 free calls, no key:

curl "https://tools.funwords.eu/validate/isbn?isbn=0-306-40615-2&trial=1"

Without a key, the same URL answers 402 Payment Required with x402 and MPP terms, so an agent with a wallet pays per call in USDC (Base, Polygon, Arbitrum or Solana over x402; Base or Tempo over MPP) and retries.

Example result

{
  "isbn": "0306406152",
  "type": "ISBN-10",
  "valid": true,
  "isbn13": "9780306406157"
}

From an AI agent (MCP)

Add the MCP server to Claude, Cursor or any MCP client. This tool is validate_isbn, and the first 25 successful calls are free:

https://tools.funwords.eu/mcp

Questions

Does it look the book up?

No, it checks the check digit; hyphens and spaces are ignored.

How do I pay?

Each successful call costs $0.01; failed calls are not charged. The first 25 calls are free. Then pay with card credits (from $1; the key goes in an Authorization header or, for clients that can only fetch a URL, as api_key= in the URL) or per call in USDC over x402 or MPP. No account is needed.

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