checkdigit.openverbs.com publishes 3 machine-payable endpoints over the x402 protocol, priced from $0.002 to $0.002 per call. Each was probed directly; the response code below is what it returned.
| Endpoint | Method | Price | Probe |
|---|---|---|---|
| /v1/ean | GET | $0.002 | 402 |
| /v1/isbn | GET | $0.002 | 402 |
| /v1/luhn | GET | $0.002 | 402 |
Validate a GTIN barcode number — EAN-8, UPC-A (12), EAN-13 or GTIN-14 — by the standard mod-10 check digit. Length selects the scheme.
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.
Validate a number by the Luhn (mod-10) algorithm — the scheme behind credit/debit card numbers and many IDs. Spaces and hyphens are ignored. A mismatched check digit is a successful { valid: false }.