Validate a GTIN check digit API

Validate GTIN-8, GTIN-12, GTIN-13, or GTIN-14 check digits with the published GS1 algorithm. Company prefix ownership is not looked up.

The Validate a GTIN check digit operation is part of the Phone GTIN ISBN Validator API on BRAINIAC.software. Validate GTIN-8, GTIN-12, GTIN-13, or GTIN-14 check digits with the published GS1 algorithm. Company prefix ownership is not looked up. It is exposed as POST /api/v1/identifiers/gtin-checksum-validate and priced at $0.01 USDC on Base. Payment uses the x402 exact scheme: an unpaid call returns HTTP 402 with payment requirements; after paying the exact USDC amount, retry the same request with a payment-signature header and Idempotency-Key. No account and no API key are required. Data sources for this product: libphonenumber-js bundled ITU/E.164 metadata GS1 check digit algorithm ISBN/ISSN check digit rules EU EORI structural rules Limitations: These endpoints validate format and checksums only. They do not prove a number, product code, or EORI is issued, assigned, or reachable. Postcode geocoding and official concordance tables from issue #81 are not in this first slice. Outputs are compliance prechecks, drafts, or data lookups for human review. They are not official determinations, legal advice, identity confirmation, or certified filings. Related operations in this group include Validate a phone number to E.164, Validate ISBN or ISSN check digits, Validate EORI number format. Open the group documentation for the full request schema, field notes, and response examples before integrating.