Check Digit Validator

Validate the check digit(s) on an IBAN, ISBN-10, ISBN-13, EAN-13, UPC-A, or Luhn-checked card number, and when it fails, try to say which digit is wrong. Use this instead of computing the checksum yourself. Mod-97 arithmetic on a rearranged 15-34 character IBAN, or a weighted sum mod 10/11 over 10-19 digits, is exactly the kind of long deterministic calculation language models get wrong silently and confidently — there is no way for a reader to tell a fabricated "valid" from a real one without redoing the math. This tool actually redoes the math, and when the checksum fails, searches for the single adjacent-digit swap or single-character substitution that would repair it, because a transposed pair is the most common real cause of a bounced payment or a mistyped barcode. IMPORTANT, and easy to get wrong: for ISBN-13/EAN-13/UPC-A and Luhn, the digit alphabet (0-9) almost exactly spans the checksum's modulus, so a failing check usually admits MANY equally valid single-digit substitutions — the math genuinely cannot narrow it to one digit unless the error was an adjacent transposition. When that happens, the response reports the full candidate list rather than picking one; treat that as the honest answer, not a failure to find it. IBAN (mod 97) is the exception and usually does localize to one digit. Inputs: `input` (the number; spaces and hyphens are stripped automatically) and `format`, one of iban | isbn10 | isbn13 | ean13 | upca | luhn — required, never auto-detected, because the same digit string can be validly checked under more than one format with different results. Refuses rather than guesses: an IBAN country code not in the ISO 13616 length registry, the wrong length for a declared format, or characters outside the format's alphabet (e.g. a letter in a card number). A passing checksum is a syntax check only — it does not confirm an IBAN names an open account, an ISBN is a real book, or a card is active or unstolen.

{'type': 'object', '$schema': 'https://json-schema.org/draft/2020-12/schema', 'required': ['input', 'format'], 'properties': {'input': {'type': 'string', 'description': 'The number to check, e.g. "DE89370400440532013000". Spaces and hyphens are stripped automatically; anything else must match the declared format exactly.'}, 'format': {'enum': ['iban', 'isbn10', 'isbn13', 'ean13', 'upca', 'luhn'], 'type': 'string', 'description': 'Which checksum to apply â\x80\x94 one of: iban, isbn10, isbn13, ean13, upca, luhn. Required; a bare number is not auto-detected because the same digits can be validly checked under more than one format with different results.'}}}