Check Digit Validator MCP server

• runwrite actionFree

  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.

• –Code scanremote-only server, no package to scann/a
• Live reliabilityremote reachable in 1264ms20/20
• Tool poisoning1 tool descriptions checked15/15
• Auth qualityopen endpoint exposes 1 write-action tools with no auth3/15
• Maintenancerepository not readable: repo not found3/15
• Maintainer identityverified namespace with website, no repo4/10

Check Digit Validator: common questions

Is Check Digit Validator MCP server safe?

With care: it is graded C, so read the findings first (60/100). Read the Check Digit Validator safety report

How do I install Check Digit Validator?

It runs remotely at check-digit.gumballtools.com. Add it to Claude Code, Claude Desktop or Cursor with the snippets above, or call it through the mcp.market gateway without installing anything.

Does Check Digit Validator need an API key?

Not as far as the registry entry and our scan can tell: no credentials are declared or required.

Is Check Digit Validator maintained?

The latest release is v0.1.0.

Is Check Digit Validator up?

100% of our last 6 checks got an answer. We check remote servers about four times a day.