Check Digit Validator

com.gumballtools/check-digitVersion 0.1.0

Validates IBAN/ISBN/EAN/Luhn checksums and finds the one wrong digit

Status

Responds847 msNov 14, 2026, 14:00 UTCAnswered our handshake without sign-in.

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Last checked: Nov 14, 2026, 14:00 UTC
Latency: 847 ms
Remotes responding: 1 of 1
Sign-in: No sign-in
Hosting: Hosted outside the EU (detected)
Registry data as of: Nov 14, 2026, 22:14 UTC

Data from the official MCP Registry, checked by run.zero. How we check

Connect

Streamable HTTP

https://check-digit.gumballtools.com/api/mcpResponds847 msNov 14, 2026, 14:00 UTC

No sign-in

Claude Code

claude mcp add --transport http check-digit https://check-digit.gumballtools.com/api/mcp

Claude Desktop and claude.ai

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Settings > Connectors > Add custom connector
Name: check-digit
Remote MCP server URL: https://check-digit.gumballtools.com/api/mcp

Cursor

{
  "mcpServers": {
    "check-digit": {
      "url": "https://check-digit.gumballtools.com/api/mcp"
    }
  }
}

VS Code

code --add-mcp '{"name":"check-digit","type":"http","url":"https://check-digit.gumballtools.com/api/mcp"}'

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Tools

1 tools

Tools of Check Digit Validator
	Description		Read-only	Idempotent	Open world
`run`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.	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.	Not declared	Yes	Not declared	No

Details

Registry status: Active
Published: Sep 5, 2026
Updated: Sep 5, 2026
Website: https://check-digit.gumballtools.com/⁠ (opens in a new tab)
Repository: https://github.com/bwalvoord/gumball⁠ (opens in a new tab)
Categories: Uncategorized
GDPR: Not declared

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