Check Digit Validator
Validates IBAN/ISBN/EAN/Luhn checksums and finds the one wrong digit
Connect it
Settings → Connectors → Add custom connector
URL: https://check-digit.gumballtools.com/api/mcpSettings → Apps & Connectors → Advanced → Developer mode
Create connector with URL: https://check-digit.gumballtools.com/api/mcp{
"mcpServers": {
"check-digit": {
"url": "https://check-digit.gumballtools.com/api/mcp"
}
}
}claude mcp add --transport http check-digit https://check-digit.gumballtools.com/api/mcp{
"servers": {
"check-digit": {
"type": "http",
"url": "https://check-digit.gumballtools.com/api/mcp"
}
}
}1 tools, verified live
runread-onlyValidate 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
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