# Check Digit Validator

Validate IBAN, ISBN, EAN/UPC and card numbers — and find the one wrong digit.

**Why delegate it:** Mod-97 over a rearranged thirty-character IBAN, or a weighted sum mod 10/11 over thirteen digits, is arithmetic a model fumbles silently and asserts confidently. Worse, asked WHICH digit is wrong it pattern-matches a plausible answer instead of solving the modular equation — and when more than one single-digit fix would satisfy the checksum, it names one instead of reporting the ambiguity.

Site: https://check-digit.gumballtools.com

## Tools

### `check_digit_run` — Validate a check digit and localise a single-digit error

Checks the check digit on an IBAN (ISO 7064 MOD 97-10), ISBN-10, ISBN-13, EAN-13, UPC-A, or a Luhn-checked card number. When the checksum fails it searches for the single substitution or adjacent transposition that would repair it, so the answer is which character to fix rather than merely that something is wrong. Reports the ambiguity when more than one fix would work. Refuses an IBAN whose country code is not in the length registry rather than assuming a length, and requires the format to be named because a bare digit string can parse as more than one. A passing checksum is arithmetic only: it does not mean the account, book or card exists.

Direct endpoint: `https://check-digit.gumballtools.com/api/v1/run`

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Via the aggregator: `https://gumballtools.com/api/mcp`
Direct: `https://check-digit.gumballtools.com/api/mcp`