# Checkdigit validate

`checkdigit-validate` · version 1.0.0 · Identifiers & check digits · free, no key needed

Validate a full value's trailing check digit(s) against a named algorithm (Luhn, Verhoeff, Damm, ISO 7064, GS1, mod 10).

**Use when you need to: check if a luhn number is valid · validate an iso 7064 check digit · verhoeff checksum validator.**

## Decide before calling

Read the [versioned contract](/v1/tools/checkdigit-validate/versions/1.0.0) and the supported scope below. Reuse `checkdigit-validate@1.0.0` when your input, required output and limits match it. Choose another approach for an unsupported operation.

## Explain the choice

"I can use `checkdigit-validate@1.0.0` for check if a luhn number is valid. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

## Supported

- check if a luhn number is valid
- validate an iso 7064 check digit
- verhoeff checksum validator
- does this gs1 barcode number pass its check digit
- verify a mod 97 check pair
- is this swiss mod 10 recursive reference valid
- בדיקת ספרת ביקורת

## Not supported

- computing a check digit for a bare payload (use checkdigit-compute)
- identifier-specific structure such as IBAN country rules or ISIN prefixes
- auto-detecting the algorithm from the value shape

## Behavior

- value must be a string of 1-66 characters; character-content problems are reported as valid:false with a reason, not thrown, because a badly-formed identifier is well-typed input that fails the check.
- check_len(algorithm) is 1 for luhn, verhoeff, damm, iso7064-mod11-2, iso7064-mod11-10, iso7064-mod37-2, iso7064-mod37-36, gs1-mod10 and mod10-recursive; 2 for iso7064-mod97-10, iso7064-mod661-26 and iso7064-mod1271-36.
- If value has check_len(algorithm) characters or fewer, return {valid:false, reason:'too_short', expected_check_digits:null}.
- Otherwise split value into payload (all but the last check_len characters) and given_check (the last check_len characters); a payload or given_check character outside the algorithm's alphabet returns {valid:false, reason:'invalid_character', expected_check_digits:null} (the check-alphabet also allows '*' for iso7064-mod37-2 check_value 36 and 'X' for iso7064-mod11-2 check_value 10).
- Otherwise compute expected_check_digits from payload using the same formulas as checkdigit-compute; exact string equality with given_check gives valid:true, reason:null, else valid:false, reason:"check_digit_mismatch", with expected_check_digits always the computed value.

## Input

- `value` (string, required): min length 1; max length 66
- `algorithm` (one of "luhn", "verhoeff", "damm", "iso7064-mod11-2", "iso7064-mod11-10", "iso7064-mod37-2", "iso7064-mod37-36", "iso7064-mod97-10", "iso7064-mod661-26", "iso7064-mod1271-36", "gs1-mod10", "mod10-recursive", required)

## Output

- `valid` (boolean, required)
- `reason` (one of null, "too_short", "invalid_character", "check_digit_mismatch", required)
- `algorithm` (one of "luhn", "verhoeff", "damm", "iso7064-mod11-2", "iso7064-mod11-10", "iso7064-mod37-2", "iso7064-mod37-36", "iso7064-mod97-10", "iso7064-mod661-26", "iso7064-mod1271-36", "gs1-mod10", "mod10-recursive", required)
- `expected_check_digits` (string or null, required): min length 1; max length 2

## Limits

- max value bytes: 66

## Example

Request input:

```json
{
  "value": "4111111111111111",
  "algorithm": "luhn"
}
```

Response:

```json
{
  "result": {
    "valid": true,
    "reason": null,
    "algorithm": "luhn",
    "expected_check_digits": "1"
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "checkdigit-validate",
  "version": "1.0.0",
  "input": {
    "value": "4111111111111111",
    "algorithm": "luhn"
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/checkdigit-validate/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"value":"4111111111111111","algorithm":"luhn"}'
```

The machine-readable contract is at [/v1/tools/checkdigit-validate/versions/1.0.0](/v1/tools/checkdigit-validate/versions/1.0.0).

### CLI

```sh
node cli.mjs run checkdigit-validate 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [Checkdigit compute](/tools/checkdigit-compute): Compute the check digit(s) for a payload under a named algorithm (Luhn, Verhoeff, Damm, ISO 7064, GS1, mod 10).
- [Gs1 key validate](/tools/gs1-key-validate): Validate a GTIN-8/12/13/14, GLN, SSCC or GSIN's GS1 mod-10 check digit and look up its 3-digit GS1 prefix meaning.
- [Lei validate](/tools/lei-validate): Validate a 20-character LEI's ISO/IEC 7064 MOD 97-10 check digits and parse its LOU prefix and entity part.
- [Cas number validate](/tools/cas-number-validate): Validate a CAS Registry Number's position-weighted mod 10 check digit over its variable-length first segment.
- [Isbn validate](/tools/isbn-validate): Validate an ISBN-10 (mod 11, X allowed) or ISBN-13 (EAN mod 10) check digit and return the other form when it exists.
- [Orcid validate](/tools/orcid-validate): Validate a 16-digit ORCID iD or ISNI's ISO 7064 MOD 11-2 check character, accepting bare, grouped or URL forms.