# Checkdigit compute

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

Compute the check digit(s) for a payload under a named algorithm (Luhn, Verhoeff, Damm, ISO 7064, GS1, mod 10).

**Use when you need to: compute a luhn check digit · calculate iso 7064 check digits · verhoeff check digit calculator.**

## Decide before calling

Read the [versioned contract](/v1/tools/checkdigit-compute/versions/1.0.0) and the supported scope below. Reuse `checkdigit-compute@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-compute@1.0.0` for compute a luhn check digit. 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

- compute a luhn check digit
- calculate iso 7064 check digits
- verhoeff check digit calculator
- gs1 check digit for a barcode payload
- damm algorithm check digit
- mod 97 check digits for an identifier payload
- swiss mod 10 recursive check digit
- חישוב ספרת ביקורת

## Not supported

- validating a full value that already has a check digit attached (use checkdigit-validate)
- identifier-specific structure such as IBAN country rules, ISIN prefixes, or GS1 key lengths
- lowercase payload letters or separators such as spaces or hyphens

## Behavior

- payload must match ^[0-9A-Z]+$ (uppercase ASCII digits and letters only, 1-64 characters); lowercase letters, separators, or any other character throw invalid_input before any algorithm-specific check runs.
- Per-algorithm payload alphabet: luhn, verhoeff, damm, gs1-mod10, mod10-recursive, iso7064-mod11-2 and iso7064-mod11-10 accept decimal digits 0-9 only. iso7064-mod661-26 accepts letters A-Z only. iso7064-mod37-2, iso7064-mod37-36, iso7064-mod97-10 and iso7064-mod1271-36 accept digits 0-9 and letters A-Z. A payload character outside the named algorithm's alphabet throws invalid_input.
- full = payload concatenated with check_digits, in that order; no separators and no rearrangement.
- Pure arithmetic on the payload string; no locale, clock, or randomness; identical output across runs for the same input.

## Input

- `payload` (string, required): min length 1; max length 64; pattern `^[0-9A-Z]+$`
- `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

- `check_digits` (string, required): min length 1; max length 2; pattern `^[0-9A-Z*]{1,2}$`
- `full` (string, required): min length 2; max length 66; pattern `^[0-9A-Z*]+$`
- `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)

## Limits

- max payload bytes: 64

## Example

Request input:

```json
{
  "payload": "411111111111111",
  "algorithm": "luhn"
}
```

Response:

```json
{
  "result": {
    "check_digits": "1",
    "full": "4111111111111111",
    "algorithm": "luhn"
  }
}
```

## How to call it

### MCP

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

```json
{
  "id": "checkdigit-compute",
  "version": "1.0.0",
  "input": {
    "payload": "411111111111111",
    "algorithm": "luhn"
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/checkdigit-compute/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"payload":"411111111111111","algorithm":"luhn"}'
```

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

### CLI

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

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

## Related tools

- [Checkdigit validate](/tools/checkdigit-validate): Validate a full value's trailing check digit(s) against 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.
- [Upu s10 validate](/tools/upu-s10-validate): Validate a 13-char UPU S10 postal identifier's weighted mod-11 check digit, split service indicator and country.
- [Iccid validate](/tools/iccid-validate): Validate a 19/20-digit ICCID's Luhn check digit, require the 89 major industry id, split issuer/account digits.
- [Imo number validate](/tools/imo-number-validate): Validate a 7-digit IMO ship identification number's weighted mod-10 check digit, with or without the 'IMO ' prefix.
- [Iso6346 container validate](/tools/iso6346-container-validate): Validate an 11-character ISO 6346 container number's mod-11 check digit and split the owner code, category and serial.