# Check Digit Validator (remote · check-digit.gumballtools.com)

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

- Trust score: 75/100 (medium)
- Change this week: +4
- Registry status: active
- Liveness: live
- Owner verified: no
- Last scored: 2026-09-25

## Components

- remote · `check-digit.gumballtools.com`: 75/100 (this document), [markdown](https://verifymcp.io/servers/com-gumballtools-check-digit/api-mcp.md), [page](https://verifymcp.io/servers/com-gumballtools-check-digit/api-mcp)

## Channel facts

- Endpoint: `https://check-digit.gumballtools.com/api/mcp`
- Transports: `streamable-http`
- Auth: `none`
- Version: `0.1.0`

## Trust breakdown

How this component scores in each security and reliability category. Every signal is checked automatically against the live server, and we only credit what we can confirm. Scores are 0–100 per category. Scoring method: https://verifymcp.io/docs/scoring (what has changed: https://verifymcp.io/docs/scoring/changelog)

Scored 2026-09-25.

- **Endpoint Security**: 63/100
  - The endpoint's TLS certificate is valid, in date, and uses a strong key.
  - Authorisation not fully verified: no authorisation is required to call this server, and 1 tool(s) never declared a destructiveHint. The MCP spec treats an absent hint as destructive by default, so we cannot call this surface safe.
  - HTTPS is enforced; there's no plaintext access path.
  - The HSTS (Strict-Transport-Security) header is present.
  - DNSSEC check failed: this domain isn't protected by DNSSEC.
- **Transport & Reachability**: 100/100
  - Verified streamable-http transport via a live MCP handshake.
- **Schema Quality & AI Usability**: 67/100
  - AI-judged instruction clarity (excellent).
  - Context-footprint check failed: tool/resource definitions use about 560 tokens (~560/item across 1 items; 1 tools + 0 resources), over budget; trim descriptions and params.
  - Usage-examples check failed: none of the tools include examples.
- **Stability & Change Management**: 67/100
  - Stability observed for 20 of 30 days with no destabilising changes; credit accrues until the full window elapses.
- **Tool Coverage**: 100/100
  - 100% of tools have a non-trivial description (not blank, and not just the tool's name).
  - 100% of tool parameters carry a description.
- **Tool Safety**: 100/100
  - No prompt-injection markers were found in the server instructions, tool names or descriptions we captured.
  - We read all 1 captured tool definition(s), and no name or description among them implies an irreversible operation.
  - An AI judge read all 1 captured unit(s) of tool text and found none that tries to manipulate the model reading it.
- **Capabilities**: 100/100
  - Implements a current MCP spec version (2026-07-28).

## Install

### How do I install the Check Digit Validator MCP server?

Check Digit Validator is a hosted endpoint at https://check-digit.gumballtools.com/api/mcp, so there is nothing to install locally. Ready-made configuration for Claude, Cursor, VS Code, Codex and 5 more is on this page, copied from each client's own documentation.

### Claude

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

### Cursor

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

### VS Code

```json
{
  "servers": {
    "com-gumballtools-check-digit": {
      "type": "http",
      "url": "https://check-digit.gumballtools.com/api/mcp"
    }
  }
}
```

### Codex

```toml
[mcp_servers.com-gumballtools-check-digit]
url = "https://check-digit.gumballtools.com/api/mcp"
```

### opencode

```json
{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "com-gumballtools-check-digit": {
      "type": "remote",
      "url": "https://check-digit.gumballtools.com/api/mcp",
      "enabled": true
    }
  }
}
```

### OpenClaw

```bash
openclaw mcp add com-gumballtools-check-digit --url 'https://check-digit.gumballtools.com/api/mcp' --transport streamable-http
```

### Hermes

```yaml
mcp_servers:
  com-gumballtools-check-digit:
    url: "https://check-digit.gumballtools.com/api/mcp"
```

### Netclaw

```json
{
  "McpServers": {
    "com-gumballtools-check-digit": {
      "Transport": "http",
      "Url": "https://check-digit.gumballtools.com/api/mcp"
    }
  }
}
```

### Vellum

```bash
assistant mcp add com-gumballtools-check-digit -t streamable-http -u 'https://check-digit.gumballtools.com/api/mcp'
```

### Other

```json
{
  "mcpServers": {
    "com-gumballtools-check-digit": {
      "type": "http",
      "url": "https://check-digit.gumballtools.com/api/mcp"
    }
  }
}
```

The mcpServers block is a cross-client convention. Remote transports vary, so check your client's docs.

## Changelog

Every change recorded for this component, newest first. Days that predate change tracking, or that we cannot explain, say so: "we were watching and nothing happened" and "we were not watching" are different claims.

### 2026-09-25 (score 75, +1)

- [functional] We updated how we score, so this day's move reflects our rubric, not a change to the server

### 2026-09-23 (score 74, +1)

No change was recorded against any check on this day. Stability & Change Management went from 57 to 60. That category is still filling its 30-day observation window: 17 days of observed history at the previous scan, 18 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-21 (score 73, +1)

No change was recorded against any check on this day. Stability & Change Management went from 50 to 53. That category is still filling its 30-day observation window: 15 days of observed history at the previous scan, 16 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-19 (score 72, +1)

No change was recorded against any check on this day. Stability & Change Management went from 43 to 47. That category is still filling its 30-day observation window: 13 days of observed history at the previous scan, 14 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-17 (score 71, +1)

No change was recorded against any check on this day. Stability & Change Management went from 37 to 40. That category is still filling its 30-day observation window: 11 days of observed history at the previous scan, 12 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-15 (score 70, +1)

No change was recorded against any check on this day. Stability & Change Management went from 30 to 33. That category is still filling its 30-day observation window: 9 days of observed history at the previous scan, 10 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-13 (score 69, +1)

No change was recorded against any check on this day. Stability & Change Management went from 23 to 27. That category is still filling its 30-day observation window: 7 days of observed history at the previous scan, 8 at this one. The score rises as the window fills, whether or not the server changes.

### 2026-09-10 (score 68, +1)

No change was recorded against any check on this day. Stability & Change Management went from 13 to 17. That category is still filling its 30-day observation window: 4 days of observed history at the previous scan, 5 at this one. The score rises as the window fills, whether or not the server changes.

## MCP tools (1)

### `run` (~560 tokens)

Check Digit Validator

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.

Input parameters:

- `format` (string, required): Which checksum to apply — one of: iban, isbn10, isbn13, ean13, upca, luhn. Required; a bare number is not auto-detected because the same digits can be validly checked under more than one format with…
- `input` (string, required): The number to check, e.g. "DE89370400440532013000". Spaces and hyphens are stripped automatically; anything else must match the declared format exactly.

## Diagnostics

Captured diagnostic sections: TLS, DNSSEC, Authorisation, Transports. The full working is on the page: https://verifymcp.io/servers/com-gumballtools-check-digit/api-mcp#diagnostics

## Score history

- 2026-09-25: 75
- 2026-09-24: 74
- 2026-09-23: 74
- 2026-09-22: 73
- 2026-09-21: 73
- 2026-09-20: 72
- 2026-09-19: 72
- 2026-09-18: 71
- 2026-09-17: 71
- 2026-09-16: 70
- 2026-09-15: 70
- 2026-09-14: 69
- 2026-09-13: 69
- 2026-09-12: 68
- 2026-09-11: 68
- 2026-09-10: 68
- 2026-09-09: 67
- 2026-09-08: 67
- 2026-09-07: 66
- 2026-09-06: 66
- 2026-09-05: 65

## Common questions

### What is the Check Digit Validator MCP server?

Check Digit Validator is an MCP server listed in the public MCP registry as com.gumballtools/check-digit. Validates IBAN/ISBN/EAN/Luhn checksums and finds the one wrong digit. This page covers its hosted endpoint (https://check-digit.gumballtools.com/api/mcp).

### Is the Check Digit Validator MCP server safe to use?

Check Digit Validator scores 75 out of 100 on VerifyMCP. That is a record of what we were able to check automatically, not an endorsement. The category breakdown on this page shows every signal behind the number, including the ones we could not confirm.

### What tools does the Check Digit Validator MCP server expose?

Check Digit Validator exposes 1 tool: run. Their descriptions and schemas cost roughly 560 tokens of context every time the server is loaded.

### Does the Check Digit Validator MCP server require authentication?

No. We connected to Check Digit Validator without credentials and it answered, so anything it exposes is reachable by anyone who knows the address.

### Is the Check Digit Validator MCP server still maintained?

Check Digit Validator is still listed as active in the MCP registry. We last reached this channel on 25 September 2026. Those dates come from our own scans of the registry and the channel itself, not from anything the publisher announced.

## Links

- Remote endpoint: https://check-digit.gumballtools.com/api/mcp
- Repository: https://github.com/bwalvoord/gumball
- Website: https://check-digit.gumballtools.com/
- Changelog RSS feed: https://verifymcp.io/servers/com-gumballtools-check-digit/api-mcp.xml
- Changelog JSON feed: https://verifymcp.io/servers/com-gumballtools-check-digit/api-mcp.json
- HTML version of this page: https://verifymcp.io/servers/com-gumballtools-check-digit/api-mcp