Check Digit Validator
REMOTE · CHECK-DIGIT.GUMBALLTOOLS.COM · SCANNED SEP 25
Validates IBAN/ISBN/EAN/Luhn checksums and finds the one wrong digit
Available components
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. How we score → Why this is hard to score →
Endpoint Security63
- The endpoint's TLS certificate is valid, in date, and uses a strong key. View diagnostics → Pass
- 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. See how to fix → View diagnostics → Unverified
- HTTPS is enforced; there's no plaintext access path. View diagnostics → Pass
- The HSTS (Strict-Transport-Security) header is present. View diagnostics → Pass
- DNSSEC check failed: this domain isn't protected by DNSSEC. See how to fix → View diagnostics → Fail
Transport & Reachability100
- Verified streamable-http transport via a live MCP handshake. View diagnostics → Pass
Schema Quality & AI Usability67
- AI-judged instruction clarity (excellent).Pass
- 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. See how to fix → Fail
- Usage-examples check failed: none of the tools include examples. See how to fix → Fail
Stability & Change Management67
- Stability observed for 20 of 30 days with no destabilising changes; credit accrues until the full window elapses.Partial
Tool Coverage100
- 100% of tools have a non-trivial description (not blank, and not just the tool's name).Pass
- 100% of tool parameters carry a description.Pass
Tool Safety100
- No prompt-injection markers were found in the server instructions, tool names or descriptions we captured.Pass
- We read all 1 captured tool definition(s), and no name or description among them implies an irreversible operation.Pass
- An AI judge read all 1 captured unit(s) of tool text and found none that tries to manipulate the model reading it.Pass
Capabilities100
- Implements a current MCP spec version (2026-07-28).Pass
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.
remote · check-digit.gumballtools.com
claude mcp add --transport http com-gumballtools-check-digit 'https://check-digit.gumballtools.com/api/mcp'
{
"mcpServers": {
"com-gumballtools-check-digit": {
"url": "https://check-digit.gumballtools.com/api/mcp"
}
}
} {
"servers": {
"com-gumballtools-check-digit": {
"type": "http",
"url": "https://check-digit.gumballtools.com/api/mcp"
}
}
} [mcp_servers.com-gumballtools-check-digit] url = "https://check-digit.gumballtools.com/api/mcp"
{
"$schema": "https://opencode.ai/config.json",
"mcp": {
"com-gumballtools-check-digit": {
"type": "remote",
"url": "https://check-digit.gumballtools.com/api/mcp",
"enabled": true
}
}
} openclaw mcp add com-gumballtools-check-digit --url 'https://check-digit.gumballtools.com/api/mcp' --transport streamable-http
mcp_servers:
com-gumballtools-check-digit:
url: "https://check-digit.gumballtools.com/api/mcp" {
"McpServers": {
"com-gumballtools-check-digit": {
"Transport": "http",
"Url": "https://check-digit.gumballtools.com/api/mcp"
}
}
} assistant mcp add com-gumballtools-check-digit -t streamable-http -u 'https://check-digit.gumballtools.com/api/mcp'
{
"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.
Every change we have recorded for this component, newest first. Security-relevant changes are always shown. ▲ marks a change for the better, ▼ a change for the worse; unmarked changes are neutral.
- 25 Sept 26 +1
- We updated how we score, so this day's move reflects our rubric, not a change to the server See what changed → functional
- 23 Sept 26 +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.
- 21 Sept 26 +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.
- 19 Sept 26 +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.
- 17 Sept 26 +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.
- 15 Sept 26 +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.
- 13 Sept 26 +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.
- 10 Sept 26 +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.
Diagnostic detail from the automated scan of this channel: what the scanner observed at each step, so you can see exactly where a check passed or failed. It is informational only and never changes the trust score.
Captured 25 Sept 2026 · Probed https://check-digit.gumballtools.com/api/mcp
TLS valid
Negotiated TLS 1.3 with TLS_AES_128_GCM_SHA256 .
| Subject | Issuer | Valid from | Valid until | Key | Signature | Serial |
|---|---|---|---|---|---|---|
| CN=*.gumballtools.com | CN=YR2,O=Let's Encrypt,C=US | 6 Sept 2026 | 5 Dec 2026 | RSA 2048 | SHA256-RSA | 60927985e79d22e7a1106139f609d74b0e4 |
| SANs: *.gumballtools.com, gumballtools.com | ||||||
| CN=YR2,O=Let's Encrypt,C=US (CA) | CN=Root YR,O=ISRG,C=US | 3 Sept 2025 | 2 Sept 2028 | RSA 2048 | SHA256-RSA | 4ebd24947e24d394802d84a52fd5b319 |
| CN=Root YR,O=ISRG,C=US (CA) | CN=ISRG Root X1,O=Internet Security Research Group,C=US | 13 May 2026 | 2 Sept 2032 | RSA 4096 | SHA256-RSA | f24b6d17f9d9ad7cb1c9fea78782699f |
Background: What to check on a remote MCP endpoint →
DNSSEC insecure
Validation of check-digit.gumballtools.com. — Not signed
| Zone | DS | Keys | Algorithms | Outcome |
|---|---|---|---|---|
| . | trust_anchor | 20326, 38696 | 8, 8 | Verified |
| com. | present | 19718 | 13 | Verified |
| gumballtools.com. | absent | Unsigned (proven) parent-signed NSEC/NSEC3 proves an unsigned delegation |
Authentication No authorisation required
The endpoint answered without asking for a token. Anyone who knows the URL can reach it.
| Result | No authorisation required |
|---|---|
| HTTP status | 200 |
| Header | Value |
|---|---|
| strict-transport-security | max-age=63072000 |
Background: How OAuth 2.1 works in the 2026 MCP spec →
Transports 2 probes
| Transport | URL | Outcome | Status | Location |
|---|---|---|---|---|
| streamable-http | https://check-digit.gumballtools.com/api/mcp | Verified | 200 | |
| http (plaintext) | http://check-digit.gumballtools.com/api/mcp | HTTPS enforced | 308 | https://check-digit.gumballtools.com/api/mcp |
The tools this component advertises to a client, with an estimated token cost for each. Expand a tool to see its parameters and schema. The per-tool counts are indicative and are not scored directly; the schema's total context footprint is one signal in Schema Quality & AI Usability. A tool's description is untrusted text the model reads on every call, which is what makes this list a security surface and not just an inventory: how tool poisoning works →
run Check Digit Validator ~560
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.
| Name | Type | Req | Description |
|---|---|---|---|
| format | string | yes | 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 | yes | The number to check, e.g. "DE89370400440532013000". Spaces and hyphens are stripped automatically; anything else must match the declared format exactly. |
No output schema declared.
No examples provided.
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.