MCP server

Check Digit Validator

com.gumballtools/check-digit

Checked 26 Oct 2026, 08:29 UTC · Connected to it and listed its tools

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

Working at last checkVerified publisher: gumballtools.comAbout 757 tokens per request once connected1 tools

https://check-digit.gumballtools.com/api/mcp

At a glance

Type: MCP server
Publisher: gumballtools.com (verified by the MCP Registry)
Status: Working at last check
Tools: 1
Token cost: About 757 tokens per request
Sets up in: Claude Code, Claude Desktop, Codex, Gemini CLI, Cursor, VS Code, Antigravity, ChatGPT, OpenAI API, Grok
Last checked: 26 Oct 2026, 08:29 UTC

Opens the app, which shows the server and asks you to confirm. For other apps, copy the setup below.

Claude Code

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

Claude Desktop

Settings → Connectors → Add custom connector, and paste the URL.

Codex (config.toml)

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

Gemini CLI

{"mcpServers":{"check-digit":{"httpUrl":"https://check-digit.gumballtools.com/api/mcp"}}}

Cursor

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

VS Code

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

Antigravity

agy mcp add check-digit https://check-digit.gumballtools.com/api/mcp

ChatGPT

Settings → Apps & Connectors → Advanced settings: turn on Developer mode, then Create a connector and paste the URL. Which plans can do this varies.

OpenAI API (Responses tools)

{"type":"mcp","server_label":"check-digit","server_url":"https://check-digit.gumballtools.com/api/mcp","require_approval":"always"}

Grok (xAI API tools)

{"type":"mcp","server_label":"check-digit","server_url":"https://check-digit.gumballtools.com/api/mcp"}

run ~757 tokens
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-

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