Check Digit Validator MCP MCP server

This MCP server exposes the Check Digit Validator engine, allowing AI agents to validate checksums for IBAN, ISBN, EAN, UPC, and Luhn formats. It serves as one of four surfaces (alongside web, markdown, and JSON API) for the same pure function, ensuring consistency. The service is free up to 250 calls per day and is part of Project Gumball.

Endpoint and access

No sign-in: your client connects and lists tools immediately, with no account or key.

No sign-inEndpoint checked 2026-10-05Operated by Gumball Tools

MCP endpoint (Streamable HTTP)

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

Vendor documentation

What you can do with it

Validate IBAN bank account numbers
Verify ISBN-10 and ISBN-13 book identifiers
Check EAN-13 and UPC-A product codes
Validate Luhn checksums for credit cards

Tools advertised by the last live check:

run

Connect from your client

Each snippet registers the same endpoint; pick the client you use.

This server is operated by the named third party. Listing and endpoint checks are not an endorsement or a security audit. Review the operator's permissions, terms, and data handling before connecting. How listings are checked and corrected.

Claude Code

Run this once in a terminal. It registers a Streamable HTTP server. Use --scope project to write .mcp.json for the repo.

Claude Code configuration

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

Codex

Run this once. Codex stores MCP servers in ~/.codex/config.toml (or .codex/config.toml in a trusted project).

Codex configuration

codex mcp add check-digit-validator-mcp --url https://check-digit.gumballtools.com/api/mcp

Cursor

Merge this server into .cursor/mcp.json in your project, then enable check-digit-validator-mcp in your MCP settings.

Cursor configuration

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

Cline

Merge this into cline_mcp_settings.json (Cline → MCP Servers → Configure). The type must be streamableHttp for a remote HTTP server.

Cline configuration

{
  "mcpServers": {
    "check-digit-validator-mcp": {
      "type": "streamableHttp",
      "url": "https://check-digit.gumballtools.com/api/mcp",
      "disabled": false
    }
  }
}

VS Code

Merge this server into .vscode/mcp.json. Use MCP: List Servers to start check-digit-validator-mcp and inspect its tools.

VS Code configuration

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

Gemini CLI

Run this once. It writes .gemini/settings.json in the current project; pass --scope user for ~/.gemini/settings.json.

Gemini CLI configuration

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

Claude Desktop

Merge this into claude_desktop_config.json (macOS: ~/Library/Application Support/Claude/, Windows: %APPDATA%\Claude\), then restart Claude Desktop.

Claude Desktop configuration

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

Windsurf

Merge this into ~/.codeium/windsurf/mcp_config.json, then refresh MCP in Cascade. Windsurf remote servers use serverUrl, not url.

Windsurf configuration

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

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