mcp-el-salvador-dte

A small, focused Model Context Protocol (MCP) server that exposes El Salvador electronic invoicing (DTE) and fiscal helpers as tools an LLM can call: IVA calculation, the official DTE document-type catalog, and DUI / NIT validation.

Built with the official @modelcontextprotocol/sdk for TypeScript, ESM, Node 18+, and the stdio transport.

• Author: Gerar Arévalo — github.com/Gerar12 · gcoder.dev

• License: MIT

What it is

The Documento Tributario Electrónico (DTE) is El Salvador's mandatory electronic invoicing standard, administered by the Ministerio de Hacienda. This server bundles a few pure, deterministic helpers that come up constantly when building DTE integrations, and makes them available to any MCP-capable client (Claude Desktop, Claude Code, etc.).

Related MCP server: keycae-mcp

Tools

calculate_iva

El Salvador IVA (VAT) is 13%.

• includesIva = false (default): amount is the net subtotal; IVA is added on top.

• includesIva = true: amount already includes IVA; it is broken out.

Values are rounded to 2 decimals, and subtotal + iva === total always holds.

// input
{ "amount": 100 }
// output
{ "subtotal": 100, "iva": 13, "total": 113 }

// input
{ "amount": 113, "includesIva": true }
// output
{ "subtotal": 100, "iva": 13, "total": 113 }

list_dte_types

Returns the 11 official DTE document types (code — name):

// output (abridged)
{ "types": [ { "code": "01", "name": "Factura (Consumidor Final)" }, ... ] }

validate_dui

Validates a Salvadoran DUI (Documento Único de Identidad): 8 digits, a hyphen, then 1 check digit (e.g. 01234567-8).

It checks the format and the modulo-10 check digit. The 8 base digits are weighted 9, 8, 7, 6, 5, 4, 3, 2 (left to right); the expected check digit is (10 - (weightedSum % 10)) % 10.

// input
{ "dui": "01234567-8" }
// output
{ "valid": true }

// input
{ "dui": "01234567-9" }
// output
{ "valid": false, "reason": "Invalid check digit: expected 8, got 9" }

Honesty note on the DUI check digit. The modulo-10 algorithm above is a widely-used community algorithm and was verified in this repo against known-valid DUIs (00016297-5, 01234567-8). It is not reproduced from an official, government-published specification. If your use case is high-stakes (e.g. legally rejecting a real person's ID), treat a valid: false from the check-digit test as "likely a typo, please re-check" rather than an authoritative rejection. The format check (^\d{8}-\d$) is unambiguous; the check-digit step is best-effort.

validate_nit

Validates the format only of an El Salvador NIT (Número de Identificación Tributaria): 14 digits, commonly formatted NNNN-NNNNNN-NNN-N. Input is accepted with or without hyphens/whitespace, and a canonically hyphenated normalized value is returned.

// input
{ "nit": "06141234560012" }
// output
{ "valid": true, "normalized": "0614-123456-001-2" }

Honesty note on NIT. This is format/length validation only — it does not verify a check digit. NIT check-digit rules are not consistently documented in public sources, so shipping a check-digit validator here would imply a correctness guarantee that cannot be honestly backed up. A valid: true means "well-formed", not "issued by Hacienda".

Install

Requires Node.js 18+.

# clone, then:
npm install
npm run build

Run the tests (unit tests for the pure functions + an stdio smoke test that boots the server and performs a full MCP handshake):

npm test

Start the server manually (it speaks JSON-RPC over stdio and waits for a client):

npm start
# or
node build/index.js

Run with npx (after publishing to npm)

npx mcp-el-salvador-dte

Add to Claude Desktop

Edit your claude_desktop_config.json:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json

• Windows: %APPDATA%\Claude\claude_desktop_config.json

{
  "mcpServers": {
    "el-salvador-dte": {
      "command": "node",
      "args": ["/absolute/path/to/mcp-el-salvador-dte/build/index.js"]
    }
  }
}

Once published to npm you can instead use:

{
  "mcpServers": {
    "el-salvador-dte": {
      "command": "npx",
      "args": ["-y", "mcp-el-salvador-dte"]
    }
  }
}

Restart Claude Desktop, and the four tools will appear.

Add to Claude Code

# local build
claude mcp add el-salvador-dte -- node /absolute/path/to/mcp-el-salvador-dte/build/index.js

# or, after publishing to npm
claude mcp add el-salvador-dte -- npx -y mcp-el-salvador-dte

Project layout

src/lib.ts     Pure, side-effect-free domain logic (unit-tested directly)
src/index.ts   MCP server: registers the 4 tools over stdio
test/          node:test unit tests + stdio smoke test

License

MIT © 2026 Gerar Arévalo