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.).

Tools

Tool	Input	Output
calculate_iva	{ amount: number, includesIva?: boolean }	{ subtotal, iva, total }
list_dte_types	(none)	{ types: [{ code, name }] }
validate_dui	{ dui: string }	{ valid, reason? }
validate_nit	{ nit: string }	{ valid, normalized, reason? }

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):

Code	Name
01	Factura (Consumidor Final)
03	Comprobante de Crédito Fiscal (CCF)
04	Nota de Remisión
05	Nota de Crédito
06	Nota de Débito
07	Comprobante de Retención
08	Comprobante de Liquidación
09	Documento Contable de Liquidación
11	Factura de Exportación
14	Factura de Sujeto Excluido
15	Comprobante de Donación

// 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