mx-fiscal-mcp-server

An MCP server that gives an AI agent Mexican tax and banking capabilities — validate RFC, CURP, CLABE and NSS with real check digits, read a CFDI 4.0 invoice, ask the SAT whether it is still live, and look up the SAT's code tables. No API keys, no CSD certificate, no PAC contract.

npm MCP TypeScript License: MIT

Ask Claude "read this invoice and tell me whether it's still valid" and it parses the XML, labels every SAT code, checks both RFCs and the arithmetic, and queries the SAT's public status service — instead of you opening three web tools.

> Read this CFDI and check it at the SAT

  parse_cfdi(xml="<cfdi:Comprobante …>")

  CFDI A-1042 — 1160.00 MXN
  ✅ Stamped · version 4.0 · Ingreso
  Emisor:   TES150312DX2 ✅  601 — General de Ley Personas Morales
  Receptor: PELJ900521DK2 ✅  G03 — Gastos en general
  ✅ Totals add up: subtotal − discount + transferred − withheld = total

  cfdi_status(xml="<cfdi:Comprobante …>")

  CFDI status — Vigente
  ✅ The invoice exists in the SAT's records and has not been cancelled.
  Es cancelable: Cancelable con aceptación
  EFOS: returned 200 — the issuer is NOT on the SAT's definitive 69-B list.

Why this exists

Mexican electronic invoicing has two halves, and only one of them was served.

Building and stamping a CFDI needs a CSD certificate and a contract with a PAC. That half already has tooling — mcp-cfdi-mx (Python) does it, and this server deliberately does not compete with it.

Reading is the other half, and it needs nothing: the check-digit algorithms are public, the SAT's catalogues are public, and the invoice status service behind the QR code on every printed invoice is public and unauthenticated. Yet an agent asked to validate an RFC will happily invent a regex that rejects XAXX010101000 — the RFC of every invoice issued to the general public — because that one does not satisfy its own check digit. This server is the read half, done carefully.

The arithmetic comes from mx-identifiers (MIT, zero dependencies, 101 tests against public vectors); the CFDI reader is ported from the tool running at ortamarco.me; the SOAP envelope was read out of nodecfdi/sat-estado-cfdi rather than guessed. It is the third of three read-only, key-free MCP servers alongside domain-security-mcp-server and seo-geo-mcp-server.

Tools

Identifiers

Tool	What it does
`validate_rfc`	RFC for individuals (13 chars) and companies (12), full modulus-11 check digit, parsed fields, birth/incorporation date. Flags the SAT generics and says which of them satisfies the arithmetic
`validate_curp`	18-character CURP with the base-37 check digit; decodes birth date, sex, state (RENAPO keys, not INEGI/ISO) and the century marker
`validate_clabe`	18-digit CLABE with the correct 3-7-1 control digit — each product reduced modulo 10 before summing, which is the step most implementations skip — plus the Banxico bank and the plaza code
`validate_nss`	11-digit IMSS number with its Luhn digit, split into subdelegación / registration year / birth year / serial
`generate_test_data` ⭐	1-100 coherent fake people or companies: the RFC and CURP derive from the same name and birth date, the CLABE's bank is a real participant, every check digit holds

CFDI

Tool	What it does
`parse_cfdi` ⭐	CFDI 4.0 XML → JSON: header, issuer, receiver, every line item with transferred and withheld taxes, tax totals, and the Timbre Fiscal Digital (or `null`). Labels every catalogue code, validates both RFCs, checks the totals arithmetic. Walks the document by local name, so any PAC's namespace prefixes work
`cfdi_status` ⭐	Queries the SAT's public `ConsultaCFDIService` SOAP endpoint: Estado, EsCancelable, EstatusCancelación and ValidaciónEFOS, each with its meaning in plain words. Takes the four key fields or the whole XML. Fail-soft: 10 s timeout, one retry, `available: false` on failure — never an exception

Catalogues

Tool	What it does
`sat_catalog_lookup`	Nine bundled code tables — `regimen_fiscal`, `uso_cfdi`, `forma_pago`, `metodo_pago`, `tipo_comprobante`, `objeto_imp`, `impuestos`, `bancos_clabe`, `estados_curp` — by exact code (leading zeros ignored) or accent-insensitive text search. Never touches the network

Every tool is read-only, declares an outputSchema and returns structuredContent (validated by the SDK) alongside human-readable Markdown (default) or JSON (response_format="json").

Honesty notes

These are surfaced in the tool output, not buried here:

Structurally valid is not registered. A check digit that adds up says the string is well-formed and nothing more. Only the SAT can say an RFC is registered; only RENAPO that a CURP belongs to a person. This server never asks either, and its wording never implies it did.
XAXX010101000 does not satisfy its own check digit. The SAT assigned the general-public RFC by decree and the modulus-11 algorithm asks for a 4 where the SAT wrote a 0. XEXX010101000 (foreign residents) does satisfy it. The tools report is_generic and check_digit_satisfied as separate fields rather than collapsing both into "valid".
Reading a CFDI is not verifying it. parse_cfdi does not check the digital signature. A perfectly parseable invoice can be cancelled, or fabricated wholesale.
An unreachable SAT is not an invalid invoice. The status endpoint publishes no rate limit, no SLA and no status page, and it goes down. cfdi_status degrades to available: false with the reason — a statement about the SAT, never about the document.
The bank list is a subset. bancos_clabe carries the main Banxico participants, not the full catalogue; an unknown code is reported as unknown rather than given an invented name. The plaza catalogue is not bundled at all, so the plaza code is returned verbatim.
The EFOS mapping is an interpretation. The SAT documents neither the ValidacionEFOS code table nor the field itself. 200/201 meaning "not on the 69-B list" is the mapping phpcfdi and nodecfdi both use, and it is labelled as such. An empty field is reported as unknown, never as "listed".

Protocol

Built on the v2 MCP SDK, so it speaks the 2026-07-28 revision (server/discover, no initialize, per-request _meta envelope) and still serves 2025-era clients — Claude Desktop, Claude Code and Cursor — from the same server factory. The entry points own the era decision: serveStdio(factory) on stdio, createMcpHandler(factory) over HTTP with the default legacy: 'stateless'. There is no session state and no Mcp-Session-Id in either direction. npm run smoke exercises every tool on both eras and asserts the negotiated era of each connection.

Because the tool list is a compile-time constant, tools/list and server/discover advertise a real one-hour ttlMs with cacheScope: 'public' on 2026-era connections instead of the SDK's conservative ttlMs: 0.

Install

Requires Node.js 20+. Nothing to clone — every MCP client can run it with npx.

Use it with Claude Code

claude mcp add mx-fiscal -- npx -y mx-fiscal-mcp-server

Use it with Claude Desktop or Cursor

Add to claude_desktop_config.json (or ~/.cursor/mcp.json) — see examples/:

{
  "mcpServers": {
    "mx-fiscal": {
      "command": "npx",
      "args": ["-y", "mx-fiscal-mcp-server"]
    }
  }
}

On Windows use "command": "cmd" with "args": ["/c", "npx", "-y", "mx-fiscal-mcp-server"]. Restart the client, then ask: "Generate 10 Mexican customers with valid RFC and CURP for my seed file."

Self-host (HTTP transport)

The same server speaks stateless Streamable HTTP for remote or multi-client use.

TRANSPORT=http npx -y mx-fiscal-mcp-server
# POST JSON-RPC to http://127.0.0.1:3000/mcp   ·   health at /healthz

It is safe by default: it binds to 127.0.0.1 and only accepts localhost Host and Origin headers, which blocks DNS-rebinding attacks from a web page. To expose it — for example behind Coolify or Traefik — opt in explicitly:

Variable	Default	Purpose
`TRANSPORT`	`stdio`	`http` to serve Streamable HTTP
`PORT`	`3000`	Listening port
`HOST`	`127.0.0.1`	Bind address; `0.0.0.0` to accept remote connections
`ALLOWED_HOSTS`	—	Comma-separated hostnames the `Host` header may carry (e.g. `mcp.example.com`)
`ALLOWED_ORIGINS`	—	Comma-separated origins allowed to call from a browser
`MCP_AUTH_TOKEN`	—	If set, every request needs `Authorization: Bearer <token>`
`MAX_XML_CHARS`	`2000000`	Largest CFDI accepted, in characters
`MAX_CONCURRENT_CFDI`	`4`	`parse_cfdi` / `cfdi_status` calls served at once; the rest are told to retry

A self-hosted instance makes requests to the SAT on its callers' behalf, so do not expose it without MCP_AUTH_TOKEN: an open one is a free relay that can get your IP rate-limited. With Docker (the image sets HOST=0.0.0.0):

docker build -t mx-fiscal-mcp .
docker run -p 3000:3000 -e ALLOWED_HOSTS=mcp.example.com -e MCP_AUTH_TOKEN=change-me mx-fiscal-mcp

Develop

npm run dev        # tsx watch (stdio)
npm run typecheck  # tsc --noEmit
npm test           # deterministic offline unit tests, HTTP transport defaults included
npm run smoke      # every tool over the real protocol, on BOTH eras
npm run inspect    # MCP Inspector against the built server
npm run build      # type-check + emit dist/

How it works

src/
├── index.ts        # transport selection: serveStdio | createMcpHandler + Express
├── server.ts       # the server FACTORY — registers every tool, holds the instructions
├── schemas.ts      # Zod 4 outputSchema for each tool
├── constants.ts    # the single external host, timeouts, limits
├── format.ts       # markdown/JSON response shaping, findings
├── core/           # pure logic, no MCP coupling — reusable & testable
│   ├── identifiers.ts # reporting layer over mx-identifiers (errors, labels, findings)
│   ├── cfdi.ts        # the CFDI 4.0 reader, ported 1:1 from the web tool
│   ├── sat.ts         # SOAP envelope, expression builder, response interpretation
│   ├── catalogs.ts    # the nine code tables + lookup
│   └── testdata.ts    # fixture generation
└── tools/          # thin MCP wrappers (Zod schemas, descriptions, formatting)

The core/ layer carries no MCP types, so the same logic backs both this server and a web UI. All the check-digit arithmetic lives in mx-identifiers and is not reimplemented here.

Network surface: exactly one host, consultaqr.facturaelectronica.sat.gob.mx, hardcoded in constants.ts. No tool accepts a URL from the caller, so there is no SSRF surface to guard. The other seven tools make no network calls at all.

Untrusted XML: a document that declares a DOCTYPE is refused before parsing (a CFDI never has one), and size and element-count caps bound the memory one parse can take. cfdi_status screens the RFCs, total and UUID before spending a request on them. Found a problem? Please open a private security advisory.

En español

Un servidor MCP que le da a un agente de IA las capacidades fiscales mexicanas que todo el mundo acaba reimplementando mal: validar RFC, CURP, CLABE y NSS con sus dígitos verificadores de verdad, leer un CFDI 4.0 en XML, consultar su estado en el SAT y buscar en los catálogos del SAT. Sin API keys, sin CSD y sin PAC.

Esto es la mitad de LECTURA. Para construir y sellar un CFDI hace falta un certificado de sello digital y un PAC, y eso ya lo cubre mcp-cfdi-mx (Python). Este servidor no compite con él: lo complementa.

Lo que sí hace bien y casi nadie:

XAXX010101000 no satisface su propio dígito verificador. El SAT lo asignó por decreto y el módulo 11 pide un 4 donde el SAT puso un 0. XEXX010101000 (residentes en el extranjero) sí lo satisface. Por eso tantos formularios rechazan las facturas al público en general. Las tools devuelven is_generic y check_digit_satisfied por separado.
El dígito de control de la CLABE reduce cada producto módulo 10 antes de sumar. Las implementaciones que suman primero, al estilo Luhn, aceptan y rechazan cuentas equivocadas.
Las claves de entidad de la CURP son de RENAPO, no de INEGI ni ISO 3166-2:MX. DF es Ciudad de México, MC es Estado de México, NE es nacido en el extranjero.
El servicio del SAT se cae y no publica límite de tasa. cfdi_status degrada a available: false con el motivo; nunca lanza una excepción, y nunca hay que leer un fallo del SAT como "la factura es inválida".
Estructuralmente válido no es dado de alta. Que el dígito cuadre dice que la cadena está bien formada, nada más.

Se instala sin clonar nada: claude mcp add mx-fiscal -- npx -y mx-fiscal-mcp-server.

El motor aritmético es mx-identifiers (MIT, cero dependencias); el lector de CFDI viene de la herramienta que corre en ortamarco.me; el sobre SOAP se leyó del código de nodecfdi/sat-estado-cfdi, no se inventó.

Habla la revisión 2026-07-28 del protocolo y sigue atendiendo a los clientes de 2025 (Claude Desktop, Claude Code, Cursor) desde la misma factoría de servidor.