Mexican Tax & Banking

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.

[ci] [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

CFDI

Catalogues

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"). Only cfdi_status and cfdi_batch_status are annotated openWorldHint: true: they are the two that reach the network.

Resources and prompts

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 SLA and no status page, and it goes down. Its documentation states capacity for up to 2 million queries per hour and asks callers not to raise their query volume, since every query reads the SAT's transactional databases. 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.
• EFOS codes are read from the SAT's own table. The SAT documents ValidacionEFOS in its Documentación del Servicio de Consulta de CFDI v1.4, section 3: 100, 101 and 104 put the issuer on the definitive 69-B list; 102 and 103 mean the issuer is not on it but a third-party RFC the invoice was issued on behalf of (a cuenta de terceros) is; 200 and 201 mean the issuer is not on it (201: nor any third party). efos_state speaks of the issuer only, and efos_third_party_state (listed / not_listed / not_reported / unknown) of the third parties. An empty field or an undocumented code is reported as unknown with the raw code kept in validacion_efos, never as "listed". Versions up to 1.0.1 reported 102 and 103 as an issuer on the list; 1.0.2 fixes it.
• Bineo CLABEs (bank 165). 1.0.3 requires mx-identifiers ^1.0.1, which carries the fix for them.

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

Add --scope user (all your projects) or --scope project (writes a shared .mcp.json) before the --; the default is local to the current project.

Use it with Claude Desktop

Open Settings → Developer → Edit Config and add to claude_desktop_config.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."

Use it with Cursor

The same JSON block goes in ~/.cursor/mcp.json (every project) or .cursor/mcp.json (this project only).

Use it with Codex

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

or in ~/.codex/config.toml (a project-scoped .codex/config.toml also works):

[mcp_servers.mx-fiscal]command = "npx"args = ["-y", "mx-fiscal-mcp-server"]

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:

No CORS headers are ever sent, so a web page cannot read its responses cross-origin; ALLOWED_ORIGINS only decides which Origin headers are let through to the handler (a request without one, as from an MCP client, always is). The request body is capped at about 1.25× MAX_XML_CHARS.

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 --noEmitnpm test           # deterministic offline unit tests, HTTP transport defaults includednpm run smoke      # every tool over the real protocol, on BOTH eras (queries the SAT)node scripts/smoke-all.mjs --offline   # the same, minus the calls that reach the SATnpm run inspect    # MCP Inspector against the built servernpm 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; attribute values are cut at 4,000 characters in the result. 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 se queda con la última cifra de cada producto ponderado 3-7-1, no con la suma de sus cifras. De 9 × 7 = 63 cuenta el 3; las implementaciones que copian a Luhn suman 6 + 3 = 9 y sacan otro dígito: para la base 09000000000000000 el algoritmo da 7 y el estilo Luhn da 1. Reducir módulo 10 antes o después de sumar da lo mismo; el error no está ahí.
• 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 SLA. Su documentación declara capacidad para hasta 2 millones de consultas por hora y pide no aumentar el volumen de consultas, porque cada una toca sus bases de datos transaccionales. 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.

License

MIT © Marco Orta