@beel_es/mcp

BeeL MCP server

Facturación electrónica española con VeriFactu, para agentes de IA.
Spanish e-invoicing with VeriFactu compliance, built for AI agents.
beel.es · API docs · MCP guide · npm

An MCP server that lets an AI agent issue legally compliant Spanish electronic invoices — VeriFactu registration with AEAT, F1/F2 invoice types, R1–R5 correctives, NIF validation against the census, and the regime keys the regulation requires.

It is not a generated wrapper around an API. Three things make it usable by a model:

• Tools are derived from the public OpenAPI contract, so each tool's input schema is the operation's real schema — enums, line items, regime keys and all. The surface cannot drift from the API.
• A tool-inclusion policy decides what an agent should actually be given. Binary downloads, multipart uploads, webhook plumbing and deprecated operations are excluded by rule, not by hand.
• Fiscal guardrails travel with the tools: the invariants a generated wrapper would miss, both as documentation the model reads and as pre-flight checks that stop a non-compliant request before it becomes a fiscal document.

One codebase, two transports: the hosted remote server at https://mcp.beel.es/mcp (Streamable HTTP + OAuth — one login per user, nothing to install), and a local stdio server built from this repository for headless use, where an API key works and a browser-based login does not.

Quick start

Add https://mcp.beel.es/mcp as a connector in Claude, ChatGPT, Cursor or VS Code and log in with your BeeL account. Nothing to install and no API key to handle: the server acts with your own credentials, and the OAuth flow is discovered from the URL.

# Claude Code
claude mcp add --transport http beel https://mcp.beel.es/mcp

That is the whole setup for interactive use. Read on only if you need the local server.

Running it locally

Use the local server when OAuth cannot: a scheduled job that issues invoices, a CI pipeline, or any headless process where no one is present to complete a browser login. It authenticates with an API key instead.

Requires Node ≥ 20.

// Claude Desktop / Claude Code MCP config
{
  "mcpServers": {
    "beel": {
      "command": "npx",
      "args": ["-y", "@beel_es/mcp"],
      "env": { "BEEL_API_KEY": "beel_sk_test_xxx" }
    }
  }
}

# Claude Code
claude mcp add beel --env BEEL_API_KEY=beel_sk_test_xxx -- npx -y @beel_es/mcp

Keys prefixed beel_sk_test_ are safe to experiment with; beel_sk_live_ issues real fiscal documents.

Releases are published from CI through npm trusted publishing, so they carry provenance: npm records the exact commit and workflow each build came from. Verify it with npm audit signatures.

What it provides

• 119 API tools derived from openapi/public-api.yaml — invoices, customers, products, recurring invoices, series and tax configuration, NIF validation, companies.
• 4 synthetic tools the API has no single endpoint for: beel_docs_search, beel_docs_get, beel_docs_list over the documentation, and beel_get_setup_status, which reports per NIF exactly what is missing before it can issue and the one next action to take.
• Guardrail resources under beel://guardrails/* — the fiscal invariants, plus beel://guardrails/errors, a catalogue of every error code with the action it calls for. Their summaries are woven into the description of every tool they constrain.
• 7 workflow prompts encoding the safe order of operations for the flows where the order is what makes them safe: issue-invoice (validate NIF → choose F1/F2 → check the VeriFactu gates → issue), fix-invoice (void vs correct), onboard-nif, setup-representation, invite-member, connect-payments and upgrade-integration.
• Inline invoice PDF viewer (MCP Apps): generating an invoice PDF opens it in a side panel in hosts that support it.

A generated catalogue of every tool, with the scopes each requires, lives at docs.beel.es/mcp/tools (npm run tools:catalog).

What is deliberately not a tool

Binary downloads (PDF preview, bulk ZIP, Excel/CSV export), multipart uploads (CSV/Holded import, signed-PDF submission), webhook infrastructure, and every deprecated operation. An agent cannot drive them, and each one costs context that a usable tool needs. The rules are in src/policy/tool-policy.ts.

The fiscal guardrails

Spanish e-invoicing has invariants an LLM will get wrong from the schema alone — voiding an invoice that should have been corrected, using R1 on a simplified invoice, editing one AEAT has already registered. The server addresses that in three layers, and the difference between them matters:

1. Advisory — src/guardrails/rules/*.md, one Markdown file per topic: the invoice lifecycle, void vs rectify, invoice types, invoice lines, regime keys, series numbering, NIF validation, the VeriFactu gates, multi-NIF accounts. Each is exposed as an MCP resource under beel://guardrails/* and its one-line summary is appended to the description of every tool it constrains, so the constraint travels with the call.

2. Enforced — src/guardrails/validate.ts, checked before the request is sent, so a bad payload never even consumes an idempotency key:

Check	Code
Exactly one pricing field per line	LINE_UNIT_PRICE_XOR_DECLARED_TOTAL
No discount on a declared total	LINE_DECLARED_TOTAL_FORBIDS_DISCOUNT
No IRPF withholding on a simplified (F2) invoice	SIMPLIFICADA_FORBIDS_IRPF
Equivalence surcharge only under regime 18, and 18 only with one	SURCHARGE_REQUIRES_REGIME / REGIME_REQUIRES_SURCHARGE
Series format can tell its reset periods apart	SERIES_ANNUAL_REQUIRES_YEAR / SERIES_MONTHLY_REQUIRES_MONTH_AND_YEAR
Numbering is only seeded in the call that activates the company	NUMBERING_REQUIRES_ACTIVATION
SUPLIDO lines carry their source reference	checked locally
Exemption text only under reason OTRO	checked locally
Correctives go through their own operation, not type: CORRECTIVE	checked locally

3. Explained — the BeeL API already answers well: its message is written for a human in the caller's language, error.details carries the specifics, and the RFC 7807 type field links to a documentation page for that exact code (around 357 of them). The server relays all of that untouched, and adds only the two things a response cannot carry: the remedy as a tool call — the docs address someone with the dashboard open ("create a series in settings"), an agent needs beel_set_default_series — and whether retrying can possibly help, which is what stops an agent looping on a 403 that needs an administrator. src/guardrails/catalog.ts holds only codes where one of those applies; anything else passes through, because a paraphrase would be worse than the original and would drift from it. The nested blockers[] of EMISSION_NOT_READY are the clearest case: they arrive as bare strings with no message and no link, and each comes back out naming the tool that clears it.

The BeeL API is the authority on all of it. Every enforced rule mirrors a rejection the contract documents, so the pre-flight is a strict subset of what the API refuses: it can only make failure faster and better explained, never permit something the API would reject. Rules that depend on server-side state — AEAT census matching, the €3 000 F2 ceiling, whether a series exists — stay advisory on purpose, because guessing at them locally would reject valid invoices. Set BEEL_DISABLE_PREFLIGHT=1 to bypass the local checks entirely.

Hand-curated lists are anchored by tests: every catalogued code must still appear in the contract, every checked operationId must still resolve to a real tool, and every guardrail reference must point at a guardrail that exists. An API rename fails CI instead of silently switching a fiscal check off.

Configuration

Local server only

Variable	Purpose
BEEL_API_KEY	API key. The prefix selects the environment: beel_sk_test_ → Test, beel_sk_live_ → Live.
BEEL_ENV / BEEL_CONFIG_DIR	Optional. With BEEL_API_KEY unset, falls back to the CLI's ~/.config/beel/config.json (beel login); BEEL_ENV (test/live, default test) picks which stored key.

Shared

Variable	Purpose
BEEL_BASE_URL	API base URL. Default https://app.beel.es/api.
BEEL_DOCS_URL	Documentation source for the docs tools. Default https://docs.beel.es.
BEEL_REQUEST_TIMEOUT_MS	Hard ceiling on a single API call. Default 30000.
BEEL_DISABLE_PREFLIGHT	Set to 1 to skip the enforced guardrails.

Every default lives in src/shared/defaults.ts; nothing is hardcoded twice. Remote deployment variables are documented in DEPLOY.md.

The server starts and lists tools with no credentials at all — it only errors when an API tool is actually called. POST requests carry a stable Idempotency-Key derived from the request itself, so an agent retrying "create invoice" can never mint a second invoice.

Self-hosting

The remote server runs on Cloudflare Workers. See DEPLOY.md for the KV namespace, the OAuth client BeeL must have registered, and the secrets involved.

Development

npm ci
npm run dev          # stdio server from source
npm test             # vitest
npm run typecheck    # both the Node and the Worker configs
npm run build        # single-file bundle to dist/index.js
npm run inspect      # MCP Inspector against the local build
npm run spec:verify  # the vendored contract still matches its lock

openapi/public-api.yaml is a generated copy of the API contract, and openapi/spec.lock.json records its version, operation count and hash. CI fails if the two disagree, which is what keeps a vendored contract honest. See CONTRIBUTING.md.

The rest of the BeeL developer ecosystem

Everything below derives from the same OpenAPI contract, so the vocabulary — invoice types, regime keys, series, VeriFactu states — is identical wherever you meet it.

REST API	The contract itself. Everything else is a projection of it
CLI	The same surface from a terminal, sandbox by default
n8n node	Invoicing inside a no-code workflow
Claude Code plugin	Implement, audit and maintain a BeeL integration
Machine-readable docs	llms.txt for agents that would rather read than guess

Contributing

Bug reports and pull requests are welcome — see CONTRIBUTING.md for how the project is laid out and which conventions are load-bearing. Security issues go to [e-mail] rather than a public issue; see SECURITY.md.

License

MIT © BeeL

_{beel.es · Documentación · Guía del MCP}