An MCP (Model Context Protocol) 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. Connect it to Claude, ChatGPT, Cursor or VS Code and your agent can handle Spanish invoicing — facturación electrónica and factura electrónica VeriFactu — end to end, without you writing a single API call.
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, operations only a browser session can authenticate, and deprecated ones 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.
Each release is also announced to the MCP
Registry as es.beel/mcp, listing both
transports, so clients that browse the registry find the server without being pointed at
it. The name is authenticated by a DNS record on beel.es, so it says the server comes
from us and not merely from some repository.
An earlier listing under io.github.beel-es/beel-mcp (v0.2.2) was retired when the name
moved. Registry names are identities rather than labels, so a rename is a new entry rather
than a redirect; both point at the same npm package and the same hosted server.
What it provides
- 125 API tools derived from
openapi/public-api.yaml— invoices, customers, products, recurring invoices, series and tax configuration, NIF validation, companies. - 7 synthetic tools the API has no single endpoint for:
beel_docs_search,beel_docs_get,beel_docs_listover the documentation (the docs site's search endpoint and one page's Markdown at a time, or some of its sections in one call: a long page answers with its outline);beel_rules_listandbeel_rules_getover the fiscal rules catalogue (by id, several ids at once, domain, keyword or error code);beel_schema_get, the fields of request and response schemas as compact TypeScript-like declarations from the bundled contract (by schema name, or by tool name for its query, body and return type); andbeel_get_setup_status, which reports per NIF exactly what is missing before it can issue, the one next action to take, and the ids an integration starts from (company, default series per document type, VeriFactu status and tax defaults). - Guardrail resources under
beel://guardrails/*— the fiscal rules, one resource per domain, the API usage guides, andbeel://guardrails/errors, a catalogue of every error code with the action it calls for. The domains and guides that apply are named in 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-paymentsandupgrade-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, operations that only a browser
session can authenticate, 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 — two sources, kept apart on purpose:
- The fiscal rules are the catalogue the documentation site publishes at
docs.beel.es/api/rules.json: each rule has an id (COR-002), a statement, why it exists, its legal basis, the error codes that enforce it and examples. The server reads it at runtime (cached, like the other docs files) and serves it throughbeel_rules_list/beel_rules_getand one resource per domain (beel://guardrails/corrective, …). Nothing fiscal is re-typed here. A snapshot of the catalogue ships in the package as a fallback for when the docs host is unreachable;npm run sync:rulesis its only writer, and a test fails if it was edited by hand. The formerbeel://guardrails/cancel-vs-rectify,invoice-typesandregime-keysURIs still resolve, to the domains that replaced them. - The API usage guides —
src/guardrails/rules/*.md— cover what is not a fiscal rule but still trips an agent: how a line states its price, how a series is configured, which company an operation acts on, how to read NIF validation and issuing readiness, and which tool performs each invoice operation. They link to rule ids rather than restating them.
The domains and guides that apply are named in the description of every tool they constrain, 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 |
No nif or alternative_id on a simplified (F2) invoice's recipient | SIMPLIFIED_INVOICE_FORBIDS_IDENTIFIED_RECIPIENT |
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. 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. When the code is one a published rule cites, the error also
names that rule (id, title, link). 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 and rules 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
npm run sync:rules # refresh the bundled rules snapshot from docs.beel.es
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 |
FAQ
What is the BeeL MCP server? An MCP server that exposes Spanish VeriFactu e-invoicing as tools an AI agent can call — so Claude, ChatGPT, Cursor or VS Code can create customers, issue F1/F2 invoices, register them with AEAT, and post R1–R5 correctives on your behalf.
How do I connect VeriFactu invoicing to Claude / ChatGPT / Cursor?
Add https://mcp.beel.es/mcp as a connector and log in with your BeeL account — see
Quick start. Nothing to install, and no API key to paste for interactive use.
Is it actually VeriFactu-compliant? Yes. Invoices are registered with AEAT under VeriFactu, numbering and series follow the regulation, and the fiscal guardrails stop non-compliant requests before they ever become a fiscal document.
VeriFactu or TicketBAI? This server targets VeriFactu, the national AEAT system. TicketBAI (the Basque Country regime) is out of scope.
Can I use it without an AI agent? Yes — it is a standard MCP server, so any MCP-capable client works, and the same invoicing surface is available as a REST API, CLI and n8n node.
Contributing
Bug reports and pull requests are welcome — see CONTRIBUTING.md for
how the project is laid out and which conventions are load-bearing, and
Discussions for questions. Issues
labelled good first issue
are a reasonable place to start. Everyone participating is expected to follow the
Code of Conduct. Security issues go to [email protected] rather
than a public issue; see SECURITY.md.
License
MIT © BeeL.