Kontor MCP

The sovereign e-invoice toolkit for AI agents. Official XRechnung / EN 16931 validation, ZUGFeRD in and out, 100 % offline, zero API keys.

Kontor MCP is a local, offline toolbox that lets AI assistants (Claude Desktop, Claude Code, any MCP client) handle German/EU e-invoices: XRechnung and ZUGFeRD/Factur-X under EN 16931. It can read an invoice (bare XML or XML embedded in a PDF), check it against the official KoSIT / EN 16931 rulebooks, explain cryptic errors in German or English, create valid new invoices (XML or ZUGFeRD PDF/A-3), convert between formats, and tell a business which legal obligations apply. The key promise is sovereignty: no cloud, no API keys, no network calls, no Java at runtime — proven by tests.

Why it matters: since 2025 every German business must be able to receive e-invoices, and from 2027/2028 must issue them. An e-invoice is validated XML, not a PDF, and a failed rule can cost the input-VAT deduction. Invoices carry personal and bank data, so they should be checked where they live — on your machine — with the same rule sets the public-sector receivers use.

Claude Desktop with the kontor server: validate → BR-DE-15 (missing Leitweg-ID) with fix hint · parse a ZUGFeRD PDF · explain BR-DE-18. MP4

v1.0 — 8 tools · 4 resource families · 3 prompts · KoSIT conformance 89/89 enforced as a CI gate · ZUGFeRD PDF/A-3 output verified by veraPDF and Mustang · stdio and Streamable HTTP (bearer auth) · Docker image (amd64/arm64) · kontor-agent reference client · listed in the Official MCP Registry. Install with npx, Docker, or from source.

Why

• Official rules, not approximations. The KoSIT XRechnung Schematron and the CEN EN 16931 rules run unmodified (compiled to XSLT/SEF, executed with Saxon-JS) with the KoSIT scenario model — the same verdicts the public-sector receivers produce, proven file-by-file against the official validator.
• Sovereign by construction. Pure TypeScript/WASM, no Java at runtime, no network calls, nothing stored, nothing logged. Runs where the invoices are.
• Agent-native. Every tool returns structured content and a readable summary; findings carry DE/EN explanations, affected business terms and fix hints; generation is fail-honest (valid is the real verdict, never assumed).
• ZUGFeRD both ways. Read the embedded XML out of any ZUGFeRD/Factur-X PDF; write PDF/A-3 invoices with factur-x.xml that pass veraPDF and Mustang.

Five-minute quickstart

Requires Node ≥ 20. Everything — rules, schemas, code lists, fonts — ships inside the npm package; no downloads at runtime, no Java.

Claude Desktop — Settings → Developer → Edit Config, then quit (⌘Q) and reopen:

{
  "mcpServers": {
    "kontor": {
      "command": "npx",
      "args": ["-y", "@kontor-mcp/server"]
    }
  }
}

(From a source checkout use "command": "node", "args": ["/absolute/path/to/kontor-mcp/packages/server/dist/bin.js"] instead.)

**Claude Code:**

```sh
claude mcp add kontor -- npx -y @kontor-mcp/server

Now audit a sample invoice. Ask Claude:

Audit /absolute/path/to/kontor-mcp/packages/server/samples/broken-missing-buyer-reference.xml — is it valid?

You get a verdict (invalid), the finding BR-DE-15 (missing buyer reference / Leitweg-ID) with an explanation and fix hint, the recomputed totals and VAT breakdown, and a reject recommendation with its rationale. Then try a ZUGFeRD PDF:

What is in /absolute/path/to/kontor-mcp/packages/server/samples/generated-zugferd-en16931.pdf?

(Attach XML files directly if you prefer; PDFs must be referenced by local path — Claude Desktop does not hand PDF bytes to MCP servers.)

Without a chat client, the MCP Inspector works headless:

npx @modelcontextprotocol/inspector@latest --cli node packages/server/dist/bin.js \
  --method tools/call --tool-name audit_invoice \
  --tool-arg file_path=$PWD/packages/server/samples/broken-missing-buyer-reference.xml --tool-arg lang=en

kontor-agent CLI — the reference client; audit needs no LLM and returns 0/1/2 for accept/review/reject, chat is an Anthropic agent loop that prints every tool call (needs ANTHROPIC_API_KEY):

node packages/client/dist/bin.js audit packages/server/samples/broken-missing-buyer-reference.xml --lang en
node packages/client/dist/bin.js chat -m "Prüfe $PWD/packages/server/samples/valid-zugferd-en16931.pdf"
node packages/client/dist/bin.js --url http://127.0.0.1:3333/mcp --token "$KONTOR_AUTH_TOKEN" tools   # against Docker / HTTP

Docker / Streamable HTTP — the same server over HTTP for remote agents and containers (token required, loopback-published port, TLS is your reverse proxy's job):

docker build -t kontor-mcp .              # multi-stage, non-root, ~70 MB, amd64 + arm64
docker run -d -p 127.0.0.1:3333:3333 -e KONTOR_AUTH_TOKEN="$(openssl rand -hex 24)" kontor-mcp
curl -s http://127.0.0.1:3333/healthz     # {"ok":true,"name":"kontor-mcp",...}

Or cp .env.example .env && docker compose up -d (read-only root FS, ./invoices mounted at /data). Config surface and the security posture: packages/server/README.md, SECURITY.md.

Tools

Resources: kontor://samples/{name}, kontor://reference/rules, kontor://reference/codelists/{list}, kontor://reference/cheatsheet. Prompts: audit-incoming-invoice, draft-supplier-rejection, create-invoice-interview. Full reference with inputs and conventions: packages/server/README.md.

Sovereignty — and how we prove it

Architecture

flowchart LR
  subgraph client [MCP client]
    A[Claude Desktop / Claude Code / kontor-agent]
  end
  A -- stdio / Streamable HTTP --> S

  subgraph server ["@kontor-mcp/server"]
    S[tools · resources · prompts<br/>Zod schemas, structuredContent + text]
  end

  subgraph core ["@kontor-mcp/core (MCP-free library)"]
    D[detect] --> P[parse → EN 16931 model]
    P --> V[validate: XSD → Schematron → plausibility]
    P --> G[generate / convert / preview]
    G --> Z[ZUGFeRD PDF/A-3 assembly]
    V --> AU[audit → verdict + recommendation]
    O[obligations decision tree]
  end

  subgraph rules ["@kontor-mcp/rules (bundled, checksummed)"]
    R1[XSDs UBL 2.1 / CII D16B]
    R2[EN 16931 + XRechnung Schematron as SEF]
    R3[rule knowledge base DE/EN]
    R4[code lists · legal timeline · fonts + ICC]
  end

  S --> core
  core --> rules

The validation pipeline: Layer 1 XSD (xmllint-wasm) → Layer 2 official Schematron (Saxon-JS, KoSIT scenario selection and severity overrides) → Layer 3 Kontor plausibility (KONTOR-PLAUS-*: decimal recomputation, VAT rates, IBAN/BIC, Leitweg-ID check digits, dates, duplicates — never changes the official verdict). Money math is decimal.js only; every XML parse has DTD/external entities disabled.

Conformance

Details, commands and recorded reports: docs/CONFORMANCE.md.

Packages

FAQ

Is this legal or tax advice? No. Kontor reports formal and technical checks against the published standards and the legal timeline with its sources; decisions remain yours. Every answer carries that disclaimer.

Does it support XRechnung 3.0.2 / the 2025 rules? Yes: XRechnung 3.0.2, Schematron 2.5.0, validator configuration 2026-01-31, EN 16931 1.3.16. Versions are pinned and listed by list_capabilities.

Which ZUGFeRD profiles can it read and write? Read: MINIMUM, BASIC WL, BASIC, EN 16931, EXTENDED, XRECHNUNG (profile from the XMP). Write: EN 16931 (default), BASIC, EXTENDED — see D-042 for what BASIC drops.

Can I send it a PDF from Claude Desktop? Reference it by local path. Desktop does not pass attached PDF bytes to MCP servers; XML attachments work either way.

Why no Java when KoSIT's validator is Java? Sovereignty and installability: Schematron is compiled once at build time and executed in TypeScript (Saxon-JS). The Java tools are used only as oracles in CI to prove parity.

Windows? Yes — CI runs the full test suite on Ubuntu, macOS and Windows with Node 20 and 22.

Development

pnpm install
pnpm build
pnpm -r test
pnpm lint
pnpm artifacts        # fetch pinned third-party artefacts for the oracles (dev only)
pnpm oracle --diff …  # KoSIT parity (Java 17+)
pnpm check:zugferd    # regenerate samples, veraPDF + Mustang (Java + veraPDF)

Read CONTRIBUTING.md before opening a PR; security issues go through SECURITY.md.

Docs

• Product Requirements · Implementation Plan · Decision log
• Conformance · Business-term coverage · Artefact provenance
• v0.1 Claude Desktop verification

License

Apache-2.0 — see LICENSE and NOTICE for bundled third-party components (KoSIT artefacts Apache-2.0, EN 16931 artefacts EUPL-1.2, Liberation Fonts OFL 1.1, ICC sRGB profile).