Lexware Office MCP Server

An open-source, self-hostable MCP server for the Lexware Office accounting API. Run your own instance, connect it to Claude, and let an agent read your invoices, vouchers, contacts and articles — and (optionally) draft new ones.

Bring your own Lexware API key — the server is single-tenant per deployment and never stores anyone else's credentials. It runs as a remote HTTP server on any container host, built with the Skybridge framework.

Two authentication methods — choose one:

• OAuth 2.1 — required to use this as a web MCP server / custom connector in the Claude app, claude.ai web, or ChatGPT (those clients accept only OAuth).
• Static bearer token — simpler, but works only with Claude Code / Claude Desktop (which let you send a request header); the web custom-connector UI does not support it.

Setup for both is in the Client support & authentication section below.

Related projects — local (stdio) Lexware MCP servers: lazyants/lexware-mcp-server, JannikWempe/mcp-lexware-office.

⚠️ This brokers real accounting data. Read SECURITY.md, protect your tokens, and note that finalized invoices are legally binding — you are responsible for their tax/legal correctness. No warranty (MIT).

Capabilities

65 tools across three tiers you enable via environment variables:

Set LEXWARE_READ_ONLY=true to force read-only (overrides the flags above).

Client support & authentication

The server supports two ways to protect /mcp, chosen by environment:

• OAuth 2.1 (OAUTH_ISSUER, …) — the recommended path. Use any OAuth provider (e.g. WorkOS AuthKit, Stytch, Auth0, Clerk; or self-hosted Keycloak/Zitadel) as the authorization server. This makes the server work as a custom connector in the Claude app, on claude.ai web, and in ChatGPT, with a real sign-in. Optionally restrict access with OAUTH_ALLOWED_EMAIL_DOMAINS (enforced server-side via the token's email / the provider's userinfo endpoint).
• Static bearer token (MCP_AUTH_TOKEN) — the simpler fallback. Works with Claude Code and Claude Desktop (which let you set a request header), but not the custom connector UI / claude.ai web / ChatGPT (those require OAuth).

OAuth takes precedence when OAUTH_ISSUER is set; otherwise the static token is used. With neither set, the server refuses to start unless MCP_ALLOW_UNAUTHENTICATED=true.

Connecting as a custom connector (OAuth)

1. In your provider, create an app, enable Dynamic Client Registration (or pre-register Claude's redirect https://claude.ai/api/mcp/auth_callback), and set this server's URL as the Resource Indicator / audience.
2. Deploy with OAUTH_ISSUER, OAUTH_RESOURCE (= the public URL), and optionally OAUTH_ALLOWED_EMAIL_DOMAINS.
3. In the Claude app → Connectors → Add custom connector, enter the server URL (https://…/mcp). Claude discovers the authorization server via /.well-known/oauth-protected-resource and walks you through sign-in.

Quick start (Docker)

git clone https://github.com/marselsel/lexware-mcp && cd lexware-mcp
cp .env.example .env          # set LEXWARE_API_KEY and MCP_AUTH_TOKEN
docker compose up --build     # serves on http://localhost:8080/mcp

Generate a strong auth token:

openssl rand -hex 32

Without Docker:

npm install
npm run build
LEXWARE_API_KEY=... MCP_AUTH_TOKEN=... npm start

Configuration

¹ The server needs either OAUTH_ISSUER (OAuth) or MCP_AUTH_TOKEN (static). It refuses to start with neither, unless MCP_ALLOW_UNAUTHENTICATED=true.

Connect to Claude (Code / Desktop)

Add to your MCP config (e.g. ~/.claude.json or the Desktop config):

{
  "mcpServers": {
    "lexware": {
      "type": "http",
      "url": "https://<your-host>/mcp",
      "headers": { "Authorization": "Bearer <MCP_AUTH_TOKEN>" }
    }
  }
}

Deploy

The server is a standard Docker container (Dockerfile) — run it on any host that can serve HTTPS (a VPS, Fly.io, Render, Railway, Cloud Run, Kubernetes, …):

docker build -t lexware-mcp .
docker run -p 8080:8080 --env-file .env lexware-mcp

Production notes:

• Serve over HTTPS (terminate TLS at your platform or a reverse proxy).
• Set auth via env (OAUTH_ISSUER… or MCP_AUTH_TOKEN) — the server fails closed otherwise.
• Run a single instance (or cap autoscaling to 1): the ~2 req/s rate limiter is per-process, so multiple instances would aggregate beyond Lexware's limit.
• Health check: GET /status (returns 200).

Google Cloud Run: a step-by-step recipe (Secret Manager + gcloud run deploy + custom domain) is in docs/cloud-run.md.

How it works

• src/config.ts — env parsing/validation, fail-closed auth, capability tiers.
• src/auth.ts — constant-time static-bearer middleware on /mcp.
• src/lexware/ — rate-limited (~2 req/s, token bucket), retry-aware client with safe error mapping; never retries non-idempotent POSTs on ambiguous failures (no duplicate documents).
• src/tools/ — tools registered conditionally by tier.
• src/server.ts — wires it together on the Skybridge Express server.

Development

See CONTRIBUTING.md. npm run dev starts the Skybridge dev server + DevTools at http://localhost:3000.

License

MIT © marselsel.