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

62 tools across three tiers you enable via environment variables, plus one opt-in tool outside them (upload-file-from-url, see below):

Tier	Default	What it covers
Read	always on	Profile; contacts & articles (list/get); the voucherlist (plus summarize-vouchers for server-side totals); full documents (invoices, quotations, credit notes, order confirmations, delivery notes, dunnings, down-payment invoices, vouchers); render any document type to PDF and download files/receipts (returned inline as embedded resources); batch & type-dispatched reads (get-vouchers, get-document, get-voucher-file, get-document-file); payments; reference data (countries, payment conditions, posting categories, print layouts); recurring templates (get & list); event subscriptions; document deeplinks
Drafts/writes (LEXWARE_ENABLE_DRAFTS)	on	Create draft invoices/quotations/credit-notes/order-confirmations/delivery-notes/dunnings (the Lexware API has no update endpoint for these — set every field, including payment terms, at creation); create & update contacts, articles, and bookkeeping vouchers; upload files and attach receipts to vouchers — inline as base64, or without base64 via a short-lived upload ticket (create-upload-ticket → browser drag-and-drop or a curl one-liner → get-upload-result); create documents as follow-ups (precedingSalesVoucherId)
Finalize (LEXWARE_ENABLE_FINALIZE)	off	Issue legally binding finalized documents in one step via the dedicated create-finalized-* tools (confirmation-gated); irreversible article deletes; manage webhook event subscriptions (create + delete — a webhook streams financial events to an external URL, so it's opt-in). Enabling this tier also enables Drafts.

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

upload-file-from-url — outside the tiers, off by default

One tool sits outside this table: upload-file-from-url fetches a file from a share link server-side and stores it in Lexware, which is how a receipt already sitting in OneDrive/SharePoint gets into the books without a round trip through the model. It is enabled with LEXWARE_ENABLE_URL_UPLOAD=true and needs the drafts tier (it will not turn that tier on for you).

It has its own switch because it has its own risk: it is the only tool that makes this server originate an outbound request to a destination the model chose — server-side request forgery, in the general case. Three things bound it, applied at every redirect hop:

1. A host allow-list, matched on a dot boundary so evilsharepoint.com cannot pass as sharepoint.com. Configure with LEXWARE_UPLOAD_ALLOWED_HOSTS; unset means the Microsoft file-sharing defaults, a set value replaces them, an empty value blocks everything.
2. A resolved-address check rejecting loopback, private, link-local (including the 169.254.169.254 metadata endpoint), CGNAT, multicast and reserved space, in every IPv4, IPv6 and IPv4-in-IPv6 spelling.
3. Connection pinning. The socket connects to an address from the very lookup that step 2 approved, rather than letting the HTTP client resolve the name again. Without this, steps 1 and 2 describe one lookup and the connection uses another, and a DNS answer that changes in between (rebinding) slips past a check that looked correct. TLS is unaffected: the certificate is still validated against the hostname.

Only https is accepted, the download is capped at 20 MB and 30 s, and redirects are followed manually — at most three — so no hop skips the checks.

What that looks like in practice

"Summarize my open invoices for this quarter — who still owes what?"

"Draft an invoice to Müller GmbH for 12 consulting hours at 140 €, due in 14 days."

"Here's the Hetzner receipt for June — file it in the bookkeeping." → see Uploading receipts

"Render invoice RE-2026-0042 as a PDF and give me the deeplink to it."

Uploading receipts (no base64 through the model)

Attaching a file through a chat has an awkward default: upload-file needs the bytes as base64 inside the model's tool call, so the whole receipt travels through the conversation — every byte billed as tokens (base64 adds ~33% on top), the file's contents sitting in the transcript, and a ~8 MB practical ceiling. The model never needed the bytes; it only needs the file id that comes back.

The ticket flow routes the bytes around the model:

1. You ask Claude to file a receipt. It calls create-upload-ticket and hands you a link like https://your-server…/upload/<ticket>.
2. You open the link and drag the file onto the page — or run the ready-made curl one-liner next to a local file (replace only the FILE= path; nothing else needs to be installed or edited). The bytes go client → server → Lexware directly.
3. Claude reads the resulting file id with get-upload-result (the curl variant prints it directly) and carries on with the actual bookkeeping — e.g. create-voucher with files: [fileId], or linking the receipt to an existing voucher.

The ticket is the credential. It is minted only through the authenticated /mcp endpoint (drafts tier), is 24 random bytes (192-bit) rendered base64url, valid for 15 minutes, single-use, and write-only — it authorizes exactly one file drop and grants no read access. The /upload/:ticket endpoint sits outside the OAuth gate by design (a plain browser or curl holds no MCP token); this is the same trust model as a pre-signed upload URL. Filenames travel as X-Filename-B64 (base64url of the UTF-8 bytes), so umlauts, dashes, typographic quotes and emoji arrive in the books intact.

The ticket store is in-memory: run a single instance (see Deploy), and note that a restart voids open tickets — they answer 410, and you simply issue a new one.

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-Server && cd Lexware-MCP-Server
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

Env var	Default	Purpose
LEXWARE_API_KEY	— (required)	Your Lexware API key (create one)
OAUTH_ISSUER	—	OAuth authorization-server issuer URL. Setting it enables OAuth mode¹
OAUTH_RESOURCE / SERVER_URL	http://127.0.0.1:$PORT	This server's public URL. Required in OAuth mode (token audience / Resource Indicator), and used in every mode to build the upload links create-upload-ticket hands out (browser URL and curl command). Set it whenever the server is reachable under a real domain — without it those links point at the loopback fallback, which only works on the server itself
OAUTH_ALLOWED_EMAIL_DOMAINS	—	Comma-separated allow-list of email domains (e.g. example.com)
OAUTH_VERIFY_AUDIENCE	true	Verify the token aud matches OAUTH_RESOURCE. Keep true. Setting false accepts any valid token from the issuer — including one minted for a different app on the same issuer (a confused-deputy risk). Only disable for a dedicated, single-audience issuer that has no Resource Indicator
OAUTH_AUDIENCE	—	Comma-separated additional accepted aud values, on top of OAUTH_RESOURCE. For IdPs that ignore the Resource Indicator: Microsoft Entra always puts the API's client ID (a GUID) in aud, never the Application ID URI, so without this every token is rejected. Prefer this over OAUTH_VERIFY_AUDIENCE=false — the check stays on, just against a value your IdP actually issues. Values are matched exactly: they are opaque identifiers, so no normalisation is applied (unlike OAUTH_RESOURCE, which also accepts its trailing-slash form)
OAUTH_SCOPES_SUPPORTED	—	Scopes advertised as scopes_supported, telling clients what to request. Separate with commas or spaces (a scope value can never contain a space). Applies to both well-known documents, so they can't contradict each other. Unset: the protected-resource doc advertises nothing and the authorization-server doc keeps its openid email profile default — i.e. unchanged behaviour. Set it for IdPs that reject an authorization request with no scope parameter (Microsoft Entra: AADSTS900144)
OAUTH_JWKS_URL / OAUTH_USERINFO_URL	derived from issuer	Override the JWKS / OIDC userinfo endpoints (defaults use the WorkOS-AuthKit layout)
OAUTH_AUTHORIZATION_ENDPOINT / OAUTH_TOKEN_ENDPOINT / OAUTH_REGISTRATION_ENDPOINT	derived from issuer	Override the endpoints advertised in the authorization-server metadata. Defaults use the WorkOS layout ({issuer}/oauth2/*); set these for other IdPs (e.g. Auth0: /authorize, /oauth/token, Entra: /oauth2/v2.0/authorize). Set OAUTH_REGISTRATION_ENDPOINT=none if your issuer does not support Dynamic Client Registration — the field is optional in RFC 8414, and advertising an endpoint that rejects every request makes clients attempt DCR and fail rather than use a pre-registered client
MCP_AUTH_TOKEN	— (required¹)	Static bearer token clients send to reach /mcp (used when OAuth is off)
MCP_ALLOW_UNAUTHENTICATED	false	Opt out of auth (trusted local use only — bind to localhost/private network)
LEXWARE_READ_ONLY	false	Register only read tools (hard override)
LEXWARE_ENABLE_DRAFTS	true	Enable create-draft tools
LEXWARE_ENABLE_FINALIZE	false	Enable finalize / legally-binding tools (also enables Drafts)
LEXWARE_ENABLE_URL_UPLOAD	false	Enable upload-file-from-url (server-side fetch). Requires the Drafts tier; does not enable it
LEXWARE_UPLOAD_ALLOWED_HOSTS	Microsoft file-sharing hosts	Hosts upload-file-from-url may fetch from, comma-separated. Replaces the defaults; empty blocks everything
LEXWARE_API_BASE_URL	https://api.lexware.io	API base URL
LEXWARE_APP_BASE_URL	https://app.lexware.de	Web-app base for document deeplinks
PORT	8080	Listen port (your platform may inject this)
LEXWARE_DEBUG_LOGGING	false	Verbose logs (never secrets/bodies)

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), for two reasons: the ~2 req/s rate limiter is per-process, so multiple instances would aggregate beyond Lexware's limit — and upload tickets live in process memory, so behind a load balancer without sticky sessions an upload can land on an instance that never issued its ticket.
• 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/uploads/ — the single-use ticket store and the raw-body /upload/:ticket routes (mounted only with the drafts tier; the ticket is the credential).
• 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.