Unofficial Lexware Office MCP Server

Disclaimer

• This project is not affiliated with, endorsed by, or sponsored by Lexware or Haufe-Lexware GmbH & Co. KG. "Lexware" and "Lexware Office" are trademarks of their respective owners.
• It uses the documented public API with an API key you generate and can revoke yourself. Use of that API is governed by Lexware's own terms, which you accept independently of this project. The API can change at any time, and requests may be rate limited or blocked.
• It reaches real accounting records. Write access is off by default. If you enable it, anything created through the API is a real and legally relevant record — a finalized document cannot be withdrawn through the API.
• Data may be incomplete or out of date. Nothing here is tax, accounting or legal advice. Do not rely on it for filings, audits, or your bookkeeping obligations.
• Provided "as is", without warranty. Intended for personal and professional use at your own risk. See LICENSE.
• For commercial use, review Lexware's API terms and your own retention and documentation duties.

An MCP server that connects an MCP client such as Claude Desktop to a Lexware Office account through the official public REST API. Ask about invoices, contacts, articles and vouchers in plain language, and let the client fetch them for you.

Status: 0.1.0, the first release. The server runs over stdio and handles contacts, vouchers and documents: find them, read them, create them, change them, see what is still unpaid, download a PDF and upload a receipt. get_profile answers which account is connected. Every tool in the table below is built, and each was exercised against a live account. An HTTP transport, a container image and a Compose file are what 0.2.0 is for. See SPECS.md for the full technical specification and the roadmap.

Why this exists

Lexware Office holds the day-to-day accounting of a small business. Most questions about it are read questions — what is still unpaid, what did this customer order, which receipt belongs to that expense — and those are exactly the questions an assistant answers well once it can see the data. This server makes that possible without exporting anything, using an API key the account owner generates and can revoke.

Safety first

The server points at a real accounting system, so the defaults are cautious.

Run it read-only unless you have a reason not to. This server can change real accounting records - create a contact, record a voucher, issue an invoice, attach a receipt - and it is the assistant that decides when to call such a tool, not you. --tools read-only gives it everything it needs to answer questions about the books, which is what most people want it for: search, read, and download. Nothing in that set writes.

Turn a write tool on when you have a job for it, and know what it leaves behind. This API cannot delete a bookkeeping voucher at all, so a wrong one is corrected in the web app rather than withdrawn here, and a finalized invoice is a real document with a number that has been used. If you are not sure which tools you need, read-only is the honest starting point - the permissions page adds one later in a click, and a client that honours notifications/tools/list_changed, as Claude Desktop does, picks it up without a restart.

• Nothing is enabled until you say so. A fresh installation has no policy file, and a server without one offers no tools at all. What this server may do is a decision somebody made, never a default that happened.
• One flag per tool, in a JSON file you write with --tools, tick through setup, or edit by hand. Not a level, not a group: create_contact on and upload_file off is an ordinary thing to want, and there is no combination the file cannot express.
• What a tool costs you is visible while you decide. Every enabled tool is sent to the assistant on every request, and the permissions page puts that number on each row.
• The file is checked twice, once when the tool list is built and again when a call arrives, so a stale tool list on the client cannot slip past it.
• The API key is never logged, never returned in a tool result, and redacted from error messages. It belongs in the .env and nowhere else — not in your client's configuration file, which another program owns and rewrites, and which is the one people screenshot when they ask for help. Nor does any path from your machine reach the assistant.

Tools

Built means it works today. The rest are specified in SPECS.md and not implemented yet.

Read tools:

Write tools. These change real accounting records, so enable them one at a time and against an account you are willing to have changed:

update_contact and update_voucher cost two API calls rather than one. The API replaces a record instead of patching it, so the current one is read first and the change is laid on top. Without that, changing only an email address would empty out the addresses, the note and everything else. Both also need the version you last read: if the record changed in between, the update is refused and nothing is written.

One tool deletes, and it is the only one:

It is the only member of the --tools irreversible step so far, so that step is the only way to switch it on. An article is also the only thing this API lets you delete, which is the other half of the point:

--tools write is not the same as undoable. Nothing that preset enables deletes a record, but two of its tools create one that cannot be removed afterwards.

A bookkeeping voucher cannot be deleted through the API. There is no endpoint for it, so a wrong create_voucher has to be corrected in the Lexware Office web app. Pass unchecked to record an entry for review rather than booking it straight away. The same applies to upload_file: uploading a receipt also creates the voucher that goes with it, so it leaves a record behind even though its name only mentions the file.

Downloads are written into the download directory on the machine the server runs on, and reported two ways: a path, which is what you want when the client and the server share that machine, and a resource URI, which the client can read to get the bytes wherever the server is. The file itself never travels inside the tool result, because base64 costs roughly 1.37 times the file size in context and no model can read a PDF anyway. An existing file is never replaced: a second download is saved beside the first with a counter in its name.

The resource list is filled from the download directory when the server starts, so a URI stays readable after a restart. What the server cannot do is announce a new download: the MCP SDK gives it no way to send a list-changed notification, so a client that lists once at startup will not see anything fetched later in the session.

Between that and Claude Desktop not following resource links at all, read_download is the route that always works. It takes the same URI and puts the content into the answer. What arrives depends on the file:

A PDF is rendered rather than passed through because Claude Desktop turns an embedded binary into an image block when it calls the API, and application/pdf is not a permitted image type there, so the whole request is refused. Rendering costs no API call either, since the file is already on the server.

A link into the web app is a separate tool. get_deeplink turns an id into a URL for a browser, costs no API call, and is the route that still works when the client can display neither the file nor a resource link: somebody opens it themselves. A download does not carry one — it answers where the bytes are, which is a different question, and the two were joined once long enough for a broken link to ride along with a working download.

upload_file accepts PDF, JPEG, PNG and XML, at most 5 MiB per file, which is what the API takes. An XML file is treated as an XRechnung and is rejected if it is not one.

Requirements

• Python 3.11 or newer. Installing pulls in the MCP SDK, httpx, platformdirs and pypdfium2, the last of these to render PDF pages
• A Lexware Office account with the public API add-on enabled
• An API key from https://app.lexware.de/addons/public-api

Getting an API key

1. Sign in to Lexware Office as the account owner.
2. Open the public API add-on at https://app.lexware.de/addons/public-api.
3. Create a key and copy it once — it is shown a single time.
4. Keep it out of any file that goes into version control. Put it in config/.env, which is gitignored, or pass it as an environment variable. A key in config/.env is found no matter which directory the server is started from, so a client such as Claude Desktop needs no key of its own in its configuration file.

A key can be revoked on the same page at any time, which is the fastest way to cut access if anything looks wrong.

Installation

With uv:

uv tool install benethos-lexware-office-mcp

Then configure it in a browser:

benethos-lexware-office-mcp setup

That opens the interface described under Configuring it in a browser: key, settings and one checkbox per tool. Everything it does can also be done by hand — copy benethos-lexware-office-mcp --settings-sample > config/.env, put the key in it, and use --tools as described below.

Check that it works:

benethos-lexware-office-mcp --help

Then point Claude Desktop at it in claude_desktop_config.json:

{
  "mcpServers": {
    "lexware-office": {
      "command": "uvx",
      "args": ["benethos-lexware-office-mcp"]
    }
  }
}

No path from your machine appears in there, which is the point: uvx looks the package up by name.

From the sources instead, to develop or to run something unreleased:

git clone https://github.com/benethos-hub/lexware-office-mcp
cd lexware-office-mcp
uv sync
uv run benethos-lexware-office-mcp setup

A client then needs the interpreter of that checkout's virtual environment, command pointing at .venv/Scripts/python.exe on Windows or .venv/bin/python elsewhere, with args of ["-m", "benethos_lexware_office_mcp"].

No key in there, on purpose. The server finds it in the .env. A client's configuration file is the wrong place for a credential: it is not yours — another program owns it, decides where it lives and when it rewrites it. It is the file people screenshot when they ask for help with an MCP setup, it is readable in the client's own settings view, and it travels to the next machine with the rest of that client's configuration. The .env is at least a file this project documents, that nothing syncs on your behalf, and that the configuration interface writes without ever showing the key back to you.

That .env is already the part worth being careful with. It holds a credential for a live accounting system, so keep it out of version control, out of shared folders and out of backups other people can read. When you stop using the server, delete it and revoke the key under Extensions, Public API — revoking is the only step that actually ends access.

Restart Claude Desktop fully — quit it from the tray rather than closing the window — so the tool list is reloaded.

Configuring it in a browser

benethos-lexware-office-mcp setup

Three pages on 127.0.0.1, closed with Ctrl+C. They write the same files the command line does, so you can use either or both. The screens are in German, because Lexware Office is sold for German companies only, and each is named below by what it does with its label in brackets.

Overview (Übersicht) — which .env and which tools.json are actually in effect, what every setting resolves to and where that value came from, whether each file exists yet, how many tools are on and what they cost. A connection test on the button, never on page load.

Credentials (Zugangsdaten) — the API key, checked against the API before it is saved unless you say otherwise, and the settings that are not secret. The key is never shown back to you, never logged and never exported. If an environment variable is setting it, the page says so, because that would override whatever you save.

Permissions (Rechte) — one checkbox per tool, grouped, with the presets as buttons. On a fresh installation with no policy file yet, the reading tools come pre-ticked as a starting point — a proposal in a form, not a permission: there is still no file and therefore still no tool until you press save, and the page says so. Each row carries what that tool costs the assistant in context, and the total follows your ticks: every enabled tool is sent to the model on every request, so switching one on is a budget decision as well as a permission one. Writing tools are marked, and the ones whose result the API cannot take back are marked separately: nur App for a contact, which Lexware Office deletes without ceremony, and nur App · Buchhaltung for a record that enters the books. Neither means it is stuck — nothing is festgeschrieben when it is created, and a legend on the page names the four things that do bind a record later.

Profiles live here too. Save the current selection under a name, load it later. Loading only fills the boxes: nothing reaches tools.json until you press save. A name that is already taken is refused rather than quietly replacing what is there — case and spacing do not make a second profile — and replacing one is its own button beside the list. They are stored in tool_profiles.json beside the policy file.

The policy file itself can be downloaded and read back in from the same page — the file as it is, so it works on another installation with or without this interface, and a tools.json written by --tools reads here. Reading one only ticks the boxes, and saving is still a separate press. A tool the file does not mention stays off and the page says how many those are, which is what --tools sync does on the command line.

Two things worth knowing. It binds 127.0.0.1 and nothing else — the pages have no password, which is only defensible while they cannot be reached from another machine, so there is no option to change it. And it is a separate command: the MCP server never serves HTTP, and a client such as Claude Desktop starts that one, not this.

--port N moves it, --no-browser only prints the address, and --env-file and --tools-file say which files it edits. Unlike everywhere else those files do not have to exist yet.

If your client starts the server with --tools-file, give setup the same argument — otherwise it edits a different file and reports success. Both processes fix their files when they start and never change them afterwards, and neither can see how the other was started. The overview prints the "args" line that makes your client match the files the interface is holding, which is the easier direction.

Switching individual tools off

One JSON file decides what this server offers, and nothing else does. Either tick the boxes under setup above, or start the file with

benethos-lexware-office-mcp --tools read-only

which writes every tool into tools.json, reading ones on and the rest off, and prints what it did. Three presets, each containing the last:

--tools show only reports. --tools-file PATH says where to write, and works with all of them — --tools write --tools-file ./tools.json creates the file there.

A preset overwrites the whole file, so hand edits are lost. Use one to start a file, not to update one. After an upgrade brings new tools, run --tools sync: it writes them in as off, leaves every flag you set alone, and never switches anything on. That last part is why it is the only one of these safe to run from a script.

The third step is its own because it is its own decision: what is deleted is gone, so it should be chosen by naming it rather than by picking the largest option. Exactly one tool carries such an effect, delete_article, and that is not a temporary state of affairs — an article is the only thing this API can delete, and there is no way to book, finalize or void anything after the fact either.

Without --tools-file, the file is searched exactly like the .env, lowest precedence first:

1. the per-user configuration directory
2. config/ of a checkout, when you are running from the sources
3. config/ and then the root of the working directory

The last one found wins, and a file nobody has created yet resolves to the first. After that, edit it:

{
 "create_contact": false,
 "search_contacts": true,
 "upload_file": false
}

A tool set to false is not listed and cannot be called. A tool the file does not mention is also off — silence is a refusal, so a tool that arrives with an upgrade waits for you rather than appearing on its own. No file at all means no tools at all, which is why --tools is part of setting the server up.

The file is read as the tool list is built and again on every call, so an edit takes effect at once in both directions — no restart. The server also tells the client when the set of enabled tools changes, so it fetches the list again by itself: Claude Desktop picks a change up while it is running. Nothing depends on it either way, since a tool that has been switched off cannot be called whatever list the client is still showing. If yours does not notice, restart it — Claude Desktop by quitting it from the tray.

Each tool also declares what it is — reading or writing, which group it belongs to, and whether what it writes can be removed again. That classification is what --tools read-only selects on and what the browser interface groups and marks by. It never decides a call: only the file does.

Configuration

Settings come from a .env file, found the same way the policy file is, or from real environment variables, which win over it. --env-file PATH names one instead of searching, and pairs with --tools-file so that one entry in a client's configuration carries its own account and its own permissions:

"args": ["--env-file", "/path/to/test.env",
         "--tools-file", "/path/to/test-tools.json"]

A path that does not exist is refused rather than quietly falling back to the search — except under setup, which exists partly to create one.

setup writes this file for you and shows which of the sources below each value is coming from, which is the quickest way to answer "why is this setting being ignored".

Every setting above is in use. LXO_MCP_PAGE_SIZE is capped at 250, which is the lowest page size any endpoint accepts, and a larger value is refused at startup rather than turning into an API error later.

Transport

The first releases speak stdio only, which is what Claude Desktop and comparable local clients use. An HTTP transport is planned for 0.2.0 and will ship with its own bearer authentication in front of the API key, because anyone who can reach an unprotected port could otherwise spend your Lexware credentials. Until that authentication exists, HTTP stays unavailable rather than insecure.

Example prompts

Once the server is connected, prompts like these are the intended use:

• "Which invoices are still open, and which of them are overdue?"
• "Show me everything we billed customer Muster GmbH this quarter."
• "What does invoice RE-2024-0142 contain, and has it been paid?"
• "Find the article with number A-1007 and tell me its current price."
• "Download the PDF of the last credit note we issued."
• "Give me a link to open voucher X in Lexware Office."

Rate limits

The Lexware API allows two requests per second, enforced with a token bucket. That budget is global — it covers all endpoints of the API at the same time, so reading a contact and reading an invoice draw from the same allowance.

The server mirrors this with a single token bucket shared by every request in the process, refilling slightly below the documented rate by default. Lexware notes that enforcing the limit exactly, without a buffer, tends to produce 429s anyway once network jitter shifts the arrival times, so the default leaves headroom. Requests are serialized through that bucket rather than fired in parallel, which means a broad question touching many documents gets slower instead of blocked.

Two things worth knowing:

• The budget belongs to your account, not to this process. A second instance of the server, another integration, or a script you run yourself all spend from the same two per second.
• Lexware warns that a client which keeps hammering after a 429 can stay blocked permanently. The server therefore backs off exponentially and gives up after a few attempts rather than retrying harder.

The account's bucket was measured on 2026-08-21 and holds four: five requests fired at once got four through and one refused. The default of 2 leaves half of that for everything else drawing on the same account — the web app, another integration, a second instance of this server. Raise it to 4 only if you know this server is the only consumer.

Both limiter values are configurable via LXO_MCP_RATE and LXO_MCP_BURST if your account behaves differently.

Development

uv sync --extra dev
uv run pytest -q
uv run ruff check .
uv run ruff format --check .
uv run mypy

The test suite is fully offline. It mocks the HTTP layer and needs no API key, so it runs anywhere. Two kinds of test leave the process without leaving the machine: three start the server as a real subprocess and speak MCP to it over stdio, which is also what proves that nothing writes to stdout on the startup path, and the configuration interface is driven through a real loopback HTTP server with a real cookie jar, because its CSRF guards are only worth testing the way a browser meets them.

No API key ships with this repository and none belongs in CI, so a checkout can never talk to Lexware on its own. Checking the server against the real API is therefore always a deliberate local run with a key you supply, separate from the suite above and never part of it:

uv run python tests/smoke.py
uv run python tests/smoke.py --env-file path/to/.env

It reads your account and writes nothing to it. The server it builds gets the read-only preset, so the writing tools are not there to be called at all. It prints what it checked, what the account had nothing for, and what failed, and it masks record ids so the report can be pasted somewhere. pytest never runs it. See SPECS.md section 14.1 for why a live check is not a gate.

Contributions and issues are welcome once the first release is out. Until then, SPECS.md is the place where design decisions are recorded, including the open questions still to be resolved against the live API.

License

MIT. See LICENSE.

Trademarks and affiliation

This project is not affiliated with, endorsed by, or sponsored by Lexware, Haufe-Lexware GmbH & Co. KG, or any of their subsidiaries. "Lexware" and "Lexware Office" are trademarks of their respective owners and are used here only to name the API this software integrates with, in a descriptive sense.

The software talks exclusively to the documented public API using credentials that the account owner supplies and can revoke. Use of that API is governed by Lexware's own terms, which you accept independently of this project.