Claude desktop extension for bookkeeping: “Buchhaltung für Claude” (.mcpb)

The simplest way: Claude in four steps

No technical knowledge needed — the technical options follow below.

• 1. Install Claude: Download the Claude app for Mac or Windows from claude.ai/download, install it and sign in.
• 2. Create a key: In your bookkeeping at jahresabschluss.io: “AI agent & API” → “New key”. If Claude should post entries, tick “Post”. Create and copy — the key works like a password for Claude and only for your company.
• 3. Connect your books: Download “Buchhaltung für Claude” and double-click the file in your “Downloads” folder. Claude asks for the key: paste it and click “Install”. Nothing else to install.
• 4. Get started: Ask Claude: “Book the new receipts in the inbox.” Claude asks for permission and then posts to your journal.

Claude asks for permission before it uses your bookkeeping, and asks again before invoices, cancellations or locking a period. Every entry lands in the real journal — with the same checks as in the app. Claude Code and Codex work too, for technical users.

Download “Buchhaltung für Claude”: https://jahresabschluss.io/downloads/buchhaltung-fuer-claude.mcpb

What is inside the file

A .mcpb file is an archive with a manifest (here in format version 0.2). This is the part of ours that tells Claude what to install and asks for your key:

"display_name": "Buchhaltung · jahresabschluss.io",
"server": { "type": "node",
            "entry_point": "server/buchhaltung-mcp.mjs" },
"compatibility": { "platforms": ["darwin", "win32"] },
"user_config": {
  "api_key": {
    "title": "Ihr Schlüssel",
    "description": "Beginnt mit jab_live_ …",
    "sensitive": true,
    "required": true
  }
}

The server part is a small bridge program. It holds neither the bookkeeping logic nor a tool list of its own: it forwards each request from Claude to the hosted jahresabschluss.io server with your key, so Claude always sees the current tools your key allows. The install dialog is in German because the manifest is — “Ihr Schlüssel” simply means “your key”.

Install in four steps

• 1. Install the Claude app for Mac or Windows from claude.ai/download and sign in.
• 2. Create a key at jahresabschluss.io: Bookkeeping › Settings › “AI agent & API” › “New key”. Tick “Post” if Claude should book, choose an expiry date (at most 366 days ahead), confirm with your second factor and copy the key — only a hash is stored, so it is shown just this once.
• 3. Download “Buchhaltung für Claude” (buchhaltung-fuer-claude.mcpb) and double-click it in your Downloads folder. Claude opens its install dialog: paste the key and click “Install”.
• 4. Ask Claude: “Book the new receipts in the inbox.” Claude asks for permission before it uses the extension.

If a double-click does not open Claude, Claude's help center (checked 30 October 2026) describes installing a .mcpb file by hand: Settings › Desktop app, “Manage” next to Extensions, then “Advanced settings” and “Install Extension…”.

Where your key is kept

The manifest marks the key field as sensitive. According to Claude's help center, Claude Desktop encrypts such values with the operating system's secure storage — the Keychain on macOS, the Credential Manager on Windows. When Claude starts the extension, the key reaches the bridge as an environment variable (JAB_API_KEY), not as a command-line argument and never through the chat. If you paste it with a leading “Bearer” or stray spaces, the bridge trims them.

The key works like a password for one company. It stays valid until its expiry date or until you revoke it under “AI agent & API”, whichever comes first.

What runs on your computer, and what does not

• On your computer: the bridge, started by Claude with the Node.js runtime that ships with Claude Desktop, per Claude's help center. Nothing else to install.
• On the server: every check and every posting — balanced entries, VAT keys, locked periods, duplicates. The bridge only passes requests on and keeps no copy of your books.
• The local extra: upload_receipt also accepts file_path, a file on your computer (PDF, PNG, JPEG, WebP or an XML e-invoice, up to 20 MB). The bridge reads the file and sends it on; a file too large for the hosted server's request limit goes straight to the REST interface with the same key.

Example: a folder of receipts

Because the extension can read local files, you can point Claude at a folder; file names and results are an example:

You:     Upload the PDFs in Downloads/receipts-october and tell me
         which ones were booked automatically.

Claude → upload_receipt   file_path …/receipts-october/hosting.pdf
       → upload_receipt   file_path …/receipts-october/train.pdf
       → upload_receipt   file_path …/receipts-october/hotel.pdf
       → list_receipts    received from today

Claude:  The Autopilot booked hosting.pdf and train.pdf. hotel.pdf
         is waiting for review: the proposal flags a possible
         private share. Shall I show you the proposal?

Sending the same file twice returns the existing receipt rather than a second one, so a repeated upload does no harm. Receipts read by the recognition count towards your quota of AI-read receipts, just as in the app.

Update, change the key, remove

• Updates: Claude's help center notes that .mcpb files you install yourself are not updated automatically. When a new version is published, download it and install it the same way.
• New key: when your key expires or you want different rights, create a new key and enter it in the extension's settings in Claude, or reinstall the extension with it.
• Remove: uninstall the extension in Claude's extension settings — and revoke the key under “AI agent & API”, because removing the extension does not end the key.

Extension or browser connector?

Limits

• Mac and Windows only; the manifest names no other platform.
• The extension holds one key, so it reaches one company.
• No filing with ELSTER: Claude can check and prepare a VAT return, and you file it in the app with your PIN.
• Claude asks before using the extension; our server additionally requires your explicit yes before issuing or emailing invoices, cancelling invoices, deleting receipts and locking periods.
• What Claude reads through the extension is processed by the provider of Claude under your account's terms.

Frequently asked questions

Do I need to install Node.js for the extension?

No. Claude Desktop ships its own Node.js runtime and starts the extension with it.

Where is my key stored?

In your operating system's secure storage — Keychain on macOS, Credential Manager on Windows — because the manifest marks it as sensitive. It is not written into a configuration file and never appears in the chat.

Does the extension update itself?

No. A .mcpb file you install yourself has to be installed again when a new version comes out.

Can I use the extension for two companies?

One installation holds one key, and each key belongs to one company. For several companies side by side, set up Claude Code with one entry per key; the browser connector also covers one company per connection.

Does it work on Linux or on my phone?

No. The extension is for the Claude app on Mac and Windows. On the phone and in the browser, use the connector with https://jahresabschluss.io/mcp.

What happens when my key expires?

Claude's calls to your books are refused. Create a new key under “AI agent & API” and enter it in the extension's settings.