Writing an invoice with ChatGPT sounds simple: enter sender, customer, service and amount, copy the text and save it as a PDF. For a German e-invoice that is not enough. ChatGPT alone does not maintain a reliable invoice number sequence, finalise an immutable document or automatically produce structured invoice data.
Rechnungstool closes that gap through the Model Context Protocol (MCP). The assistant receives specific invoice tools. It can create a draft, ask follow-up questions, finalise after your approval and return the completed ZUGFeRD PDF as a file in chat.
What ChatGPT can do on its own
A language model can organise unstructured details. From “two consulting days in August at €900 each”, it can draft invoice lines and a clear service description. It can flag obviously missing information. But that is not an executed invoice transaction. Without a connected tool, the chat lacks:
- a persistent or time-limited invoice record,
- a defined transition from editable draft to final invoice,
- a reliably assigned sequential invoice number,
- a structured file under the e-invoice standard,
- a retrievable PDF tied to that exact final record.
A remote MCP server exposes these operations as tools. ChatGPT uses their names, descriptions and parameter schemas to choose when to call them. Rechnungstool also states the limits: it is not general tax advice, complete bookkeeping or a guarantee of legal correctness for every case.
The complete anonymous workflow
No Rechnungstool account is required for the first invoice. After connecting the MCP server, you can finish in chat:
- Describe the job. Share known invoice details and ask the assistant to request missing data before finalisation.
- Create a draft. Rechnungstool creates an invoice in an anonymous workspace; lines remain editable.
- Check details. Review sender, recipient, description, service period, amounts and VAT.
- Approve finalisation. Only then does the draft become a fixed invoice. This cannot be reversed.
- Receive the file. Rechnungstool returns the ZUGFeRD PDF as a file resource in chat.
The anonymous workspace retains invoices and PDFs for 30 days. If you later need history, reusable details and consistent numbering, you can optionally transfer the workspace to a free account.
Why ZUGFeRD rather than a regular PDF?
Under the German VAT definition, a simple PDF has not counted as an e-invoice since 1 January 2025, because it lacks structured data for automatic processing. The German Federal Ministry of Finance lists ZUGFeRD from version 2.0.1, except MINIMUM and BASIC-WL profiles, and XRechnung as common German formats that can meet the VAT requirements.
ZUGFeRD is hybrid: people see a normal PDF view, while accounting software can read embedded XML. The chat result is therefore readable and machine-processable.
Support for a format does not replace review of the individual transaction. The actual details, recipient and applicable rules still matter.
Why finalisation is separate
You can change a draft before finalisation. Afterwards it is fixed. This separation prevents a wording change from silently creating a new final invoice. In a connected account, finalisation takes the next sequential number. Repeated identical finalisation requests are idempotent, so a dropped connection does not automatically create a duplicate.
When an account helps
Anonymous use is suited to one invoice in a conversation. A free account is helpful when you reuse sender profiles and invoice settings, customers and customer numbers, items and tax rates, and when you need persistent invoice history, retention workflows and consistent future numbers. OAuth 2.1 with PKCE connects the account; the assistant does not receive your Rechnungstool password. Sign-in is required only for account features or for transferring an anonymous workspace.
A useful starting prompt
“Create a German e-invoice with Rechnungstool for two consulting days in August 2026 at €900 net each plus 19% VAT. Ask me for all missing details. Show me the full draft and finalise only after I confirm. Then give me the ZUGFeRD PDF.”
This separates data collection, review and the final action, and asks for a file.
Getting started
Until Rechnungstool is published as a confirmed listing in the universal OpenAI plugin directory, add its remote MCP server in ChatGPT Developer mode. The current MCP guide gives the endpoint and instructions for ChatGPT and Claude.
Connect Rechnungstool to ChatGPT
Add the public MCP endpoint once, then start invoice work in natural language.
