About Rechnungskern
Rechnungskern is an independent project by Abbas Albukhari, Senior Software Engineer. It creates and validates e-invoices according to EN 16931 – free of charge and without storing any data.
Why this project exists
Germany has been phasing in mandatory electronic invoicing for B2B since 2025. Large companies solve this with their ERP systems; small businesses, freelancers and tradespeople are often left with a PDF that legally is not an e-invoice. The goal of Rechnungskern is simple: enter invoice data, get a compliant ZUGFeRD/Factur-X PDF or XRechnung XML back – in the browser or via API – and be able to check any e-invoice against the same rules that authorities apply.
Design decisions
- No data retention. Invoice data is processed in memory and discarded once the response is delivered. This is not a feature toggle but the architecture: there is no database.
- Compliance is verified, not assumed. Every XML is checked against the official schema; the EN 16931 and XRechnung Schematron rules as well as PDF/A-3 are checked with the Mustang validator, and the test suite contains negative tests that prove the validator catches violations.
- Amounts are calculated server-side with decimal arithmetic and the rounding rules of the standard (BR-CO-*), so line, document and VAT totals are always consistent – including price discounts, line and document level allowances and charges.
- Boring, proven technology. Python, FastAPI, WeasyPrint (HTML to PDF/A-3), drafthorse (CII XML), Caddy for TLS, Docker for deployment. No JavaScript framework on the site; the invoice editor is plain JavaScript that keeps your entries in your own browser.
Who is behind it
Abbas Albukhari – Senior Software Engineer. Rechnungskern is developed and operated privately in Herne. Questions, feedback or an API key: eng.abbas.it@gmail.com.