einvoicekit

Why we built it

Electronic invoices in Europe follow one standard, EN 16931, and EN 16931 alone is over 200 business rules. Each national format adds its own on top: XRechnung in Germany, Factur-X and ZUGFeRD in France and Germany, Peppol BIS across many countries. In France, since 1 September 2026, every VAT-registered business must be able to receive electronic invoices, and large and mid-size companies must issue them. Small companies must issue them from 1 September 2027.

Developers who build invoicing into their own software need two things: to know exactly why an invoice fails, and to produce one that passes. einvoicekit does both, by API.

What “valid” means here

Validation runs in two steps, in the order KoSIT’s official validator uses. First the XML schema (XSD): is the document built correctly? Then schematron, the official business rules: do the totals add up, are the mandatory fields there, does the VAT breakdown match the lines? A document that fails the schema stops at the first step.

We do not rewrite those rules. einvoicekit runs the official rule files: EN 16931, the KoSIT configuration for XRechnung 3.0.2, the Factur-X 1.09 rules and Peppol BIS 3.0. Every business rule an invoice breaks comes back with its identifier, such as BR-CO-15, so a developer can look it up and fix it. For French invoices, the BR-FR rules published for the French mandate can be added with one option.

Generating a valid invoice

POST /v1/generate takes the invoice as JSON and returns a Factur-X: a PDF with the XML invoice inside. einvoicekit computes the totals itself, then puts the result through the same validation, plus a PDF/A-3 check with veraPDF, before it is returned. An invoice that fails is never handed back as if it were fine.

The PDF/A-3 check applies to the PDFs einvoicekit generates. For uploaded files, the validator checks the invoice data, not the PDF/A conformance of the file.

Free tools, and where your file goes

The web validator and generator are free and need no account, with a daily limit. The validator sends your invoice to our validation service, where it is processed in memory and not stored. It does not run in your browser, and einvoicekit’s site says so.

The client packages are open source under the MIT licence: @einvoicekit/einvoicekit on npm and einvoicekit on PyPI, with their code on GitHub. The website, the API and the validation service are not open source.

How it runs, and how we watch it

The website and the API run on Cloudflare Workers; the validation engine is a Java service on Cloudflare Containers. A monitor running outside Cloudflare, on a Hetzner server, checks the website and the API every 60 seconds and runs a deeper check every hour. Its status page is public: anyone can see the result of every check.

Every change runs the formatter, type checks, linter, the website and API tests, the build and the Java tests; each deployment then sends a real validation request to the live API. The site is available in English, French and German, and has been live since August 2026.

Visit einvoicekit.com