How we check e-invoices
Every e-invoice file the Clockaroo app makes is checked by a validator before the invoice is issued. If the check fails, the invoice isn't issued and its number stays free. This page names the validators and their versions, and what our tests cover.
This page describes technical checks. It isn't legal or tax advice.
Which validator checks which format
| File | Checked with | What it checks |
|---|---|---|
| XRechnung (CII and UBL) | KoSIT validator 1.6.3 with the configuration for XRechnung 3.0.2, released August 31, 2026 | The XML schema, the rules of EN 16931 and the XRechnung rules |
| ZUGFeRD (profile EN 16931) | Mustang 2.26.0 with veraPDF 1.30.2 | veraPDF checks the PDF/A-3 file; Mustang checks the embedded XML against the rules of EN 16931 |
| Peppol BIS Billing 3.0 (UBL), for sellers in the Netherlands | KoSIT validator 1.6.3 with the OpenPeppol rules of BIS 3.0.21 | The UBL schema, the rules of EN 16931 and the Peppol rules |
The KoSIT validator is published by the Koordinierungsstelle für IT-Standards (KoSIT), with the official configuration for XRechnung. Mustang and veraPDF are open source. A plain PDF invoice isn't an e-invoice, so no validator checks it.
The versions in this table are read from the file our server uses to download the validators, where each version is pinned with a checksum. When we update a validator, this page changes with it.
When a file is checked
- When you issue an invoice or a credit note, the app makes its file and the validator checks it. The invoice is issued only if the file passes.
- If the check finds an error, nothing is issued and no number is used. The app tells you what to fix.
- If a validator can't be reached, the invoice isn't issued either, and you can try again.
- An issued file never changes afterwards, so it stays the file that passed the check.
- Our free e-invoice creator checks every file the same way before you can download it.
What our tests cover
Besides the check at issue, a test matrix makes invoices the way the app does, for sellers and clients in different countries, and runs every file through the same validators. Each case is checked as an invoice and as its credit note.
| Seller in | VAT cases | E-invoice files checked |
|---|---|---|
| Germany | Standard rate (S), Zero-rated (Z), Exempt (E), Reverse charge (AE), Not subject to VAT (O) | ZUGFeRD, XRechnung CII and XRechnung UBL |
| Austria | Standard rate (S), Exempt (E), Reverse charge (AE), Not subject to VAT (O) | ZUGFeRD, XRechnung CII and XRechnung UBL |
| Netherlands | Standard rate (S), Zero-rated (Z), Exempt (E), Reverse charge (AE), Not subject to VAT (O) | Peppol BIS |
| United Kingdom | Standard rate (S), Not subject to VAT (O) | ZUGFeRD, XRechnung CII and XRechnung UBL |
| Ireland | Standard rate (S), Reverse charge (AE), Not subject to VAT (O) | ZUGFeRD, XRechnung CII and XRechnung UBL |
| United States | Not subject to VAT (O) | ZUGFeRD, XRechnung CII and XRechnung UBL |
Small businesses in the matrix: § 19 UStG in Germany, the Kleinunternehmerregelung in Austria and the KOR in the Netherlands. KOR invoices are plain PDFs and have no e-invoice file. For every case the matrix also checks that the PDF copy renders.
The matrix runs on every full test run, together with the rest of the app's tests. On our test server a missing validator counts as a failure, so a full run can't pass without checking the files.
Last full test run passed:
What a passed check means
A passed check means the file meets the technical rules these validators test. It doesn't cover:
- the content of the invoice. Amounts, VAT treatment and the service described are still yours to check.
- a certification. Clockaroo holds no certificate for e-invoices.
- legal or tax advice.
- sending over the Peppol network. The app makes the Peppol BIS file and checks it; the file goes with your email and you can download it.
Check a file yourself
- Check an e-invoice file with our free validator: XRechnung with the same KoSIT validator, ZUGFeRD and other EN 16931 files with the same Mustang version. No account needed.
- Create an XRechnung or ZUGFeRD file with our free creator. It checks the file before you download it.
The tools
- KoSIT validator 1.6.3 (GitHub, KoSIT)
- Configuration for XRechnung 3.0.2, release August 31, 2026 (GitHub, KoSIT)
- Mustang 2.26.0 (GitHub, Mustang project)
- veraPDF (the version inside Mustang: 1.30.2)
- Peppol BIS Billing 3.0 (OpenPeppol; rules of BIS 3.0.21)