Solution 3 · free
One shortcode puts the official KoSIT validator on your page. A visitor drops an e-invoice XML onto it, the plugin runs it through exactly the Schematron and XSD scenarios the XRechnung ecosystem itself relies on, and shows the verdict with every finding. Nothing leaves your server: no third-party validation API, no account, no key.
Real output, not a mockup
A rejected report: four errors, each with its rule identifier, the rule text in plain language and the exact location in the document. Separated into errors, warnings and notices.
Not a reimplementation
The plugin runs the Java application the German XRechnung project ships itself — and the one PEPPOL and Factur-X publish their own scenario configurations for. Not a rebuilt approximation of the rules.
XRechnung 3.0.2, ZUGFeRD / Factur-X EN16931 including EXTENDED, and PEPPOL BIS Billing 3.0. Every configured scenario is tried in turn, so one upload is checked against every format it could plausibly be. A format that is not installed is skipped rather than treated as an error.
The KoSIT validator also exits with an error code when it correctly finds a document invalid — which, for a faulty invoice, is the expected outcome. So the verdict never hangs on the exit code alone, but on whether a report was produced at all.
What comes out
The raw KoSIT report XML is always offered as a download — for anyone who needs the authoritative artefact itself, to file or to pass on.
With the Validation Explainer active alongside, the same report appears in plain language instead: every finding labelled with its EN 16931 business term, inside a nested Shadow DOM so its typography never leaks into the surrounding page. If the Explainer cannot make sense of a report, the raw KoSIT HTML is shown rather than nothing.
Accepted: the validation steps listed one by one – XML schema, Schematron rules, val.xml.
The document content alongside – the offending line is marked right in the tree.
A process that takes real seconds
Unlike the Viewer and the Explainer, this plugin cannot avoid a temporary file: the validator command line wants a file path, not data on standard input. The compensating control is a fresh, unpredictably named directory per request — outside the uploads folder, deleted together with the report in a finally block that runs whether the check succeeded, failed or threw.
A hanging validator cannot wedge a worker: the process output is drained non-blocking in the same loop that watches the timeout, not only once the process is done. The process itself is invoked in array mode rather than as a composed shell string, which avoids Windows path-escaping pitfalls entirely.
The Validator delivers the verdict, the Validation Explainer turns it into sentences you can read without knowing Schematron. Together that is two plugins and one upload.