KSeF FA(3) Invoice Lint

It reads a Polish structured invoice XML and reports every line the KSeF gateway will reject, with the value the FA(3) schema wants instead. 18 rules cover the namespace, KodFormularza attributes, WariantFormularza, NIP format, currency code, ISO dates, dot decimal separators, the P_16 to P_18A annotation flags and correction references.

Polish VAT-registered sellers and the ERP or billing developers who generate their KSeF XML, in SAP, Microsoft Dynamics, Comarch or an in-house system. It is aimed at the person who has an invoice file open and needs to know why the gateway said no, not at the accountant who only sees the finished document.

Generative models still emit the FA(2) namespace and the FA(2) kodSystemowy, because most published examples predate the 2025/06/25/13775 template. The KSeF gateway does validate, but it answers after the call, with a schema code rather than a line, and each round trip runs through your integration. The lint answers in the editor.

Free and complete for one file: open an invoice XML, run the lint, get every finding with line numbers, in the editor or on the web page, with no key and no account. The licence key opens a different axis, scope: sweeping every invoice XML in the workspace in one run and writing a dated REPORT.md you own and keep.

A mid-size KSeF integration project in Poland runs PLN 10,000 to PLN 50,000 over 2 to 4 months, and most of that budget is spent making the XML pass validation. From 2027-01-01 a KSeF failure can carry up to 100% of the invoice VAT. The lint is $29 once, for one person or one CI seat.