Free tool
Will KSeF accept this invoice?
Your own system, OMS or SaaS now has to send invoices to KSeF. Drop an FA(3) XML here: it is checked against the official schema in your browser, every error is explained in plain words, and you see the invoice the way a person would. The file never leaves your computer.
Who has to use KSeF and when
According to the Ministry of Finance, issuing invoices in KSeF is mandatory from 1 February 2026 for companies with sales above PLN 200 million (including VAT) in 2024, and from 1 April 2026 for everyone else. Until 31 December 2026 invoices can still be issued outside KSeF if their monthly sales including VAT stay within PLN 10,000.
What this tool checks
The invoice is validated against the official FA(3) schema published by the Ministry of Finance in ksef-docs, using libxml2 compiled to WebAssembly. On top of the schema it checks NIP checksums and whether the total matches the net and VAT fields, which the schema does not do. The production environment accepts FA(3) (environments).
What it does not do
It does not send anything to KSeF, does not log in and does not give a KSeF number or an UPO. Passing the schema does not mean the tax content is right; that stays with your accountant.
Questions
Is my invoice uploaded anywhere?
No. The schema, the validator and the preview all run in your browser. Analytics records only whether the invoice passed and how many errors it had.
Which schema version is used?
FA(3), file schemat_FA(3)_v1-0E.xsd from the Ministry of Finance documentation, with its base schemas.
My system is not ready for KSeF. What now?
Integration usually means: authentication, an online session, sending FA(3) invoices, collecting UPOs, handling corrections and offline mode, tested on the KSeF test environment first. The configurator shows the price and the start date.
Can I validate FA(2) here?
No. Production accepts FA(3), so this tool checks FA(3).
Sources
- Ministerstwo Finansów, KSeF 2.0: od kiedy trzeba wystawiać faktury w KSeF
- Ministerstwo Finansów, ksef-docs: FA(3) schema (schemat_FA(3)_v1-0E.xsd), MIT license
- Ministerstwo Finansów, ksef-docs: KSeF API 2.0 environments (TEST, DEMO, PROD) and accepted formats
- CIRF, ksef-pdf-generator: sample FA(3) invoice used as the example, MIT license
- xmllint-wasm: libxml2 compiled to WebAssembly, MIT license
Need KSeF in your own system?
I add sending and receiving invoices through KSeF to your system, OMS, CRM or SaaS, test it on the KSeF test environment, and keep it working when the schema or the API changes.
