One Month with Mandatory KSeF — What We Know
Since April 1, 2026, all active VAT taxpayers in Poland must issue invoices through the National e-Invoice System (KSeF). After the first month, clear patterns have emerged — both in common problems and best practices that help businesses transition smoothly.
Most Common Issues
1. Delays in KSeF Number Assignment
During the first days of April, the system experienced delays of up to several minutes in assigning KSeF numbers, especially during peak hours (10 AM–2 PM). The Ministry of Finance confirmed increased server capacity, and the situation stabilized by the second week. Remember that the invoice issue date is when KSeF accepts it, not when you send it.
2. XML Structure Errors
Many businesses using older accounting software versions encounter FA(2) structure validation errors. Common causes include missing required fields, incorrect NIP formats, and character encoding issues. Solution: Update your accounting software and test on the KSeF test environment first.
3. Authorization and Access Tokens
Companies that didn't prepare in advance struggle with obtaining authorization tokens. The process requires a qualified signature or trusted profile. Tip: Tokens can be generated ahead of time at podatki.gov.pl/ksef.
Best Practices
- Send invoices in the morning — system load is lowest between 6–9 AM
- Keep UPO confirmations — they prove the invoice was issued
- Monitor statuses — no KSeF number means the invoice wasn't issued
- Train your staff — even with automated software
Penalties — Lenient for Now
The Ministry announced that until September 30, 2026, minor formal errors won't be penalized. However, not using KSeF at all can result in a fine of up to 100% of the VAT amount on the invoice.
Need help with KSeF implementation? Contact us.