KSeF: What It Is and Who It Affects

KSeF stands for Krajowy System e-Faktur — the Polish state system for electronic invoicing. From 2026 it stops being voluntary and becomes the mandatory route that most business invoices in Poland pass through. If you invoice into Poland or are VAT-registered there, this may affect you. This article explains what KSeF is, who it affects, and which deadlines apply.

What KSeF Is

KSeF is a central government platform run by the Polish tax administration. The principle is simple: an invoice is not issued as a PDF and emailed, but sent in a structured format (the FA(3) schema) directly to the state system. KSeF accepts the invoice, assigns it a unique number, and returns a confirmation of receipt — UPO (Urzędowe Poświadczenie Odbioru). Only the UPO is proof that the invoice is valid.

From a company's point of view, three things change:

• Format. The invoice is no longer a free-form PDF document, but structured XML following a fixed schema.
• Channel. The invoice travels through the state system, not directly between supplier and customer.
• Confirmation. Without UPO, an invoice has no legal effect. It is not an optional step.

In return, the state gets a real-time view of transactions. The company pays for that by having to connect its invoicing to KSeF — either through its accounting software or through its own integration.

Who KSeF Affects

The obligation is not introduced for everyone at once. Poland phased it in by payer size and distinguishes two things: issuing invoices and receiving them.

Issuing invoices through KSeF:

• From 1 February 2026 it is mandatory for large payers — companies with 2024 turnover above 200 million PLN. That is roughly 4,200 entities.
• From 1 April 2026 it is mandatory for all other VAT payers. With one exception: the smallest companies (micro-enterprises) and payers whose monthly volume of invoices outside KSeF does not exceed 10,000 PLN have a deferral.
• From 1 January 2027 these micro-enterprises and payers under the limit join as well. If someone exceeds the 10,000 PLN monthly limit during 2026, the obligation applies from the invoice that breaks the limit.

Receiving invoices through KSeF:

• From 1 February 2026 all VAT payers must be able to receive invoices through KSeF — even those who do not yet issue them. When a supplier sends you an invoice through KSeF, you have to retrieve it from there.

So the shorthand "everyone else from April" is not accurate. Micro-companies and the group under 10,000 PLN per month actually join only in 2027.

What Is Outside KSeF

Not everything goes through KSeF. The following stay outside the system:

• Invoices to end customers (B2C). Toward consumers, the structured-invoice obligation does not apply in this sense.
• Purchase invoices from foreign suppliers. If a company established outside Poland invoices you, that invoice is not reported to KSeF. A foreign supplier has no obligation to use the Polish structured e-invoice.

A special case is invoicing a foreign customer. If a Polish payer issues an invoice to a foreign customer, the invoice is still issued in KSeF — it is an invoice within the meaning of Polish rules. But the foreign customer has no access to KSeF, so you hand them the invoice outside the system, typically as a PDF visualization or a shared link.

What It Means for a Czech Company Invoicing into Poland

What matters is whether you are VAT-registered in Poland and invoice under the Polish regime.

• If you do not have Polish VAT registration and only sell into Poland as a Czech payer, your foreign-supplier invoice does not have to be reported to KSeF. Your Polish customer processes it normally, outside the system.
• If you do have Polish VAT registration — typically when you have a warehouse, branch, or local sales in Poland — and issue invoices under Polish VAT law, the KSeF obligation applies to you just as it does to Polish companies. Then you face the same deadlines and the same technical side: structured XML, logging in to KSeF, UPO.

The second thing everyone selling into Poland runs into is receiving. From February 2026, Polish suppliers can send you invoices through KSeF. To retrieve them, you need access and permissions in the system.

How You Log In to KSeF

For completeness — KSeF has several login methods, and they matter for a company. A short overview:

• A qualified electronic seal identifies the organisation, not a specific person. If the seal contains the company's VAT number (NIP), the company gets access on its own, without filing a ZAW-FA notification. For a company this is the most direct route.
• A qualified electronic signature belongs to a natural person and has the force of a handwritten signature.
• ZAW-FA. A company that does not use a seal designates, via a ZAW-FA notification, a natural person (a director, an accountant, an employee) to act for it in KSeF. That person can then assign permissions to other people or to accounting software inside the system.
• Token and KSeF certificate are used for machine login from applications and integrations. The deadline here matters: the token and the certificate coexist from 1 February 2026, the token works until 31 December 2026, but from 1 January 2027 KSeF accepts only the certificate. An integration set up only on a token will not connect after New Year 2027. For a long-term connection, the switch to the KSeF certificate needs to be planned ahead of time.

When This Turns into a Technical Problem

2026 is a grace period — the obligation is in force, but without penalties. That is exactly why it is easy to underestimate. A company connects its invoicing "somehow", it looks like it works, and the matter is postponed. From 1 January 2027, hard enforcement arrives and token support drops at the same time. Two things at once: you must have an integration that reliably delivers every invoice, and you must be switched to the certificate.

And KSeF is not an ordinary API. It is asynchronous, has call-count limits, is occasionally unavailable, and holds its own state and session. An integration that just sends an invoice in an HTTP request and hopes it worked will lose the first invoice the moment the system responds with an error or a timeout. And no one finds out — until an audit. I wrote about how to build a connection that does not lose a single invoice in the article Robust KSeF Integration.

We Can Help with the Connection

This is not theory. Our KSeF integration runs in production: over 40,000 documents sent with 100% delivery, and during a forensic recovery we tracked down 15,141 invoices that a previous fire-and-forget pipeline had silently lost. We build the connection as a durable workflow with retry, reconciliation, and monitoring — and with the switch to the certificate before the 2027 boundary.

If you invoice into Poland and are working out whether and how KSeF affects you, get in touch. We will go through your situation and tell you what you specifically need.

FAQ

What is KSeF?

KSeF (Krajowy System e-Faktur) is the Polish state system for electronic invoicing. An invoice is sent in the structured FA(3) format to the government platform, which assigns it a number and returns a confirmation of receipt (UPO). That is what gives the invoice legal effect.

When does KSeF become mandatory?

Issuing invoices through KSeF is mandatory from 1 February 2026 for large VAT payers (2024 turnover above 200 million PLN) and from 1 April 2026 for all other VAT payers. The smallest companies and payers under the 10,000 PLN monthly limit join from 1 January 2027. Receiving invoices through KSeF is mandatory for all VAT payers from 1 February 2026.

Does KSeF affect a Czech company that invoices into Poland?

It depends on whether the company is VAT-registered in Poland. KSeF applies to invoices issued under Polish VAT law by entities established in Poland or registered for Polish VAT. Invoices from a foreign supplier without Polish registration are not reported to KSeF. But if a Czech company holds a Polish VAT number and invoices under the Polish regime, the obligation applies.