The First Month of KSeF Operation: Pros and Cons “Phantom invoices” reveal problems with integrating ERP systems with KSeF

Despite initial issues with system performance and the smooth integration of KSeF with taxpayers’ financial systems, the first month of KSeF’s operation should be assessed positively.

Above all, no critical irregularities threatening the functioning of the KSeF platform as a whole have been identified. On the other hand, minor operational (technical) issues were gradually eliminated by the Ministry of Finance in real time.

Among the selected problems of a system-wide nature were situations in which invoices that had been assigned a KSeF number were not visible to the invoice recipient. It often happened that the issuer received a UPO message (official receipt confirmation), while the recipient was unable to locate the invoice in the system. In extreme cases, this led to situations where taxpayers had to postpone their right to deduct VAT due to the lack of purchase documentation (i.e., the absence of structured invoices in the VAT purchase register).

According to analysts, this situation may have resulted from problems with integrating KSeF with taxpayers’ financial and accounting systems (ERP – enterprise resource planning). The lack of invoice visibility in KSeF could also have been caused by communication errors occurring already at the login stage (e.g., verification of authorization tokens) or during incorrect XML file encoding. There may also have been improper handling of API (application programming interface) messages or inconsistent behavior between the test and production APIs.

The Ministry of Finance advised that in such cases invoices should be resent to KSeF. If a large number of invoices were rejected, it was recommended to implement XML file validation tools in invoicing software. However, the real issue lay elsewhere. Before the taxpayer realized what caused the error, the original structured invoice had already been posted and had begun its journey through the taxpayer’s ERP system.

The National Revenue Administration (KAS) assures that it is continuously working on every problem that arises. It is therefore worth monitoring the Ministry of Finance’s announcements on an ongoing basis, as they may provide advance notice of upcoming difficulties with KSeF.

Two months of the so-called KSeF phasing create difficulties for micro-businesses

As of 1 February 2026, Polish taxpayers have been divided into three groups:

1. those who must issue invoices via KSeF,
2. those who are not required to do so, and
3. those who may issue invoices through KSeF but are not obliged to.

To make matters worse, from February onward every taxpayer must receive invoices sent to them via KSeF and continuously verify whether a document received from a contractor should indeed be in KSeF.

As a result, in the second month of the KSeF transition phase a taxpayer (e.g., a micro-entrepreneur) may receive, alongside an invoice issued in KSeF, its visualization (just in case), an electronic invoice (because it was allowed), a paper invoice (because there was no alternative), or a simplified invoice (because it is not prohibited). In all of the above cases, the right to deduct VAT will also indirectly depend on the contractor’s correct interpretation of the KSeF regulations. The most difficult situation is faced by small taxpayers who do not run a structured business (with an internal accounting team or an external accounting office), as a dual document flow may simply go unnoticed or the taxpayer may not be familiar with KSeF.

Although the legislator does not provide penalties for errors made within the KSeF system itself, penalties still apply for incorrect tax settlements or improper reporting of JPK structures. A taxpayer exempt from the obligation to issue invoices in KSeF must regularly check their KSeF account to avoid a situation in which, due to double deduction of VAT, they are summoned to provide explanations. It should be remembered that from 1 April 2026 KSeF will become mandatory for another group of taxpayers—those with monthly turnover exceeding PLN 10,000.

Emergency procedures have not yet been widely tested

At the beginning of February, sporadic difficulties in the functioning of KSeF were observed. These included errors such as “401 Unauthorized” or “504 Gateway Timeout,” incorrect XSD validation, or problems with assigning KSeF numbers in real time. Later the situation stabilized, but there has essentially been no opportunity to test all emergency procedures described in the VAT Act on a larger scale.

Nevertheless, it is important to have a contingency plan in case KSeF emergency modes occur. It should be recalled that despite a system failure, taxpayers are required to submit invoices to KSeF no later than:

• the next working day after their issuance (offline24 mode),
• the next working day after the unavailability of KSeF ends (offline – KSeF unavailability mode),
• within 7 working days after the end of the KSeF failure (emergency mode).

For businesses that make intensive use of KSeF, it may be worth considering:

• signing a contract with an alternative Internet provider (as a backup) or obtaining an additional Internet source (e.g., a modem),
• designating a person responsible for monitoring communications from the Ministry of Finance, checking Internet stability, ERP functionality, and updates to invoicing software,
• appointing a person who, in the event of various emergency modes, will take over KSeF operations (remotely or from another device with Internet access).