KSeF number on an invoice: where to find it and what to store

The KSeF number is assigned only after an invoice has been accepted by the system. The seller does not create it, does not enter it in the invoice draft and should not confuse it with the invoice's own P_2 number.

The KSeF number is not an element of the same FA(3) invoice XML. It can appear in XML as a reference to another invoice, for example in a correction, but the number of the current invoice is created after KSeF verification.

A small business should store the KSeF number together with the invoice number, UPO, status, acceptance date, contractor, amount and data needed for JPK and payments. Without this set, it is easy to confuse an invoice draft with a document accepted by the system.

A service company issues an invoice, sends it to KSeF and then accounting asks for the KSeF number. The owner can see FV/06/2026, XML, PDF and a sending status, but does not know which identifier belongs in JPK, UPO or a payment process.

That confusion is understandable because KSeF uses several identifiers. The seller assigns one number in its own invoice series, a technical session may have another reference and KSeF assigns a separate number after accepting the document. This guide reflects the state of information checked on June 23, 2026 and focuses on the daily process of a small business.

The core rule is simple: record the KSeF number only when the invoice has been accepted by KSeF and the system has returned that number. With that starting point, the rest of the article separates each identifier and its use.

1. What the KSeF number is and when it appears

2. KSeF number, invoice number, UPO and session reference

3. Whether the KSeF number is in XML or on a PDF

4. Where to find the KSeF number after issuing or receiving an invoice

5. What to store for accounting, JPK and payments

6. The process from UPO to payment

7. How KSeFGPT helps organize KSeF numbers

8. Frequently asked questions

The table below summarizes the decisions a company should make before copying a KSeF number into accounting, JPK or a payment title.

The KSeF number is a unique invoice identifier in Poland's National e-Invoicing System. It appears after positive verification and processing of XML, when the system accepts the document. The KSeF 2.0 manual describes this point as successful processing, assignment of the KSeF number and availability of UPO.

In practice, an invoice prepared in invoicing software does not yet have a KSeF number. It can already have the seller's invoice number, buyer data, line items and payment term, but the system identifier appears only after KSeF acceptance.

The technical format should be described carefully. The current KSeF 2.0 OpenAPI indicates that `KsefNumber` accepts 35 or 36 characters, while KSeF 2.0 generates numbers only in the 35-character format. The documentation example is `5555555555-20250828-010080615740-E4`.

Operational check: verify acceptance status, save or download UPO, copy the KSeF number from the system, connect it with the P_2 invoice number and never create the number manually from a pattern. Once the creation moment is clear, the next step is to separate it from other identifiers.

The most common mistake is treating a number such as FV/06/2026 as the KSeF number. These are two different identifiers. The invoice number in field P_2 is assigned by the taxpayer. The KSeF number is assigned by the system after acceptance.

UPO is not the KSeF number either. It is a separate XML receipt confirming that KSeF accepted the invoice and containing data that links the invoice to the submission. A session or batch reference helps trace the technical operation, but it does not identify the invoice like a KSeF number.

If accounting asks about sales invoice FV/12/06/2026, the company should be able to provide both P_2, the KSeF number and UPO. One identifier finds the document in the company's numbering, the other in KSeF, while UPO confirms acceptance. Once the identifiers are separated, the next question is where the number appears.

The KSeF number assigned to a given e-invoice is not an element of that invoice. The FA(3) structure does not contain a field where the seller enters the KSeF number of the same document before submission. The number is created only after system acceptance.

In XML, a KSeF number can appear as a reference to another invoice, for example in a correction or settlement invoice. This does not change the rule: the number of the invoice currently being submitted is not assigned by the taxpayer inside XML.

PDF needs even more caution. A visualization may show process information, but it should not be the only source of truth for acceptance. If you only have a PDF from a contractor, check the invoice in KSeF, UPO or metadata. For the difference between human preview and structured data, see how to read KSeF XML.

The safest places are UPO, invoice metadata in KSeF and the application or integration used to send or receive invoices. KSeF 2.0 OpenAPI shows the number in invoice retrieval by number, metadata queries, session and invoice statuses and UPO retrieval for an invoice.

For API exports, the documentation also gives a practical rule: invoices are added to a package as files named `{ksefNumber}.xml`, with a `_metadata.json` file containing invoice metadata. This matters when documents are passed to accounting or an archive.

In the KSeF 2.0 Taxpayer Application, the KSeF number is also needed for anonymous invoice access without logging in. The procedure requires the KSeF number, the P_2 invoice number, buyer data and the total amount due. This article does not describe exact clicks because the user interface can change.

Operational check: look for the number in UPO, metadata, export, invoice status or the system that handles KSeF. Do not look for it in the draft before submission. Once the number is found, decide what to do with it.

A small business should not store the KSeF number in isolation. The minimum set is P_2 invoice number, KSeF number, UPO, status, acceptance date, contractor, amount, payment term and any collective identifier. This set helps reconstruct the process during accounting, a dispute or a tax review.

JPK_V7M(3) and JPK_V7K(3) include NrKSeF and the OFF, BFK and DI markers for situations without the number. A safe operational rule is: if the invoice has a KSeF number on the JPK filing date, enter it in NrKSeF. For the details, see KSeF and JPK.

For payments made from January 1, 2027, Article 108g of the VAT Act covers an obligation to include the KSeF invoice identifier or a collective identifier for certain payments for structured invoices between active VAT taxpayers, if the payment instrument allows a transfer title. For one invoice, the reference point is the KSeF number; for several invoices, the collective identifier. For payment timing, see KSeF invoice payment term.

Operational check: keep both numbers, attach UPO, verify status, determine the payment term, decide on NrKSeF or the correct JPK marker and do not enter a number in a transfer until it has been confirmed.

Treat the KSeF number as a control point in the process, not as a single field to copy. The issuer or the person receiving the invoice should check status first, then store the KSeF number, and only then pass the document to accounting or payment handling.

Example: a service company issues FV/24/06/2026 but does not send it to accounting immediately after generating a PDF. It waits for acceptance, downloads UPO, stores the KSeF number in the same record and only then marks the document as ready for JPK and receivables.

The biggest risk is manual copying. If invoice number, KSeF number, UPO and status live in different folders, the error usually appears at month-end close. One operational table for incoming and outgoing invoices reduces that risk.

This process does not replace accounting decisions. It only organizes the information that accounting, the business owner and the person making payments need to read in the same way.

KSeFGPT should be treated as a workspace for KSeF invoice data, not as a source of new tax rules. The application can help organize incoming and outgoing invoices, statuses, XML, PDF, UPO and data needed by accounting.

For KSeF numbers, the key value is reducing manual work. The company should see the invoice number, KSeF number, status, UPO, contractor, amount and payment term next to one invoice record instead of combining them from emails, folders and exports.

KSeFGPT is not a Ministry of Finance service and does not replace an accountant. It can help with the operational problem: checking whether a document was accepted, whether UPO exists, whether the KSeF number is attached to the correct invoice and whether accounting received complete data.

To continue without duplicating the whole KSeF process, read these four guides:

KSeF for small businesses - how a small company can organize invoices, UPO, KSeF numbers and accounting cooperation.

UPO in KSeF - what UPO confirms and how it differs from a KSeF number.

KSeF and JPK - how the KSeF number works with NrKSeF and the OFF, BFK and DI markers.

KSeF invoice payment term - when the KSeF number date matters for invoice receipt and payment.