How to choose KSeF software for your business?

I signed up for five KSeF software tools, issued the same invoice in each, and I'm not going to rank them.

Here's why. I build a competing product — Biurko — so my verdict on "which one is best" would be about as trustworthy as a baker judging a bread contest. But the exercise still paid off. Tools for KSeF (Krajowy System e-Faktur, Poland's national e-invoicing system) don't differ in what's on the pricing page. They differ in how they behave in five situations no sales demo will ever show you.

What follows is a decision framework: nine questions and a two-hour test protocol you can run on your own data. Apply it to any tool, Biurko included. At the end I show where Biurko fails it.

A ranking answers a question you don't have

Nobody wakes up wondering which KSeF tool is objectively best. You wake up wondering whether it will break on a Wednesday at 4 p.m., when the government gateway returns an error and your client is waiting for the invoice.

The calendar doesn't leave much room either. Since 1 February 2026, businesses with turnover above PLN 200 million have had to issue invoices through KSeF; since 1 April 2026, the obligation covers practically every active VAT payer in Poland. Transitional arrangements — including one for sellers invoicing up to PLN 10,000 gross a month — end on 31 December 2026.

On 16 September 2026, the Ministry of Finance announced it is extending the grace period on KSeF penalties until 31 December 2027 (in Polish). Read it carefully. What's deferred is the penalties, not the obligation — the tax administration says it will still act on invoices issued outside the system. And the extension needs a statutory amendment that the ministry has only just started drafting.

In practice: you have more room for mistakes, not more room for implementation. Switching tools in January 2027, while new payment rules kick in, is the worst possible timing. Q4 2026 is the window.

Layer 1: will the invoice arrive, and can you prove it?

This is the only layer where software can do real damage. Start here.

Test 1. Disconnect from the internet and issue an invoice.

Polish VAT law defines three special modes, not one: offline24 (your choice, e.g. your connection is down), offline-unavailability (announced maintenance) and emergency mode (an outage announced by the ministry). Each has its own deadline for submitting the invoice afterwards: the next business day for offline24 and unavailability, seven business days after the outage ends for emergency mode (official overview, in Polish).

What to check: does the tool distinguish between modes? Does the printout carry two QR codes — one that gives access to the invoice and verifies its data, one that confirms the issuer's identity and requires a KSeF certificate? And once the system is back, does the tool resubmit on its own and show you the deadline?

Red flag: a single "offline" button and a note saying "submit manually later". The deadline is yours, not theirs.

Test 2. Ask the tool for proof.

"Sent" on a screen is not proof. Proof is the UPO (the official receipt KSeF issues for every accepted invoice) plus the KSeF number. Check three things: can you download the UPO; can you download the exact XML that went to the gateway (not a file regenerated from the database — those become two different documents the moment you edit a customer's details); and does the KSeF number flow into your records automatically?

Test 3. Click "send" twice.

Send an invoice, close the tab mid-submission, send it again. You should end up with one transmission. A duplicate in KSeF can't be deleted — you're left with a correction invoice and an awkward email to your customer.

Layer 2: will the tool survive January 2027?

Test 4. Ask how the tool authenticates with KSeF.

The realistic options are: a trusted profile or qualified seal, a KSeF certificate, or the authorised-entity model (you grant permissions to the vendor's tax ID and never buy a certificate yourself). Separately, KSeF tokens are scheduled to expire at the end of 2026. If the integration relies on tokens only, ask in writing: "What happens on 1 January 2027?" "We're waiting for the ministry" is an answer too.

Test 5. Pay an invoice.

From 1 January 2027, Article 108g of the Polish VAT Act requires payments between active VAT payers to reference the invoice's KSeF number (or a collective identifier) in the transfer title; in split payment, the KSeF number replaces the invoice number. Check whether the KSeF number shows up where you actually need it — the invoice list, the bank export, the payment reminder — and whether the tool can generate a collective identifier for several invoices from one seller.

Test 6. Issue 200 invoices at once and add an attachment.

Single submission and batch sessions are different paths on the gateway side, with different limits. A good tool shows you those limits instead of discovering them alongside you. Attachments to FA(3) invoices require separate consent from the tax administration (KAS), requested through the online tax office, and the answer can take up to three business days. The tool should warn you before your first attempt, not after the document is rejected.

Layer 3: the purchase side everyone forgets

Test 7. Receive a cost invoice you've already entered.

Enter a supplier invoice manually, then pull the same one from KSeF. A good tool recognises it as the same document. A weak one books the same cost twice.

Same test, second part: JPK_V7, Poland's monthly VAT return file. Every entry needs either a KSeF number or one of the substitute codes. See whether the tool fills them in or leaves you to do it in a spreadsheet.

Layer 4: what happens after the invoice, and how you leave

Test 8. Ask for a full export.

Invoice XML files, UPOs, registers. If all you get is PDFs, that's a souvenir, not an export. KSeF keeps the original invoices for 10 years, but your records and your proof of submission are your responsibility — including on the day you decide to switch tools.

Test 9. Check who still maintains it — and in what language.

KSeF is a moving target: the FA(3) schema, new API versions, ministry announcements, deadlines that shift on a Friday afternoon. Find a public changelog and the date of the last update. No changelog doesn't prove nobody works there — it just means you can't check. If your team doesn't work in Polish, check two more things: is the interface fully available in your language (not just the invoice template), and does support answer in it?

Only then does price make sense — and only together with the question of what the tool does after the invoice: VAT registers, JPK files, tax calculation, social security contributions. Simply issuing and receiving invoices in KSeF is covered today by the ministry's free app and several banks. What you pay for is everything that comes next.

The protocol: two hours, your own data

1. Create a test account and issue one real invoice to a real customer.

2. Go offline and issue a second one — count the QR codes on the printout and check who tracks the resubmission deadline.

3. Click "send" twice in a row and count the transmissions.

4. Download the UPO and the submitted XML. Not "the invoice PDF" — the XML.

5. Enter a cost invoice manually, then pull it from KSeF and check for a duplicate.

6. Ask support in writing how the tool authenticates and what its plan is for 1 January 2027. Time the reply.

7. Request a full data export and see what arrives.

If a tool passes all seven, pricing becomes a conversation about money rather than risk.

Where this framework hurts Biurko

Fair's fair — I built it.

Biurko passes layers one and two: it supports all three special modes with their deadlines, prints both verification codes, submits the frozen bytes of the document and shows their fingerprint next to the UPO, guards against duplicates on resubmission, connects to KSeF in three ways without relying on tokens, displays gateway limits and builds collective identifiers. The interface is fully available in English and Ukrainian.

What Biurko doesn't do: full accounting for limited companies (sp. z o.o.) — for them it covers invoices, KSeF, VAT registers and JPK_V7, not "no accountant needed". It doesn't file social security (ZUS) declarations for you; it prepares the file and the printout. Annual income tax returns arrive in 2027. If your requirements start with statutory bookkeeping, run this protocol elsewhere and save yourself two hours.

Conclusion

A ranking of KSeF software ages as fast as the API changes. A test protocol doesn't. Seven steps on your own data tell you more than any feature comparison, because they check exactly what goes wrong in real life: no connection, a double click, proof of submission, and the way out.

Run the protocol on Biurko before you pay us anything — there's a 14-day free trial at biurko.io. If we fail a step, I'd rather hear it from you than read it in someone else's ranking.

FAQ

How do I choose KSeF software? Skip the feature lists and test one failure scenario instead: issue an invoice offline, check both QR codes on the printout, download the UPO and the submitted XML, then click "send" twice. A tool that passes these four steps has a solid integration. Compare prices only after that.

Do I need to switch KSeF software before 1 January 2027? You don't have to, but it's the last convenient moment. On 1 January 2027 the transitional periods end and new rules require KSeF numbers in payment titles. Migrating your data in the same month the rules change gives you two sources of problems at once instead of one.

What happens when KSeF is down? Polish VAT law provides three special modes: offline24, offline-unavailability and emergency mode. You issue the invoice as usual, then submit it to KSeF — by the next business day for offline24 and unavailability, within seven business days after the outage ends for emergency mode. Tracking the deadline is the taxpayer's job.

Is the Ministry of Finance's free KSeF app enough? For issuing, receiving and viewing invoices in KSeF — for many micro-businesses, yes, and that's the honest answer. Paid software earns its price with what happens after the invoice: VAT registers, JPK files, calculated tax, social security, proof of filing. If you don't need that, you don't need paid software.