KSeF integration: how to connect your system to Poland's e-invoicing hub

Connecting your invoicing to KSeF is an API integration with a specific shape: authenticate, open a session, submit FA_VAT invoices, and collect the confirmation. Here's what an integration involves and the choices to get right first.

Environments

KSeF exposes separate environments — a test and a demo environment that behave like production without legal effect, and production. Build and validate against test/demo before touching production. Never trial with real invoices.

Authentication

You authenticate as the taxpayer (or their authorised software) using one of:

• a qualified electronic signature or qualified seal,
• a trusted profile (Profil Zaufany), or
• an authorisation token generated inside KSeF for automated/software access.

For system-to-system automation, the token flow is the usual choice. Manage permissions carefully — who/what may issue invoices on the company's behalf.

Sessions: interactive vs batch

KSeF works in sessions:

• Interactive — open a session, send invoices one by one, receive each

result, then close.

• Batch — package many invoices and submit them together (useful for volume).

Your integration picks the mode based on how you invoice.

The core loop

□ Authenticate → obtain a session token
□ Build a valid FA_VAT XML invoice
□ Submit it in the session
□ Receive the KSeF number (invoice is now "issued")
□ Collect the UPO (proof of receipt)
□ Store the KSeF number + UPO against your invoice record
□ Handle errors/rejections and retry safely (idempotency)

What to store on your side

• The KSeF number and UPO for every issued invoice.
• The exact FA_VAT submitted (for audit).
• Status transitions and any rejection reasons.

Decisions before you build

1. Direct vs provider. Integrate against the KSeF API yourself, or go through

a provider/software that already speaks KSeF. Direct gives control; a provider gives speed and maintenance cover as the schema evolves (FA(2) → FA(3)).

1. Signature/token strategy — how software authenticates without a human each

time.

1. Retry + idempotency — never double-issue on a network hiccup.
2. Schema versioning — KSeF evolves; plan for format updates.

Don't forget the EU side

KSeF is domestic. If you also invoice across the EU, you still need Peppol/EN 16931 for cross-border — a separate channel with its own format. Many Polish exporters run both.

Where Eurobillr fits

Eurobillr integrates with Recommand for EU e-invoicing over Peppol (Belgium, France's accredited-platform flow, and more) — the same "authenticate, build a valid structured document, send, confirm" pattern KSeF uses, applied to the cross-border side. KSeF is Poland's national hub; for domestic Polish issuance you'll connect to KSeF directly or via a KSeF provider. If you need both, we're happy to hear where the gaps are.

Next: KSeF API · how to issue a KSeF invoice.