Integrating with Poland's KSeF: five things that aren't in the docs

Poland switched to mandatory structured e-invoicing this year. Since 1 February 2026 for the largest companies, since 1 April 2026 for all active VAT payers, and from 1 January 2027 for everyone else, including the smallest and VAT-exempt.

The system is called KSeF (Krajowy System e-Faktur). You send an invoice as XML in a schema called FA(3), it validates it, assigns it a number, and returns a signed receipt. Conceptually simple. There is an OpenAPI spec and official SDKs in C# and Java.

I've spent this year building an integration against it in TypeScript on Deno, and it now files invoices in production. Below are five things that cost me real time and are not in the documentation, or are in it in a way you only recognise afterwards.

1. The receipt is issued per invoice, but fetched through the session

This is the one I'd most want to tell my past self, and it has two halves that are easy to get backwards.