KSeF API 2.0 in Next.js on Vercel

KSeF (Krajowy System e-Faktur) is Poland's mandatory national e-invoicing platform: domestic invoices are submitted as structured XML (schema FA(3)) to the Ministry of Finance API, which assigns a KSeF number and issues a UPO (Urzędowe Poświadczenie Odbioru — the official receipt). The statutory mandate phases in from 1 February 2026 — verify current cohort dates at podatki.gov.pl/ksef.

Official SDKs exist only for C# and Java. This skill provides equivalent TypeScript/Node.js implementations and the deployment patterns for Vercel.

When to use

• Building or debugging any KSeF integration in a Next.js / Node.js / Vercel codebase: issuing sales invoices, ingesting purchase invoices, UPO handling, QR codes, offline modes, credentials.
• Questions about KSeF API 2.0 mechanics: auth flows, sessions, encryption, rate limits, test environment.