Can I mix multiple seller NIPs in one KSeF batch?

No. KSeF 2.0 requires every invoice inside a single batch session to have the same seller (Podmiot1) NIP. Mixed batches are rejected with code 21181.

How should accounting firms structure batches?

One batch per client NIP. You can run those batches in parallel with the same technical token, so throughput is not affected as long as your integration parallelises correctly.

Does the buyer NIP matter for 21181?

No. The rule only checks the seller identifier. Buyer NIPs can vary freely within a single batch.

Can Invoice Navigator detect this before submission?

Yes — Invoice Navigator groups invoices by Podmiot1 NIP during pre-flight and blocks batches with mixed sellers so KSeF never sees a rejectable submission.

Error codes · KSeF 2.0 · Poland

KSEF-21181Partly

Sellers are not identical for all invoices in a KSeF batch

This is the rejection, word for word: „[KSEF-21181] The KSeF 2.0 API rejected a batch session because at least one invoice inside the batch has a Podmiot1 (issuer) NIP that differs from the others. KSeF requires every invoice in a single batch session to belong to the same seller.“

Triggered on POST /sessions/batch when the aggregated invoice set contains more than one distinct issuer NIP. The API returns HTTP 400 with code 21181 and message "Nieprawidłowe żądanie eksportu faktur" or "Sprzedawcy nie są identyczni we wszystkich dostarczonych fakturach". No invoices in the batch are ingested — the entire batch is rejected atomically.

What to have readyThe correct figures from your bookkeeping; we show you which field is affected.

What we doSplit the batch so that every invoice inside a single session belongs to one seller NIP.

Choose the rejected invoiceor drop it here

XRechnung XML or ZUGFeRD PDF. Checking is free · corrected file €1.90 · the first every 30 days is free · no sign-up.

What the finding looks likeExample

FindingNeeds your check · KSEF-21181

From youThe correct figures

ThenChecked again

ProofSHA-256 and /verify link after the check

Often appears together with

KSEF-21180KSeF session status does not allow this operation KSEF-21182KSeF concurrent export limit reached PL-R-004KSeF session token invalid