A deliberately misplaced XML block proves Poland's FA(3) validator can still fail

FA(3) is built from `xsd:sequence`, so where a block sits is part of the contract. Under `Faktura` the order runs `Naglowek`, `Podmiot1` for the seller, `Podmiot2` for the buyer, up to 100 `Podmiot3` third parties, an optional `PodmiotUpowazniony`, then `Fa`, an optional `Stopka` and an optional `Zalacznik`, with `Platnosc` inside `Fa` after the rows. The change under test added three blocks to every invoice: a third party, a payment section with bank accounts, and a footer. They land in three different places, one between the buyer and the invoice body, one deep inside the body after the line items, one after the body. A builder that emits the right content one slot early produces an invalid document.

The XSD declares limits the guides omit. A factor may have at most 20 bank accounts. An account number is any string of 10 to 34 characters. The third party's role is a number from a closed list, where 1 means factor. "When I read these rules in the XSD instead of in a PDF guide, I found them faster and trusted them more," the author wrote on dev.to.

Four documents, including fetching the two imported schema files, take about 1.3 seconds. That is roughly 0.33 seconds per document, and since the imports are fetched once, the steady-state cost per invoice is lower than that. For the figure to transfer you need lxml compiling the same three files locally, the schema already cached, and invoices of comparable size; there is no token, no login and no network round trip per invoice in that path. The script downloads the XSD from a hardcoded URL to `tmp/fa3/schemat.xsd` only if the file is missing.

The samples come out of the application's own code. A hand-written valid invoice proves that a person can write one, so a small script calls the same serializer function the application calls and writes the output to disk. Four cases cover the new paths: a baseline, one with everything at once (a foreign buyer in euros with two factor accounts and a footer), one with an own bank account only, one with a footer only. The baseline carries as much weight as the rest: a change that adds optional blocks must not change the document when those blocks are empty.