Validation rules

BR-21

An invoice line has no line identifier (BT-126).

Rule

BR-21 — official EN 16931 / XRechnung rule

Severity

error

Business term

BT-126 — Line identifier

Field in your JSON

invoice.lines[*].id

Detected by

tier 3 — KoSIT validator (authoritative)

Reachability

Only reachable when validating your own XML — normbill-generated documents cannot trigger it

The serializer writes line.id, falling back to the 1-based line number, so generated lines always carry BT-126.

How to fix it

Give every cac:InvoiceLine a non-empty cbc:ID.

Official sources

• XRechnung specification (KoSIT / xeinkauf.de)
• KoSIT validator on GitHub and the XRechnung validator configuration

Every 422 from the API lists the rule id, the path in your request JSON, a plain-language message and a suggested fix — see the docs overview for the report format.