Today, on December 18th, 2025, version 2.21.0 of the open source e-invoicing toolkit Mustangproject and version 1.8.0 of the Software-as-a-Service REST API Mustangserver have been released.

They both support validation of the new Factur-X 1.08 version, in Germany also referred to as ZUGFeRD 2.4. Additional information on changes of Mustangproject can be obtained in it’s history file. The new version of Mustangproject can, as usual, be integrated into software projects using Maven Central, or be downloaded as a command line tool.

Mustangserver additionally now makes Factur-X invoices designable: A new, so called “StyledInvoice” allows to send a Factur-X invoice along with a HTML Template and additional resources in a single request, which are then converted into a Factur-X file. The resources can be ordinary files, like logo images or fonts, to be used in HTML, or references thereof in a corporate fileshare (i.e., a S3 bucket).

The use of HTML (mustache)-templates makes it possible for customers to offer their users to customize and design the optics of the resulting PDF part of the invoice e.g. using standard embeddable visual live design tools. The feature is further described in the Mustangserver manual.

Mustangserver 1.8 offers a free 4-week-evalution version.

Screenshot of a (german) demo on possible “StyledInvoice” designs: