As you found, setting .setProfile("EN16931") or .setProfile("EXTENDED") is mandatory for French Factur-X compliance. Here's a quick reference for choosing the right profile:

Factur-X Profiles:

• MINIMUM - Invoice number and amount only (limited use)
• BASIC - Essential invoice data (no line items)
• EN16931 - European standard (recommended for French e-invoicing 2026)
• EXTENDED - Full invoice details with extensions
• XRECHNUNG - German B2G specific standard

For the French mandatory e-invoicing (starting Sept 2026), you must use EN16931 profile minimum to pass validation with Plateformes Agréées (PA/PDP).

Common pitfall: Don't forget to validate both PDF/A-3 conformance AND XML Schematron rules (250+ business rules). VeraPDF only checks PDF structure, not invoice semantics.

Alternative approach: If maintaining Mustang integration becomes complex (especially for Schematron validation, auto-completion from SIRET, or multi-PA/PDP compatibility), you can externalize this via REST API. For example, FactPulse API accepts PDF + invoice metadata and returns compliant Factur-X (disclaimer: I'm the founder, but it's open for integration regardless of your stack).

Documentation:

• Mustang GitHub: https://github.com/ZUGFeRD/mustangproject
• French Factur-X specs: https://fnfe-mpe.org/factur-x/