Mustang Project Alternative: Open-Source Library or Managed E-Invoicing API?

Free tool: create & validate an XRechnung →

Feature comparison

Who is Mustang Project and who is it built for?

Mustang Project is the established open-source Java library for ZUGFeRD and XRechnung generation, hosted at mustangproject.org. It has been the de-facto standard in the DACH Java ecosystem for e-invoicing libraries since ~2014, with an active community and regular updates for KoSIT schemas.

The user persona: engineering teams with a Java backend and capacity for self-host. Mustang is not an API service but a library that you embed into your own stack. You're responsible for: hosting, compliance updates, integration, performance optimization.

invoice-api.xhub.io takes the opposite approach: managed API service with REST endpoints, automatic compliance updates, multi-country support, and stack agnosticism. You make HTTP calls; we handle the rest.

Where Mustang Project shines

If your requirement profile is "engineering-first, self-host, open-source":

• Full code control — you can fork, modify, customize anything
• No vendor lock-in — works without external services, no API key, no quota
• Apache 2.0 / GPL license — free for any use, including commercial
• Active community — GitHub issues, regular updates, transparent codebase
• Standalone validator — you can validate XRechnung locally without external calls
• Java-stack native — directly integrable into Spring Boot, Quarkus, JEE apps

For engineering teams with a Java stack, self-host requirement, or strict data-protection rules that don't allow external API calls, Mustang is often the right choice. A managed API would be too much vendor-reliance.

Where invoice-api.xhub.io is set up differently

invoice-api.xhub is built for teams that want to ship fast without maintaining their own library integration:

1. You don't have a Java stack

Mustang is Java. If your backend runs in TypeScript, Python, Go, Rust, PHP, or other languages, Mustang either isn't directly usable or requires complex wrappers. invoice-api.xhub is language-agnostic — REST calls work everywhere.

2. You need multi-country EU formats

Mustang focuses on DE (XRechnung, ZUGFeRD). If you also invoice in France (Factur-X), Italy (FatturaPA), Netherlands (Peppol BIS 3.0/NLCIUS), etc., you'd need either additional libraries plus integration effort, or a multi-country service. invoice-api.xhub covers 37 countries — with the national format where one exists, otherwise Peppol BIS 3.0/UBL.

3. You don't want to maintain compliance updates yourself

KoSIT schemas are updated occasionally. With Mustang, you have to track library updates, update your build, deploy. With invoice-api.xhub, compliance updates come automatically via API — no re-deploy on your side.

4. Time-to-First-Invoice in minutes, not days

Mustang setup: set up Java stack, integrate library, configuration, testing, deploy — typically 1–3 days of engineering time. invoice-api.xhub: generate API key, make first call — 5 minutes. For prototyping, MVP validation, or quick customer demos, that's a significant difference.

5. You need cloud-native features

The n8n community node and the standalone Validator as an HTTP endpoint can only be built with Mustang via your own server infrastructure. invoice-api.xhub delivers them out of the box. Webhooks are not among them: the API responds synchronously, there is no push channel.

Honest cost comparison

Our prices are net, plus VAT.

Important: at high volume (>20k invoices/month) and existing Java engineering team, Mustang can be cheaper. At low volume or stack-agnostic, invoice-api.xhub wins.

Migration: can I use both in parallel?

Theoretically yes, but rarely sensible. Realistic hybrid scenarios:

• Mustang for DE, invoice-api.xhub for other EU countries: If your volume is >20k DE invoices/month and you already use Mustang, the hybrid approach for FR/IT/ES makes sense.
• Mustang as library, invoice-api.xhub as validator backup: Edge case — you use Mustang locally, but supplement our standalone Validator for CI/CD pipelines.

In most cases, you pick one strategy and stick with it.

Decision guide

If you recognize yourself in any of these statements, invoice-api.xhub is the better choice:

• "My backend is not Java (TypeScript / Python / Go / PHP / etc.)."
• "I need multi-country EU support."
• "I want compliance updates automatically, not maintained myself."
• "Time-to-First-Invoice must be in minutes, not days."
• "n8n integration is required."

If you recognize yourself here, Mustang is the more natural choice:

• "Java stack present, engineering capacity for self-host and library maintenance."
• "Open-source requirement, no vendor lock-in desired."
• "DE-only (XRechnung, ZUGFeRD), no multi-country."
• "Strict data protection rules forbid external API calls."
• "High volume (>20k invoices/mo), where library costs are lower than managed API."

Next step

If you use Mustang and need one of our specialty endpoints (multi-country, validator-as-a-service, Parser API) — free tier with 20 invoices/month, no credit card:

• Generate a sample XRechnung from a JSON body
• Validate it against the KoSIT schemas
• Convert it to a ZUGFeRD PDF/A-3
• Browse the n8n workflow template

If you're starting without e-invoicing integration and Mustang setup overhead is too high: try the Starter plan from €23/month (€18 annually) with 2,000 invoices. All prices net, plus VAT.

What neither option solves

Mustang is a solid library, and the TCO table above is honest: at high volume with Java capacity in-house, self-hosting is cheaper. That point goes to Mustang.

But neither a library nor an API solves the part where migrations actually fail:

• The routing ID (Leitweg-ID) of your public-sector customers is nowhere in the customer record.
• Credit notes need a reference to the original invoice your system never stored.
• Tax categories work in the standard case but not for reverse charge or intra-community supply.
• Custom fields grown over years don't map onto EN 16931.
• The recipient accepts the invoice technically but rejects it on substance — and nobody notices until the payment fails to arrive.

Mustang produces valid XML from correct input data. Where the correct input data comes from is your job — and that is the larger part of the work.

If you build it yourself: the readiness check tells you upfront which fields are missing in your system. That saves you iterations regardless of whether you end up choosing Mustang or us.

→ E-invoicing integration at xhub.io