CO-Authors: Arnold Ahrer, Philipp Hochrieser

From September 21 to 25, 2026, we took part in the EUDIW Unfold Interop Week #4.1, a dedicated week focused on practical interoperability testing within the European Digital Identity Wallet ecosystem.

The event brought together wallet providers, Attestation Issuers, Attestation Verifiers and other stakeholders from across Europe. For us, it was an opportunity to put our Wallet-Unit implementation to the test with implementations from more than 30 manufacturers across Europe.

Testing Sessions

Our main objective throughout the week was straightforward: test interoperability in practice.

Our Wallet-Unit supported the following formats:

• SD-JWT
• mDoc

and the following protocols:

• ISO 18013-5
• OpenID4VP 1.0
• OpenID4VCI 1.0

We combined two approaches during the week. In several 1-on-1 testing sessions, we worked directly with Attestation Verifier manufacturers. When incompatibilities occurred, both sides could investigate the issue, exchange technical details and actively work towards a solution.

In parallel, we conducted standalone testing against registered Attestation Verifier implementations provided by the event organisers. Using their websites and API documentation, we could independently initiate verification processes and test our Wallet-Unit against a broad range of implementations.

Both approaches provided valuable insights into how different implementations interpret and apply the relevant standards in practice.

Testing Outcome

The organisers provided a dedicated website where participants could document their results in a 1-to-N matrix between Attestation Verifiers and the Wallet-Unit. This provided a structured way to record the outcomes and build a transparent picture of interoperability.

For the testing, we worked with externally provided Attestation Issuers and their registered certificates. This added another layer of complexity, as potential interoperability issues could arise on both the issuer and verifier sides.

Despite this, we were able to successfully complete verification flows with several Attestation Verifiers. These results demonstrated that our Wallet-Unit can interoperate with external implementations under the conditions of the Interop Week.

Complexity of Interoperability

Interoperability is a fundamental building block for digital identity solutions that need to operate across different implementations, ecosystems and national borders. Testing against a range of external implementations helps identify differences in how standards and protocols are interpreted and implemented in practice.

By integrating interoperability testing into our own development and testing process, we address this complexity before it becomes part of individual use cases. This includes testing different combinations of Wallets, Attestation Issuers and Attestation Verifiers, identifying compatibility issues and working through them with other implementation teams.

This is particularly relevant for cross-border use cases, where different national ecosystems and implementations need to interact reliably. Continuous interoperability testing therefore helps reduce integration effort and contributes to a more consistent technical foundation for the use of digital identity solutions across Europe.

What We Take Away

The EUDIW Unfold Interop Week #4.1 gave us the opportunity to move beyond specifications and test interoperability in real-world scenarios. The combination of direct collaboration and independent testing provided valuable insights into different implementation approaches and helped us identify and resolve compatibility issues.

For us, interoperability is an ongoing process. We will continue testing, learning from other implementations and contributing to an increasingly interoperable European Digital Identity ecosystem.
We look forward to continuing this work and to further interoperability testing with the EUDI community. See you next time at Launchpad in Brussels.

Want to Experience What We’re Building?

If you would like to explore the EUDI Wallet experience yourself, you can already try it out in our youniqx Playground.