When an interface with default methods is used with MicroProfile REST Client, the interface instance does not retain default methods and invoking a default method results in a failure.

The original issue has been reported to Eclipse MP REST Client project where a more detailed description and sample code to demonstrate the issue is available. The current status is that this particular situation is not covered well in the specification yet, but it perhaps shall be included in the next version of the specification.

This ticket is mostly meant as a pointer to the issue found – I don't really expect it to be fixed before the specification supplies exact details… although if it worked even before that, it would be a nice surprise.