jsos | An operating system written in JavaScript

It seems that your annotated Provider class is unknown to JAX-RS impl.'s context which is resteasy for this case.

Since reasteasy and other JAX-RS implementations have embedded json serialization/deserialization support, your json test surely works.

According to Jboss documentation;

The JAX-RS specification allows you to plug in your own request/response body reader and writers. To do this, you annotate a class with @Provider and specify the @Produces types for a writer and @Consumes types for a reader. You must also implement a MessageBodyReader/Writer interface respectively. Here is an example.

The Resteasy ServletContextLoader will automatically scan your WEB-INF/lib and classes directories for classes annotated with @Provider or you can manually configure them in web.xml. See Installation/Configuration

First of all, add necessary logging to your @Provider class and double check if there is a nested underlying exception that causes the class from loading/initing.

Also add configuration to your web.xml for your provider class as a workaround for bypassing (if any) auto-scanning issues.