The UndertowJSRWebSocketDeploymentProcessor looks for types annotated with both @ServerEndpoint and @ClientEndpoint. Per the Jakarta WebSocket 2.2 specification on section 6.2:

The Jakarta WebSocket implementation must use the web container scanning mechanism defined in Servlet 3.0 to find annotated and programmatic endpoints contained within the WAR file at deployment time [WSC-6.2-1]. This is done by scanning for classes annotated with @ServerEndpoint and classes that extend Endpoint. See also Section 4.8 for potential extra steps needed after the scan for annotated endpoints. Further, the WebSocket implementation must use the WebSocket scanning mechanism to find implementations of the ServerApplicationConfig interface packaged within the WAR file (or in any of its sub-JAR files) [WSC-6.2-2].

There is no reference there to needing to scan for @ClientEndpoint's. However, WildFly has been doing this since WebSockets were introduced. This came up in a TCK challenge that is likely to get rejected.