Currently by listening to FiddlerApplication.OnWebSocketMessage it's possible to modify the incoming & outgoing messages but it's not possible to send independent direct messages in or out.

Adding the ability to send direct messages will give more freedom on injecting custom messages in both directions, repeating server response messages etc.
Currently if you need to inject an outgoing message you need to wait for the client to generate a message and only then intercept, modify and forward it. Sometimes the client may wait longer times to respond and a direct message mechanism would be quite useful to generate quicker responses.

From: How to send a new web socket message instead of modifying an existing one?
 (https://groups.google.com/forum/#!topic/httpfiddler/CC5XxiWfpuI) Related to: Add properties to WebSocket object (https://fiddler.ideas.aha.io/ideas/FID-I-146)