Having taken a moment to test this in the rig this morning.
And looking at the bug you found.
Yes the whisper topic doesn’t seem to work in the developer rig.
This is likely due to the rig “pretending” to be a user, so the “user” specific topic doesn’t work for security reasons. I do 99.9% of my extension development on Twitch rather than the Rig itself.
Depending on your expected send count, you might want to be using the main channel feed.
As then you can send an array of messages in one go.
And your frontend can extract the message for that user from the array
Which then is a more efficent usage of the per channel rate limit, since the whisper topic uses the channel rate limit.
Also saves having to juggle whispertarget ID’s