how bad an idea is mutable state on a custom CoroutineContext element? like all the way bad?

If it's thread local, it's probably less risky. Just make sure worst cases are handled correctly and reasoning about code built upon this is not too hard.

If some code performs concurrent decomposition, then they all inherit this context element and may start to mutate it concurrently.

https://gist.github.com/Groostav/c0d6d7625db61045064b45b51a51b6d9 Its wrappedin an atomic reference. but it seems like Ive got a life-cycle missmatch. It could be "closed" and hang around on the object graph by virtue of a long-running co-routine for a long time. Hmm.