I’ve done some pretty trivial ones; the most notable being nrebl.middleware. I don’t recall anything particularly unusual with it, you might want to check and make sure you’re passing vars not functions to set-descriptor

Thanks, that's a good thought, but it's not that...

How are you developing it? nREPL is slightly unusual if you’re using nREPL to develop nREPL. I’m assuming you’re using the recommended interactive hacking workflow? https://nrepl.org/nrepl/0.8/hacking_on_nrepl.html#interactive-hacking

I have recently started doing it like that - and it seems to be better.

Previously I was setting the middleware with the nrepl.edn file and then starting an nrepl from cursive. It works the first time and then seems to get cached and I can't seem to make it pick up my code sensibly.

I liked that approach because that was how I imagined using the middleware from other projects, so it seemed like a good test.