Buffer never connects to REPL on some ClojureScript projects

[dgtized]

Expected behavior

Jack-in on ClojureScript project connects project buffers to the REPL

Actual behavior

Jack-in succeeds, opens the page and the REPL buffer, but does not connect any buffers in the project to the repl, leaving them in a [not connected] state.

Steps to reproduce the problem

I've got a couple projects that use figwheel, three of them startup and then instantly disconnect, and one that does successfully connect each buffer to the repl. I'm trying to figure out what's different about the one that is working from the 3 that are not.  Example public repo that work is: https://github.com/dgtized/tile-game, but it immediately disconnects for https://github.com/dgtized/shimmers and https://github.com/dgtized/sierpinski-sieve. The other repo that it disconnects on is private.

The first project this broke on (shimmers) I had also upgraded clojurescript, but that was not the case for the other three that broke and I witnessed the same behavior. For the one that works (tile-game) I was able to upgrade to latest clojurescript, so it does not appear to be the source of the issue.

Finally, I usually install Cider from MELPA and was running version 20260510.755. I tried downgrading to 1.21.0 and it started working on all projects. When I reverted back to the MELPA release it stopped working again.

So TLDR, it does strongly appear to be from a change in Cider, but there might also be a setting that is contributing that I haven't been able to isolate as yet.

Initially reported this on ClojureScript slack and summarized here.

Environment & Version information

CIDER version information

Breaks on Cider 20160510.755:
[nREPL] Starting server via /usr/local/bin/clojure -Sdeps \{\:deps\ \{nrepl/nrepl\ \{\:mvn/version\ \"1.7.0\"\}\ cider/cider-nrepl\ \{\:mvn/version\ \"0.59.0\"\}\ refactor-nrepl/refactor-nrepl\ \{\:mvn/version\ \"3.11.0\"\}\ cider/piggieback\ \{\:mvn/version\ \"0.6.1\"\}\}\ \:aliases\ \{\:cider/nrepl\ \{\:main-opts\ \[\"-m\"\ \"nrepl.cmdline\"\ \"--middleware\"\ \"\[refactor-nrepl.middleware/wrap-refactor\,cider.nrepl/cider-middleware\,cider.piggieback/wrap-cljs-repl\]\"\]\}\}\} -M:dev:cider/nrepland

Works on Cider 1.21.0:
[nREPL] Starting server via /usr/local/bin/clojure -Sdeps \{\:deps\ \{nrepl/nrepl\ \{\:mvn/version\ \"1.5.2\"\}\ cider/cider-nrepl\ \{\:mvn/version\ \"0.58.0\"\}\ refactor-nrepl/refactor-nrepl\ \{\:mvn/version\ \"3.11.0\"\}\ cider/piggieback\ \{\:mvn/version\ \"0.6.1\"\}\}\ \:aliases\ \{\:cider/nrepl\ \{\:main-opts\ \[\"-m\"\ \"nrepl.cmdline\"\ \"--middleware\"\ \"\[refactor-nrepl.middleware/wrap-refactor\,cider.nrepl/cider-middleware\,cider.piggieback/wrap-cljs-repl\]\"\]\}\}\} -M:dev:cider/nrepl

Lein / Clojure CLI version

Clojure CLI version 1.12.4.1618

Emacs version

GNU Emacs 32.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.49, cairo version 1.18.4) of 2026-05-09
(snapshot build from git)

Operating system

Ubuntu 2025.04 (plucky)

JDK distribution

openjdk version "25.0.1" 2025-10-21
OpenJDK Runtime Environment (build 25.0.1+8-Ubuntu-125.04)
OpenJDK 64-Bit Server VM (build 25.0.1+8-Ubuntu-125.04, mixed mode, sharing)

[bbatsov]

Thanks for the bisect - that narrows it significantly. We made some invasive changes in that window (the namespaced-ops switch in April being the biggest), but the fact that one of your figwheel projects works while three don't suggests it's the per-buffer friendliness check, not jack-in itself.

Could you do this in a "stuck" buffer (open a .cljs file in e.g. shimmers, with the project's REPL running) and share the output?

M-x cider-debug-sesman-friendly-session-p

It prints what classpath / classpath-roots the function sees versus the file path it's trying to match. That'll tell us whether this is a classpath-resolution issue, an ns-match issue, or something else.

Same buffer: M-x cider-describe-current-connection would also help - comparing that output between tile-game (works) and shimmers (broken) should highlight what's different about the projects.

[bbatsov]

Just merged #3913, which is my best guess at what's going wrong here.

The friendly-session matcher had a few issues that could plausibly cause buffers to stay [not connected] after a successful jack-in:

• It cached empty classpath/ns-list responses as nil, indistinguishable from "not yet cached", so it kept re-fetching and never settled.
• It had no fallback to the connection's project directory when the classpath check returned nothing -- which is exactly what happens for CLJS-only piggieback connections where the cider/classpath op isn't usefully populated.
• Classpath-root matching used string-prefix-p, so /foo/bar would spuriously match files under /foo/barber/....

The PR also restructured the matcher to precompute the caches at connect time, so it's now a pure path comparison instead of doing nREPL round trips during sesman dispatch. That hopefully also helps with any unreported edge cases in this area.

Leaving this open since I can't verify against your project. If you grab the latest from master and it still misbehaves, please re-run M-x cider-debug-sesman-friendly-session-p and paste the output -- the debug listing now includes the project-dir fallback info too.

[dgtized]

Installed cider version 20260514.1615, and restarted emacs (as I noticed some of the other connection settings appear to be sticky without a forced eval of cider):

Opened up shimmers/src/core.cljs and jacked in, unfortunately core remains unconnected. If I run cider-describe-connection (as I don't appear to have a cider-describe-current-connection), I get:

No cljs REPLs in current session "clojure/shimmers:localhost:37921"

If I switch to the *cider-repl buffer then I get:

CLJS shimmers@localhost:37921 (Java 25.0.1, Clojure 1.12.4, nREPL 1.7.0)

And cider-debug-sesman-friendly-session-p (from both the unconnected core.cljs and the *cider-nrepl buffer):

("/home/clgc/code/clojure/shimmers/src")

I also retried the shimmers test after upgrading to clojure 1.12.5 just to make sure that difference wasn't relevant, but it had the same behavior.

I then restarted emacs again just to clear all state, and jacked into the tile-game project from grid.cljs. That does successfully connect.

cider-describe-connection output is:

CLJS tile-game@localhost:44153 (Java 25.0.1, Clojure 1.12.5, nREPL 1.7.0)

and cider-debug-sesman-friendly-session-p output is:

("/home/clgc/code/clojure/tile-game/src")

I also retried it with the sierpinsky-sieve project and it did not connect the buffer. I also double checked with this latest build that it would connect on a CLJ only project and it was successful.

I don't have permission to re-open this issue, but I also see that #3914 might be a duplicate or similar issue as this?

Is there a command to manually attach a buffer to a specific nrepl session? Would there be any value in getting any failure information from running that? Let me know what else you want to try. Also, the shimmers example does have some other dependencies so could see how that might not run locally, but I believe sierpinsky-sieve has no external dependencies, so that might be easier for you to try it locally?