I've attached a patch for the emacs-guix source which works around this issue. It modifies the REPL starting process for the Internal REPL, so that the (guix gexp) module is explicitly (re)loaded, which allows to reader syntax to work when the other modules load. I do not know why this should be necessary.

I wondered if there is some subtle difference here when dealing with a REPL that is accessed over a socket, as is the case with the Internal REPL. But I can't reproduce the problem on the command-line, outside of emacs-guix, using sockets.

With this patch, the "guix shell command" popup opens, though I see there is something wrong with the popup itself. It does not ask for a command to run, and the popup options only attempt to run an empty command. But I could investigate that in a separate bug.

Troubleshooting/testing was done with GNU Emacs 30.0.92 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.41, cairo version 1.18.0) of 2024-10-29.

-- 
Christopher Howard