As it happens, I'd never used dune utop before - the man page is not entirely clear that you need an argument: SYNOPSIS only lists optional arguments and OPTIONS doesn't really hint at it. I inadvertently skipped the one line in DESCRIPTION which mentions it!

The error message when running dune utop in a directory which doesn't define a library also needs improving:

[dra@ocamldefang syslog-message]$ dune utop
Don't know how to build .utop/utop.exe

It could be:

No library found in the current directory, did you mean to specify a directory
e.g. dune utop lib

or it could even list the directories under the current one which do define libraries?