It could be useful for GHC API clients to implement their own external interpreter commands. For example, the debugger may want an efficient way to inspect the stacks of the running threads in the external interpreter. At the moment, you are limited to using the command set which is needed by GHCi. If there was a `CustomMessage`, then the debugger could perform self-execution to launch the external interpreter with support for additional commands that are needed. I have a POC branch here: https://gitlab.haskell.org/ghc/ghc/-/commits/wip/ghc-external-custom-commands