I added a typo in my `config.toml` such that the `command` for one of my `mcp_servers` did not exist and I verified that the error was surfaced in the TUI (and that I was still able to use Codex). 
codex-core
This crate implements the business logic for Codex. It is designed to be used by the various Codex UIs written in Rust.
Though for non-Rust UIs, we are also working to define a protocol for talking to Codex. See:
You can use the proto subcommand using the executable in the cli crate to speak the protocol using newline-delimited-JSON over stdin/stdout.