server.Server.command_prompt

The prompt runs command once the user answers, with %% replaced by what they typed.

This does not return when the prompt opens. tmux holds the invocation until somebody answers or dismisses it, so a caller is waiting on a person -- and on a server nobody is watching, on nobody. The dispatch timeout is what ends that wait: with the default it fails after thirty seconds having opened a prompt that is still there. Give the call a server whose default_timeout suits a human, or drive it from a task that may take that long.

Passing a client is not what decides this. Both forms wait; naming one only decides which terminal the prompt appears on.

Errors

Returns Error::ServerMismatch when the client belongs to another server, or an error when no suitable client exists, when tmux refuses, and when the dispatch timeout expires before the prompt is answered.