server.Server.format

This is display-message -p, whose target is a pane: the format is evaluated against it, so #{pane_current_command} and the session and window fields around it all resolve from there.

The result is TmuxText because a format can interpolate names that are not valid UTF-8.

A literal #(command) starts command in a shell. Recursive expansion, such as #{E:status-left}, can also expose a command stored in the expanded value. The job is asynchronous, so this call may return before it produces output. Escaping the outer template with ## does not escape text introduced by a recursive expansion. Use only validated, simple #{field} lookups with untrusted input.

A % in the template is a time conversion, not literal text. tmux expands this command through strftime before it expands #{} -- it calls format_expand_time here and plain format_expand for every listing -- so %Y becomes the year. Write %% for a literal %.

A % followed by something strftime does not define is worse than surprising: it is not portable. glibc keeps both characters, so the template survives; Apple's libc emits the conversion character alone and deletes the %. So a template separating fields with % reads back correctly on Linux and comes back unframed on macOS. Separate fields with a byte that is not %.

Errors

Returns Error::ServerMismatch when the pane belongs to another server, or an error when tmux rejects the format or the pane is gone.