Function is_held_elsewhere_error 

pub(super) fn is_held_elsewhere_error(err: &str) -> bool

Did spawn_process refuse because of session_held_elsewhere? The answer decides whether a caller may fall back to a FRESH spawn: for any other failure a blank session is a reasonable recovery, but for this one it silently hands the user’s accepted prompt to a new conversation while the real one is open next door — exactly what the guard exists to prevent (codex P1 on PR #3551).