Function unique_prefix_tag 

pub fn unique_prefix_tag(prefix: &str, run_id: u64) -> String

2026-09-26: A prompt prefix tag of the form <prefix>-<pid>-<run_id>.

run_id comes from the run’s crate::PluginHandle, which the executor numbers per run; with a prefix that is unique within the run, the tag differs from every other run’s in the process, and the pid separates processes. The cold-TTFT measurement (ttft.rs) uses it so no two requests share a cached prefix.