Function pane_history_session_id 

pub(crate) fn pane_history_session_id(
    filestore: Option<&FileStore>,
    mstore: Option<&Store>,
    block_id: &str,
    session_id_field: &str,
) -> Option<String>

The session id of the conversation a pane renders: the last provider session id in its own transcript, else in the agent’s global transcript zone. Same sources, same order, as has_prior_transcript, so “the pane shows history” and “the spawn continues it” can’t disagree.

Taken from AgentMux’s own record rather than a scan of the provider’s dir, so an archived conversation (every explicit “start fresh” action archives) is never picked up, and a subagent’s transcript can’t pass for the agent’s own. No reachability check; callers decide what an unreachable id means.