Function verified_block_id 

pub(crate) fn verified_block_id(
    state: &AppState,
    caller: Option<&Caller>,
    auth: &UiAutomationAuth,
) -> Result<String, String>

Verify auth proves the caller genuinely is auth.agent_id (via that agent’s own jekt key — an attacker without agent_id’s key cannot produce a valid signature no matter what it claims), then derive that agent’s ACTUAL current block_id server-side. Never trusts a block_id from the client — there isn’t one to trust. See this module’s own doc comment for the full rationale. pub(crate): also reused by app_api::pane::handle_close_pane (and any future own-pane-resolving handler) — the identity-verification mechanism isn’t UI-automation-specific, just first built for it. See docs/specs/SPEC_AGENT_PANE_LIFECYCLE_CONTROL_2026_09_10.md §5.0.

Identity M4a-2: caller is the request’s Caller; auth.agent_id is counted against it (m4.actor_*.ui_auth), never refused on it — M4d moves this check onto Caller (spec §6.5.4).