diff --git a/codex-rs/core/src/agent/control.rs b/codex-rs/core/src/agent/control.rs
index 7361c445ceb5..696982b0a131 100644
--- a/codex-rs/core/src/agent/control.rs
+++ b/codex-rs/core/src/agent/control.rs
@@ -18,6 +18,7 @@ use crate::session_prefix::format_subagent_notification_message;
 use crate::thread_activity::ThreadActivityGate;
 use crate::thread_activity::ThreadActivityHandle;
 use crate::thread_activity::ThreadActivityRegistrationError;
+use crate::thread_activity::ThreadActivityReservation;
 use crate::thread_manager::ResumeThreadWithHistoryOptions;
 use crate::thread_manager::ThreadManagerState;
 use crate::thread_rollout_truncation::truncate_rollout_to_last_n_fork_turns;
@@ -81,6 +82,11 @@ pub(crate) struct LiveAgent {
     pub(crate) status: AgentStatus,
 }
 
+struct CompletionWatcherChild {
+    status: watch::Receiver<AgentStatus>,
+    _activity: ThreadActivityReservation,
+}
+
 #[derive(Clone, Debug, Serialize, PartialEq, Eq)]
 pub(crate) struct ListedAgent {
     pub(crate) agent_name: String,
@@ -155,6 +161,33 @@ impl AgentControl {
         self.rollout_budget.as_ref()
     }
 
+    pub(crate) async fn has_thread_spawn_descendant_idle_shutdown_blocker(
+        &self,
+        parent_thread_id: ThreadId,
+    ) -> bool {
+        let Ok(state) = self.upgrade() else {
+            return false;
+        };
+        for descendant_id in self
+            .thread_activity_gate
+            .descendant_thread_ids(parent_thread_id)
+        {
+            let Ok(descendant) = state.get_thread(descendant_id).await else {
+                continue;
+            };
+            if !is_final(&descendant.agent_status().await)
+                || descendant
+                    .codex
+                    .session
+                    .has_local_idle_shutdown_blocker()
+                    .await
+            {
+                return true;
+            }
+        }
+        false
+    }
+
     /// Send rich user input items to an existing agent thread.
     pub(crate) async fn send_input(
         &self,
@@ -479,6 +512,7 @@ impl AgentControl {
         session_source: Option<SessionSource>,
         child_reference: String,
         child_agent_path: Option<AgentPath>,
+        completion_child: Option<CompletionWatcherChild>,
     ) {
         let Some(SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
             parent_thread_id, ..
@@ -488,19 +522,13 @@ impl AgentControl {
         };
         let control = self.clone();
         tokio::spawn(async move {
-            let status = match control.subscribe_status(child_thread_id).await {
-                Ok(mut status_rx) => {
-                    let mut status = status_rx.borrow().clone();
-                    while !is_final(&status) {
-                        if status_rx.changed().await.is_err() {
-                            status = control.get_status(child_thread_id).await;
-                            break;
-                        }
-                        status = status_rx.borrow().clone();
-                    }
-                    status
-                }
-                Err(_) => control.get_status(child_thread_id).await,
+            let mut completion_child = completion_child;
+            let status = match completion_child.as_mut() {
+                Some(child) => wait_for_final_status(&mut child.status).await,
+                None => match control.subscribe_status(child_thread_id).await {
+                    Ok(mut status) => wait_for_final_status(&mut status).await,
+                    Err(_) => control.get_status(child_thread_id).await,
+                },
             };
             if !is_final(&status) {
                 return;
@@ -509,12 +537,13 @@ impl AgentControl {
             let Ok(state) = control.upgrade() else {
                 return;
             };
-            let child_thread = state.get_thread(child_thread_id).await.ok();
-            let child_uses_multi_agent_v2 = match child_thread.as_ref() {
-                Some(child_thread) => {
-                    child_thread.multi_agent_version() == Some(MultiAgentVersion::V2)
-                }
-                None => true,
+            let child_uses_multi_agent_v2 = match completion_child.as_ref() {
+                Some(_) => false,
+                None => state
+                    .get_thread(child_thread_id)
+                    .await
+                    .ok()
+                    .is_none_or(|child| child.multi_agent_version() == Some(MultiAgentVersion::V2)),
             };
             if child_agent_path.is_some() && child_uses_multi_agent_v2 {
                 let Some(child_agent_path) = child_agent_path.clone() else {
@@ -769,6 +798,17 @@ fn agent_matches_prefix(agent_path: Option<&AgentPath>, prefix: &AgentPath) -> b
     })
 }
 
+async fn wait_for_final_status(status: &mut watch::Receiver<AgentStatus>) -> AgentStatus {
+    let mut current = status.borrow().clone();
+    while !is_final(&current) {
+        if status.changed().await.is_err() {
+            return AgentStatus::NotFound;
+        }
+        current = status.borrow().clone();
+    }
+    current
+}
+
 pub(crate) fn render_input_preview(input: &[UserInput]) -> String {
     input
         .iter()
diff --git a/codex-rs/core/src/agent/control/spawn.rs b/codex-rs/core/src/agent/control/spawn.rs
index 9078434f1fe8..af71bac8bd95 100644
--- a/codex-rs/core/src/agent/control/spawn.rs
+++ b/codex-rs/core/src/agent/control/spawn.rs
@@ -328,6 +328,27 @@ impl AgentControl {
             (None, _, _) => Box::pin(state.spawn_new_thread(config.clone(), self.clone())).await?,
         };
         agent_metadata.agent_id = Some(new_thread.thread_id);
+        let completion_child = if multi_agent_version != MultiAgentVersion::V2
+            && matches!(
+                &notification_source,
+                Some(SessionSource::SubAgent(SubAgentSource::ThreadSpawn { .. }))
+            ) {
+            let Some(completion_watcher) =
+                new_thread.thread.codex.session.try_reserve_turn_activity()
+            else {
+                let _ = state.remove_thread(&new_thread.thread_id).await;
+                let _ = new_thread.thread.shutdown_and_wait().await;
+                return Err(CodexErr::InvalidRequest(
+                    "parent thread is shutting down".to_string(),
+                ));
+            };
+            Some(CompletionWatcherChild {
+                status: new_thread.thread.subscribe_status(),
+                _activity: completion_watcher,
+            })
+        } else {
+            None
+        };
         reservation.commit(agent_metadata.clone());
         if let Some(residency_slot) = residency_slot {
             residency_slot.commit(new_thread.thread_id);
@@ -415,6 +436,7 @@ impl AgentControl {
                 notification_source,
                 child_reference,
                 agent_metadata.agent_path.clone(),
+                completion_child,
             );
         }
 
@@ -737,6 +759,30 @@ impl AgentControl {
                 inherited_exec_policy,
             })
             .await?;
+        let completion_child = if multi_agent_version != MultiAgentVersion::V2
+            && matches!(
+                &notification_source,
+                SessionSource::SubAgent(SubAgentSource::ThreadSpawn { .. })
+            ) {
+            let Some(completion_watcher) = resumed_thread
+                .thread
+                .codex
+                .session
+                .try_reserve_turn_activity()
+            else {
+                let _ = state.remove_thread(&resumed_thread.thread_id).await;
+                let _ = resumed_thread.thread.shutdown_and_wait().await;
+                return Err(CodexErr::InvalidRequest(
+                    "parent thread is shutting down".to_string(),
+                ));
+            };
+            Some(CompletionWatcherChild {
+                status: resumed_thread.thread.subscribe_status(),
+                _activity: completion_watcher,
+            })
+        } else {
+            None
+        };
         let mut agent_metadata = agent_metadata;
         agent_metadata.agent_id = Some(resumed_thread.thread_id);
         reservation.commit(agent_metadata.clone());
@@ -754,6 +800,7 @@ impl AgentControl {
                 Some(notification_source.clone()),
                 child_reference,
                 agent_metadata.agent_path.clone(),
+                completion_child,
             );
         }
         self.persist_thread_spawn_edge_for_source(
diff --git a/codex-rs/core/src/agent/control_tests.rs b/codex-rs/core/src/agent/control_tests.rs
index 4741c3c101a3..4057e02c909e 100644
--- a/codex-rs/core/src/agent/control_tests.rs
+++ b/codex-rs/core/src/agent/control_tests.rs
@@ -1950,6 +1950,11 @@ async fn spawn_child_completion_notifies_parent_history() {
         .get_thread(child_thread_id)
         .await
         .expect("child thread should exist");
+    harness
+        .manager
+        .remove_thread(&child_thread_id)
+        .await
+        .expect("child removal should succeed");
     let _ = child_thread
         .submit(Op::Shutdown {})
         .await
@@ -2099,6 +2104,7 @@ async fn multi_agent_v2_completion_queues_message_for_direct_parent() {
         })),
         tester_path.to_string(),
         Some(tester_path.clone()),
+        /*completion_child*/ None,
     );
     let tester_turn = tester_thread.codex.session.new_default_turn().await;
     tester_thread
@@ -2187,6 +2193,7 @@ async fn completion_watcher_notifies_parent_when_child_is_missing() {
         })),
         child_thread_id.to_string(),
         /*child_agent_path*/ None,
+        /*completion_child*/ None,
     );
 
     assert_eq!(wait_for_subagent_notification(&parent_thread).await, true);
diff --git a/codex-rs/core/src/session/inject.rs b/codex-rs/core/src/session/inject.rs
index 80f7b491a2a3..b4f65883319e 100644
--- a/codex-rs/core/src/session/inject.rs
+++ b/codex-rs/core/src/session/inject.rs
@@ -73,6 +73,12 @@ impl Session {
                 input,
             ));
         };
+        let Some(thread_activity) = self.try_reserve_turn_activity() else {
+            return Err(TryStartTurnIfIdleError::new(
+                TryStartTurnIfIdleRejectionReason::Busy,
+                input,
+            ));
+        };
         let turn_state = {
             let mut active_turn = self.active_turn.lock().await;
             if active_turn.is_some() {
@@ -82,6 +88,7 @@ impl Session {
                 ));
             }
             let active_turn = active_turn.get_or_insert_with(ActiveTurn::default);
+            active_turn.thread_activity = Some(thread_activity);
             Arc::clone(&active_turn.turn_state)
         };
         drop(reservation);
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index 404075ccf204..8b3f3a899d12 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -1048,7 +1048,7 @@ fn push_prompt_fragment(
 }
 
 impl Session {
-    async fn has_idle_shutdown_blocker(&self) -> bool {
+    pub(crate) async fn has_local_idle_shutdown_blocker(&self) -> bool {
         matches!(self.agent_status.borrow().clone(), AgentStatus::Running)
             || self.conversation.running_state().await.is_some()
             || *self.services.elicitations.subscribe().borrow()
@@ -1057,6 +1057,15 @@ impl Session {
             || !self.list_background_terminals().await.is_empty()
     }
 
+    async fn has_idle_shutdown_blocker(&self) -> bool {
+        self.has_local_idle_shutdown_blocker().await
+            || self
+                .services
+                .agent_control
+                .has_thread_spawn_descendant_idle_shutdown_blocker(self.thread_id)
+                .await
+    }
+
     pub(crate) async fn try_claim_idle_shutdown(
         &self,
     ) -> Option<session::SessionActivityReservation<'_>> {
diff --git a/codex-rs/core/src/session/session.rs b/codex-rs/core/src/session/session.rs
index 89db411437dc..66efbf5b55bc 100644
--- a/codex-rs/core/src/session/session.rs
+++ b/codex-rs/core/src/session/session.rs
@@ -612,6 +612,14 @@ impl Session {
         self.try_reserve_submission(/*close*/ false)
     }
 
+    pub(crate) fn try_reserve_turn_activity(&self) -> Option<ThreadActivityReservation> {
+        self.thread_activity.try_reserve(/*close*/ false)
+    }
+
+    pub(crate) fn try_reserve_dispatched_turn_activity(&self) -> Option<ThreadActivityReservation> {
+        self.thread_activity.try_reserve_dispatched_turn()
+    }
+
     pub(crate) fn try_reserve_idle_shutdown(&self) -> Option<SessionActivityReservation<'_>> {
         let thread = self.thread_activity.try_reserve_idle_shutdown()?;
         let submission = self.submission_lifecycle.try_reserve_idle_shutdown()?;
diff --git a/codex-rs/core/src/session/tests.rs b/codex-rs/core/src/session/tests.rs
index e7911e4b46bf..8175778b4f7a 100644
--- a/codex-rs/core/src/session/tests.rs
+++ b/codex-rs/core/src/session/tests.rs
@@ -10194,7 +10194,7 @@ async fn try_start_turn_if_idle_rejects_active_turn_without_injecting() {
         Vec::<TurnInput>::new(),
         sess.input_queue.get_pending_input(&sess.active_turn).await
     );
-
+    assert!(sess.thread_activity.try_reserve_idle_shutdown().is_none());
     sess.abort_all_tasks(TurnAbortReason::Interrupted).await;
 }
 
@@ -10235,6 +10235,38 @@ async fn interrupt_waits_for_idle_turn_start_to_install_task() {
         .expect("idle turn should be accepted");
     interrupt.await.expect("interrupt task should not panic");
     assert!(sess.active_turn.lock().await.is_none());
+    assert!(sess.thread_activity.try_reserve_idle_shutdown().is_some());
+}
+
+#[tokio::test]
+#[expect(
+    clippy::await_holding_invalid_type,
+    reason = "the test holds session state to pause replacement at a deterministic boundary"
+)]
+async fn replacement_holds_thread_activity_through_startup() {
+    let (sess, tc, _rx) = make_session_and_context_with_rx().await;
+    let task = || NeverEndingTask {
+        kind: TaskKind::Regular,
+        listen_to_cancellation_token: true,
+    };
+    sess.spawn_task(Arc::clone(&tc), Vec::new(), task()).await;
+    let replacement_context = sess.new_default_turn().await;
+    let state = sess.state.lock().await;
+    let replacement = tokio::spawn({
+        let sess = Arc::clone(&sess);
+        async move {
+            sess.spawn_task(replacement_context, Vec::new(), task())
+                .await
+        }
+    });
+    while sess.active_turn.lock().await.is_some() {
+        tokio::task::yield_now().await;
+    }
+    assert!(sess.thread_activity.try_reserve_idle_shutdown().is_none());
+    drop(state);
+    replacement.await.expect("replacement should not panic");
+    sess.abort_all_tasks(TurnAbortReason::Interrupted).await;
+    assert!(sess.thread_activity.try_reserve_idle_shutdown().is_some());
 }
 
 #[tokio::test]
diff --git a/codex-rs/core/src/state/turn.rs b/codex-rs/core/src/state/turn.rs
index d1e31b79949f..20b0408d05f8 100644
--- a/codex-rs/core/src/state/turn.rs
+++ b/codex-rs/core/src/state/turn.rs
@@ -22,6 +22,7 @@ use crate::agent::control::AgentExecutionGuard;
 use crate::session::TurnInputQueue;
 use crate::session::turn_context::TurnContext;
 use crate::tasks::AnySessionTask;
+use crate::thread_activity::ThreadActivityReservation;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_protocol::protocol::ReviewDecision;
 use codex_protocol::protocol::TokenUsage;
@@ -30,6 +31,7 @@ use codex_protocol::protocol::TokenUsage;
 pub(crate) struct ActiveTurn {
     pub(crate) task: Option<RunningTask>,
     pub(crate) turn_state: Arc<Mutex<TurnState>>,
+    pub(crate) thread_activity: Option<ThreadActivityReservation>,
 }
 
 /// Whether mailbox deliveries should still be folded into the current turn.
@@ -58,6 +60,7 @@ impl Default for ActiveTurn {
         Self {
             task: None,
             turn_state: Arc::new(Mutex::new(TurnState::default())),
+            thread_activity: None,
         }
     }
 }
diff --git a/codex-rs/core/src/tasks/mod.rs b/codex-rs/core/src/tasks/mod.rs
index 2dda4b2e29f5..82ad37fc92b0 100644
--- a/codex-rs/core/src/tasks/mod.rs
+++ b/codex-rs/core/src/tasks/mod.rs
@@ -322,9 +322,11 @@ impl Session {
         task: T,
     ) {
         let _turn_start = self.turn_start_lock.lock().await;
-        self.abort_all_tasks_locked(TurnAbortReason::Replaced).await;
+        let (_, replaced_turn_activity) =
+            self.abort_all_tasks_locked(TurnAbortReason::Replaced).await;
         self.clear_connector_selection().await;
         self.start_task_locked(turn_context, input, task).await;
+        drop(replaced_turn_activity);
     }
 
     pub(crate) async fn start_task_locked<T: SessionTask>(
@@ -333,6 +335,30 @@ impl Session {
         input: Vec<TurnInput>,
         task: T,
     ) {
+        let turn_state = {
+            let mut active = self.active_turn.lock().await;
+            if active.is_none() {
+                let Some(thread_activity) = self.try_reserve_dispatched_turn_activity() else {
+                    return;
+                };
+                *active = Some(ActiveTurn {
+                    thread_activity: Some(thread_activity),
+                    ..Default::default()
+                });
+            }
+            let Some(turn) = active.as_mut() else {
+                return;
+            };
+            debug_assert!(turn.task.is_none());
+            if turn.thread_activity.is_none() {
+                let Some(thread_activity) = self.try_reserve_dispatched_turn_activity() else {
+                    return;
+                };
+                turn.thread_activity = Some(thread_activity);
+            }
+            Arc::clone(&turn.turn_state)
+        };
+
         let task: Arc<dyn AnySessionTask> = Arc::new(task);
         let task_kind = task.kind();
         let span_name = task.span_name();
@@ -354,14 +380,7 @@ impl Session {
             .lock()
             .await
             .clear_turn(&turn_context.sub_id);
-
         let pending_items = self.input_queue.get_pending_input(&self.active_turn).await;
-        let turn_state = {
-            let mut active = self.active_turn.lock().await;
-            let turn = active.get_or_insert_with(ActiveTurn::default);
-            debug_assert!(turn.task.is_none());
-            Arc::clone(&turn.turn_state)
-        };
         turn_state.lock().await.token_usage_at_turn_start = token_usage_at_turn_start.clone();
         self.input_queue
             .extend_pending_input_for_turn_state(turn_state.as_ref(), pending_items)
@@ -371,8 +390,15 @@ impl Session {
 
         let turn_extension_data = Arc::clone(&turn_context.extension_data);
         let mut active = self.active_turn.lock().await;
-        let turn = active.get_or_insert_with(ActiveTurn::default);
-        debug_assert!(turn.task.is_none());
+        let Some(turn) = active.as_mut() else {
+            return;
+        };
+        if turn.task.is_some()
+            || turn.thread_activity.is_none()
+            || !Arc::ptr_eq(&turn.turn_state, &turn_state)
+        {
+            return;
+        }
         let agent_execution_guard = self.services.agent_control.execution_guard(
             turn_context.multi_agent_version,
             &turn_context.session_source,
@@ -494,12 +520,18 @@ impl Session {
         let Some(reservation) = self.try_reserve_activity() else {
             return;
         };
+        let Some(thread_activity) = self.try_reserve_turn_activity() else {
+            return;
+        };
         {
             let mut active_turn = self.active_turn.lock().await;
             if active_turn.is_some() {
                 return;
             }
-            *active_turn = Some(ActiveTurn::default());
+            *active_turn = Some(ActiveTurn {
+                thread_activity: Some(thread_activity),
+                ..Default::default()
+            });
         }
         drop(reservation);
 
@@ -516,14 +548,21 @@ impl Session {
     )]
     pub async fn abort_all_tasks(self: &Arc<Self>, reason: TurnAbortReason) {
         let turn_start = self.turn_start_lock.lock().await;
-        let restart_pending = self.abort_all_tasks_locked(reason).await;
+        let (restart_pending, transition_activity) = self.abort_all_tasks_locked(reason).await;
         drop(turn_start);
         if restart_pending {
             self.maybe_start_turn_for_pending_work().await;
         }
+        drop(transition_activity);
     }
 
-    async fn abort_all_tasks_locked(self: &Arc<Self>, reason: TurnAbortReason) -> bool {
+    async fn abort_all_tasks_locked(
+        self: &Arc<Self>,
+        reason: TurnAbortReason,
+    ) -> (
+        bool,
+        Option<crate::thread_activity::ThreadActivityReservation>,
+    ) {
         let mut aborted_turn = false;
         let mut active_turn_to_clear = None;
         let mut turn_context = None;
@@ -548,7 +587,12 @@ impl Session {
             // in-flight approval wait can surface as a model-visible rejection before TurnAborted.
             self.input_queue.clear_pending(active_turn).await;
         }
-        reason == TurnAbortReason::Interrupted && aborted_turn
+        let transition_activity =
+            active_turn_to_clear.and_then(|mut active_turn| active_turn.thread_activity.take());
+        (
+            reason == TurnAbortReason::Interrupted && aborted_turn,
+            transition_activity,
+        )
     }
 
     #[expect(
@@ -620,15 +664,18 @@ impl Session {
             .turn_metadata_state
             .cancel_git_enrichment_task();
 
-        let turn_state = {
+        let completed_turn = {
             let mut active = self.active_turn.lock().await;
             active.as_mut().and_then(|active_turn| {
                 let task = active_turn.task.take()?;
                 task.handle.detach();
-                Some(Arc::clone(&active_turn.turn_state))
+                Some((
+                    Arc::clone(&active_turn.turn_state),
+                    active_turn.thread_activity.take(),
+                ))
             })
         };
-        let Some(turn_state) = turn_state else {
+        let Some((turn_state, thread_activity)) = completed_turn else {
             return;
         };
         let pending_input = self
@@ -818,6 +865,7 @@ impl Session {
             })
         };
         self.send_event(turn_context.as_ref(), event).await;
+        drop(thread_activity);
         self.services
             .guardian_rejection_circuit_breaker
             .lock()
diff --git a/codex-rs/core/src/thread_activity.rs b/codex-rs/core/src/thread_activity.rs
index b2f0b4297b01..a89870f392c8 100644
--- a/codex-rs/core/src/thread_activity.rs
+++ b/codex-rs/core/src/thread_activity.rs
@@ -139,6 +139,36 @@ impl ThreadActivityGate {
         })
     }
 
+    pub(crate) fn descendant_thread_ids(&self, ancestor_id: ThreadId) -> Vec<ThreadId> {
+        let state = self
+            .state
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner);
+        state
+            .nodes
+            .iter()
+            .filter_map(|(thread_id, node)| {
+                if *thread_id == ancestor_id {
+                    return None;
+                }
+                let mut parent_thread_id = node.parent_thread_id;
+                let mut visited = HashSet::new();
+                while let Some(parent_id) = parent_thread_id
+                    && visited.insert(parent_id)
+                {
+                    if parent_id == ancestor_id {
+                        return Some(*thread_id);
+                    }
+                    parent_thread_id = state
+                        .nodes
+                        .get(&parent_id)
+                        .and_then(|node| node.parent_thread_id);
+                }
+                None
+            })
+            .collect()
+    }
+
     fn validate_ancestors(
         state: &ThreadActivityState,
         mut parent_thread_id: Option<ThreadId>,
@@ -191,6 +221,42 @@ impl ThreadActivityGate {
         })
     }
 
+    fn try_reserve_dispatched_turn(
+        self: &Arc<Self>,
+        thread_id: ThreadId,
+        generation: u64,
+    ) -> Option<ThreadActivityReservation> {
+        let mut state = self
+            .state
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner);
+        let node = state.nodes.get(&thread_id)?;
+        if node.generation != generation
+            || node.closed
+            || node.initializing
+            // One committed slot is the queued Shutdown itself; a turn may start only while an
+            // earlier FIFO submission is also still committed.
+            || (node.closing && node.committed <= 1)
+            || !Self::validate_ancestors(
+                &state,
+                node.parent_thread_id,
+                /*allow_closing*/ false,
+            )
+        {
+            return None;
+        }
+        let node = state.nodes.get_mut(&thread_id)?;
+        node.active = node.active.checked_add(1)?;
+        Some(ThreadActivityReservation {
+            gate: Arc::clone(self),
+            thread_id,
+            generation,
+            clear_closing_on_drop: false,
+            delivery_prepared: false,
+            active: true,
+        })
+    }
+
     fn try_reserve_idle_shutdown(
         self: &Arc<Self>,
         thread_id: ThreadId,
@@ -417,6 +483,11 @@ impl ThreadActivityHandle {
             .try_reserve(self.thread_id, self.generation, close)
     }
 
+    pub(crate) fn try_reserve_dispatched_turn(&self) -> Option<ThreadActivityReservation> {
+        self.gate
+            .try_reserve_dispatched_turn(self.thread_id, self.generation)
+    }
+
     pub(crate) fn try_reserve_idle_shutdown(&self) -> Option<ThreadActivityReservation> {
         self.gate
             .try_reserve_idle_shutdown(self.thread_id, self.generation)
diff --git a/codex-rs/core/src/thread_activity_tests.rs b/codex-rs/core/src/thread_activity_tests.rs
index b5b0f8d518cc..eca5d9d1042b 100644
--- a/codex-rs/core/src/thread_activity_tests.rs
+++ b/codex-rs/core/src/thread_activity_tests.rs
@@ -112,27 +112,28 @@ fn unpublication_holds_activity_through_a_provisional_close() {
 fn activity_reservations_are_atomic_and_survive_session_teardown() {
     let gate = Arc::new(ThreadActivityGate::default());
     let parent_id = ThreadId::new();
-    let child_id = ThreadId::new();
+    let intermediate_id = ThreadId::new();
+    let leaf_id = ThreadId::new();
     let parent = register(&gate, parent_id, /*parent_thread_id*/ None);
-    let child = register(&gate, child_id, Some(parent_id));
+    let intermediate = register(&gate, intermediate_id, Some(parent_id));
+    let leaf = register(&gate, leaf_id, Some(intermediate_id));
 
-    let child_activity = child
+    let completion_activity = leaf
         .try_reserve(/*close*/ false)
-        .expect("reserve child activity");
-    assert!(parent.try_reserve_idle_shutdown().is_none());
-    drop(child_activity);
-    let parent_activity = parent
-        .try_reserve(/*close*/ false)
-        .expect("reserve parent activity");
+        .expect("reserve child completion watcher");
     assert!(parent.try_reserve_idle_shutdown().is_none());
+    drop(intermediate);
+    let descendants = gate.descendant_thread_ids(parent_id);
+    assert!(descendants.contains(&intermediate_id));
+    assert!(descendants.contains(&leaf_id));
     drop(parent);
     assert!(gate.register(parent_id, /*parent_thread_id*/ None).is_err());
-    drop(parent_activity);
+    drop(completion_activity);
     let parent = register(&gate, parent_id, /*parent_thread_id*/ None);
     let shutdown = parent
         .try_reserve_idle_shutdown()
         .expect("reserve idle shutdown");
-    assert!(child.try_reserve(/*close*/ false).is_none());
+    assert!(leaf.try_reserve(/*close*/ false).is_none());
     drop(shutdown);
 
     let mut failed_shutdown = parent
@@ -164,3 +165,29 @@ fn receiver_can_finish_before_sender_observes_delivery() {
     let node = state.nodes.get(&thread_id).expect("thread node");
     assert_eq!((node.active, node.committed), (0, 0));
 }
+
+#[test]
+fn accepted_submission_can_start_before_queued_shutdown() {
+    let gate = Arc::new(ThreadActivityGate::default());
+    let thread_id = ThreadId::new();
+    let thread = register(&gate, thread_id, /*parent_thread_id*/ None);
+
+    let mut user_submission = thread
+        .try_reserve(/*close*/ false)
+        .expect("reserve user submission");
+    assert!(user_submission.prepare_delivery());
+    user_submission.commit();
+    let mut queued_shutdown = thread
+        .try_reserve(/*close*/ true)
+        .expect("reserve later shutdown");
+    assert!(queued_shutdown.prepare_delivery());
+    queued_shutdown.commit();
+
+    let turn = thread
+        .try_reserve_dispatched_turn()
+        .expect("accepted user submission starts before shutdown");
+    thread.finish_submission();
+    assert!(thread.try_reserve_dispatched_turn().is_none());
+    drop(turn);
+    thread.finish_submission();
+}
diff --git a/codex-rs/core/tests/suite/agent_execution.rs b/codex-rs/core/tests/suite/agent_execution.rs
index 69db20cc57f6..a2530c05f6c8 100644
--- a/codex-rs/core/tests/suite/agent_execution.rs
+++ b/codex-rs/core/tests/suite/agent_execution.rs
@@ -1,13 +1,19 @@
 use anyhow::Result;
+use codex_core::sandboxing::SandboxPermissions;
 use codex_features::Feature;
+use codex_protocol::models::PermissionProfile;
+use codex_protocol::protocol::AskForApproval;
+use codex_protocol::protocol::EventMsg;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
 use core_test_support::responses::ev_function_call_with_namespace;
 use core_test_support::responses::ev_response_created;
+use core_test_support::responses::ev_shell_command_call_with_args;
 use core_test_support::responses::mount_sse_once_match;
 use core_test_support::responses::sse;
 use core_test_support::responses::start_mock_server;
 use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event;
 use pretty_assertions::assert_eq;
 use serde_json::json;
 use std::time::Duration;
@@ -131,3 +137,95 @@ async fn v2_nested_spawn_checks_shared_active_execution_capacity() -> Result<()>
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn idle_parent_cannot_shutdown_while_unpublished_child_waits_for_approval() -> Result<()> {
+    let server = start_mock_server().await;
+    let spawn_args = serde_json::to_string(&json!({
+        "message": FIRST_TASK,
+        "task_name": "first",
+    }))?;
+    mount_sse_once_match(
+        &server,
+        |request: &wiremock::Request| body_contains(request, FIRST_PROMPT),
+        sse(vec![
+            ev_response_created("parent-spawn-response"),
+            ev_function_call_with_namespace(
+                "spawn-call",
+                MULTI_AGENT_V2_NAMESPACE,
+                "spawn_agent",
+                &spawn_args,
+            ),
+            ev_completed("parent-spawn-response"),
+        ]),
+    )
+    .await;
+    let shell_args = json!({
+        "command": "printf child",
+        "sandbox_permissions": SandboxPermissions::RequireEscalated,
+        "justification": "hold the child at a deterministic approval boundary",
+    });
+    mount_sse_once_match(
+        &server,
+        |request: &wiremock::Request| {
+            body_contains(request, FIRST_TASK) && !has_function_call_output(request, "spawn-call")
+        },
+        sse(vec![
+            ev_response_created("child-response"),
+            ev_shell_command_call_with_args("child-shell", &shell_args),
+            ev_completed("child-response"),
+        ]),
+    )
+    .await;
+    mount_sse_once_match(
+        &server,
+        |request: &wiremock::Request| has_function_call_output(request, "spawn-call"),
+        sse(vec![
+            ev_response_created("parent-followup-response"),
+            ev_assistant_message("parent-followup-message", "spawned"),
+            ev_completed("parent-followup-response"),
+        ]),
+    )
+    .await;
+
+    let mut builder = test_codex().with_model("koffing").with_config(|config| {
+        for feature in [Feature::Collab, Feature::MultiAgentV2] {
+            config
+                .features
+                .enable(feature)
+                .expect("test config should allow feature update");
+        }
+    });
+    let test = builder.build(&server).await?;
+    test.submit_turn_with_approval_and_permission_profile(
+        FIRST_PROMPT,
+        AskForApproval::OnRequest,
+        PermissionProfile::read_only(),
+    )
+    .await?;
+
+    let child_thread_id = test
+        .thread_manager
+        .list_thread_ids()
+        .await
+        .into_iter()
+        .find(|thread_id| *thread_id != test.session_configured.thread_id)
+        .expect("spawned child thread");
+    let child = test.thread_manager.get_thread(child_thread_id).await?;
+    wait_for_event(&child, |event| {
+        matches!(event, EventMsg::ExecApprovalRequest(_))
+    })
+    .await;
+    assert!(!test.codex.try_shutdown_if_idle().await?);
+    let child = test
+        .thread_manager
+        .remove_thread(&child_thread_id)
+        .await
+        .expect("remove child while preserving its lifecycle");
+
+    assert!(!test.codex.try_shutdown_if_idle().await?);
+
+    child.shutdown_and_wait().await?;
+    test.codex.shutdown_and_wait().await?;
+    Ok(())
+}