diff --git a/codex-rs/tui/src/app/app_server_event_targets.rs b/codex-rs/tui/src/app/app_server_event_targets.rs
index 5e5fe181a..291082000 100644
--- a/codex-rs/tui/src/app/app_server_event_targets.rs
+++ b/codex-rs/tui/src/app/app_server_event_targets.rs
@@ -39,6 +39,19 @@ pub(super) enum ServerNotificationThreadTarget {
     Global,
 }
 
+impl ServerNotificationThreadTarget {
+    /// The thread this notification targets, if it is addressed to a specific
+    /// thread. Non-thread targets (app-scoped, global, invalid) return `None`.
+    pub(super) fn thread_id(&self) -> Option<ThreadId> {
+        match self {
+            ServerNotificationThreadTarget::Thread(thread_id) => Some(*thread_id),
+            ServerNotificationThreadTarget::InvalidThreadId(_)
+            | ServerNotificationThreadTarget::AppScoped
+            | ServerNotificationThreadTarget::Global => None,
+        }
+    }
+}
+
 pub(super) fn server_notification_thread_target(
     notification: &ServerNotification,
 ) -> ServerNotificationThreadTarget {
diff --git a/codex-rs/tui/src/app/app_server_events.rs b/codex-rs/tui/src/app/app_server_events.rs
index 8c18ef3f9..b4d7ad8f8 100644
--- a/codex-rs/tui/src/app/app_server_events.rs
+++ b/codex-rs/tui/src/app/app_server_events.rs
@@ -127,10 +127,21 @@ impl App {
                     self.note_session_recap_turn_completed(thread_id, &notification.turn);
                 }
 
-                let result = if self.primary_thread_id == Some(thread_id)
-                    || self.primary_thread_id.is_none()
-                {
-                    self.enqueue_primary_thread_notification(notification).await
+                let result = if self.primary_thread_id == Some(thread_id) {
+                    self.enqueue_thread_notification(thread_id, notification)
+                        .await
+                } else if self.primary_thread_id.is_none() {
+                    // Startup pre-primary window: the primary thread has not
+                    // finished starting yet. Only buffer as a pending primary
+                    // event when the target thread is not already tracked; a
+                    // known loaded/scheduled thread routes to its own channel
+                    // so its events never leak into the primary buffer.
+                    if self.is_tracked_thread(thread_id) {
+                        self.enqueue_thread_notification(thread_id, notification)
+                            .await
+                    } else {
+                        self.enqueue_primary_thread_notification(notification).await
+                    }
                 } else {
                     self.enqueue_thread_notification(thread_id, notification)
                         .await
@@ -202,12 +213,20 @@ impl App {
             return;
         };
 
-        let result =
-            if self.primary_thread_id == Some(thread_id) || self.primary_thread_id.is_none() {
-                self.enqueue_primary_thread_request(request).await
-            } else {
+        let result = if self.primary_thread_id == Some(thread_id) {
+            self.enqueue_thread_request(thread_id, request).await
+        } else if self.primary_thread_id.is_none() {
+            // Startup pre-primary window: only buffer requests for threads that
+            // are not already tracked. Known loaded/scheduled threads route to
+            // their own channel instead of the pending primary buffer.
+            if self.is_tracked_thread(thread_id) {
                 self.enqueue_thread_request(thread_id, request).await
-            };
+            } else {
+                self.enqueue_primary_thread_request(request).await
+            }
+        } else {
+            self.enqueue_thread_request(thread_id, request).await
+        };
         if let Err(err) = result {
             tracing::warn!("failed to enqueue app-server request: {err}");
         }
diff --git a/codex-rs/tui/src/app/tests.rs b/codex-rs/tui/src/app/tests.rs
index 5b72dda2d..a7f99335c 100644
--- a/codex-rs/tui/src/app/tests.rs
+++ b/codex-rs/tui/src/app/tests.rs
@@ -315,6 +315,142 @@ async fn resolved_buffered_approval_does_not_become_actionable_after_drain() ->
     Ok(())
 }
 
+#[tokio::test]
+async fn enqueue_primary_thread_session_drops_unrelated_pre_primary_notifications() -> Result<()> {
+    let (mut app, mut app_event_rx, _op_rx) = make_test_app_with_channels().await;
+    let primary_thread_id = ThreadId::new();
+    let unrelated_thread_id = ThreadId::new();
+
+    // Pre-primary startup window: no primary thread is set yet, so events are
+    // buffered as pending primary events.
+    assert!(app.primary_thread_id.is_none());
+    app.enqueue_primary_thread_notification(agent_message_delta_notification(
+        unrelated_thread_id,
+        "turn-x",
+        "item-x",
+        "unrelated",
+    ))
+    .await?;
+    app.enqueue_primary_thread_notification(agent_message_delta_notification(
+        primary_thread_id,
+        "turn-1",
+        "item-1",
+        "primary",
+    ))
+    .await?;
+
+    app.enqueue_primary_thread_session(
+        test_thread_session(primary_thread_id, test_path_buf("/tmp/project")),
+        Vec::new(),
+    )
+    .await?;
+    while app_event_rx.try_recv().is_ok() {}
+
+    // The unrelated thread must not leak a channel out of the primary buffer flush.
+    assert!(
+        !app.thread_event_channels.contains_key(&unrelated_thread_id),
+        "unrelated thread should not receive a channel from the primary flush"
+    );
+
+    let rx = app
+        .active_thread_rx
+        .as_mut()
+        .expect("primary thread receiver should be active");
+
+    let event = time::timeout(Duration::from_millis(50), rx.recv())
+        .await
+        .expect("timed out waiting for buffered primary notification")
+        .expect("channel closed unexpectedly");
+    assert!(
+        matches!(
+            &event,
+            ThreadBufferedEvent::Notification(ServerNotification::AgentMessageDelta(delta))
+                if delta.thread_id == primary_thread_id.to_string() && delta.delta == "primary"
+        ),
+        "expected the primary thread's buffered notification to replay first, got {event:?}"
+    );
+
+    // The unrelated-thread notification must have been dropped, not replayed onto
+    // the primary thread's stream.
+    let extra = time::timeout(Duration::from_millis(50), rx.recv()).await;
+    assert!(
+        extra.is_err(),
+        "unrelated pre-primary notification should be dropped, not replayed onto the primary"
+    );
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn enqueue_primary_thread_session_routes_known_thread_pre_primary_notifications() -> Result<()>
+{
+    let (mut app, mut app_event_rx, _op_rx) = make_test_app_with_channels().await;
+    let primary_thread_id = ThreadId::new();
+    let known_thread_id = ThreadId::new();
+
+    // A previously loaded/scheduled thread is already tracked before the primary
+    // thread finishes starting.
+    app.thread_event_channels.insert(
+        known_thread_id,
+        ThreadEventChannel::new(THREAD_EVENT_CHANNEL_CAPACITY),
+    );
+    // Mark the known thread active so routed events are delivered on its channel
+    // receiver rather than parked in its store buffer.
+    app.set_thread_active(known_thread_id, /*active*/ true)
+        .await;
+
+    assert!(app.primary_thread_id.is_none());
+    app.enqueue_primary_thread_notification(agent_message_delta_notification(
+        known_thread_id,
+        "turn-k",
+        "item-k",
+        "known",
+    ))
+    .await?;
+
+    app.enqueue_primary_thread_session(
+        test_thread_session(primary_thread_id, test_path_buf("/tmp/project")),
+        Vec::new(),
+    )
+    .await?;
+    while app_event_rx.try_recv().is_ok() {}
+
+    // The buffered event for the already-tracked thread is delivered on that
+    // thread's own channel, not on the primary thread's stream.
+    let mut known_rx = app
+        .thread_event_channels
+        .get_mut(&known_thread_id)
+        .expect("known thread channel should still exist")
+        .receiver
+        .take()
+        .expect("known thread receiver should be available");
+    let known_event = time::timeout(Duration::from_millis(50), known_rx.recv())
+        .await
+        .expect("timed out waiting for known-thread notification")
+        .expect("known thread channel closed unexpectedly");
+    assert!(
+        matches!(
+            &known_event,
+            ThreadBufferedEvent::Notification(ServerNotification::AgentMessageDelta(delta))
+                if delta.thread_id == known_thread_id.to_string() && delta.delta == "known"
+        ),
+        "expected known-thread notification to route to its own channel, got {known_event:?}"
+    );
+
+    // Nothing from the known thread should have leaked onto the primary stream.
+    let rx = app
+        .active_thread_rx
+        .as_mut()
+        .expect("primary thread receiver should be active");
+    let leaked = time::timeout(Duration::from_millis(50), rx.recv()).await;
+    assert!(
+        leaked.is_err(),
+        "known-thread notification must not be replayed onto the primary stream"
+    );
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn enqueue_primary_thread_session_replays_turns_before_initial_prompt_submit() -> Result<()> {
     let (mut app, mut app_event_rx, _op_rx) = make_test_app_with_channels().await;
diff --git a/codex-rs/tui/src/app/thread_routing.rs b/codex-rs/tui/src/app/thread_routing.rs
index 3f3d8b5b7..51da4b3a7 100644
--- a/codex-rs/tui/src/app/thread_routing.rs
+++ b/codex-rs/tui/src/app/thread_routing.rs
@@ -5,6 +5,8 @@
 //! when the visible thread changes.
 
 use super::*;
+use crate::app::app_server_event_targets::server_notification_thread_target;
+use crate::app::app_server_event_targets::server_request_thread_id;
 use crate::app::app_server_requests::AppServerRequestResolution;
 use crate::session_resume::read_session_model;
 use codex_app_server_protocol::ThreadExternalAgentStartStatus;
@@ -1194,11 +1196,37 @@ impl App {
         for pending_event in pending {
             match pending_event {
                 ThreadBufferedEvent::Notification(notification) => {
-                    self.enqueue_thread_notification(thread_id, notification)
-                        .await?;
+                    let event_thread_id =
+                        server_notification_thread_target(&notification).thread_id();
+                    match self.resolve_pending_primary_flush_target(event_thread_id, thread_id) {
+                        Some(target_thread_id) => {
+                            self.enqueue_thread_notification(target_thread_id, notification)
+                                .await?;
+                        }
+                        None => {
+                            tracing::debug!(
+                                ?event_thread_id,
+                                %thread_id,
+                                "dropping buffered notification for unrelated untracked thread while starting primary thread"
+                            );
+                        }
+                    }
                 }
                 ThreadBufferedEvent::Request(request) => {
-                    self.enqueue_thread_request(thread_id, request).await?;
+                    let event_thread_id = server_request_thread_id(&request);
+                    match self.resolve_pending_primary_flush_target(event_thread_id, thread_id) {
+                        Some(target_thread_id) => {
+                            self.enqueue_thread_request(target_thread_id, request)
+                                .await?;
+                        }
+                        None => {
+                            tracing::debug!(
+                                ?event_thread_id,
+                                %thread_id,
+                                "dropping buffered request for unrelated untracked thread while starting primary thread"
+                            );
+                        }
+                    }
                 }
                 ThreadBufferedEvent::HistoryEntryResponse(event) => {
                     self.enqueue_thread_history_entry_response(thread_id, event)
@@ -1215,6 +1243,42 @@ impl App {
         Ok(())
     }
 
+    /// Whether `thread_id` is already tracked by the TUI (has an event channel
+    /// or is a registered side thread). Used during the startup pre-primary
+    /// window to distinguish known loaded/scheduled threads from the primary
+    /// thread that is still starting.
+    pub(super) fn is_tracked_thread(&self, thread_id: ThreadId) -> bool {
+        self.thread_event_channels.contains_key(&thread_id)
+            || self.side_threads.contains_key(&thread_id)
+    }
+
+    /// Resolve which thread a buffered pre-primary event should flush onto once
+    /// the primary thread (`primary_thread_id`) has started.
+    ///
+    /// - Events that target the started primary thread flush onto the primary.
+    /// - Events for another already-tracked thread flush onto that thread's own
+    ///   channel so they are not misrouted onto the primary's stream.
+    /// - Events for an unrelated, untracked thread return `None` and are dropped
+    ///   by the caller; they never belonged to the primary being started.
+    /// - Events with no resolvable thread target flush onto the primary, matching
+    ///   the historical behavior (such events are not buffered here in practice).
+    fn resolve_pending_primary_flush_target(
+        &self,
+        event_thread_id: Option<ThreadId>,
+        primary_thread_id: ThreadId,
+    ) -> Option<ThreadId> {
+        match event_thread_id {
+            None => Some(primary_thread_id),
+            Some(event_thread_id) if event_thread_id == primary_thread_id => {
+                Some(primary_thread_id)
+            }
+            Some(event_thread_id) if self.is_tracked_thread(event_thread_id) => {
+                Some(event_thread_id)
+            }
+            Some(_) => None,
+        }
+    }
+
     pub(super) async fn enqueue_primary_thread_notification(
         &mut self,
         notification: ServerNotification,