feat: enable ma through /agent (#13246)

<img width="639" height="139" alt="Screenshot 2026-03-02 at 16 06 41"
src="https://github.com/user-attachments/assets/c006fcec-c1e7-41ce-bb84-c121d5ffb501"
/>

Then
<img width="372" height="37" alt="Screenshot 2026-03-02 at 16 06 49"
src="https://github.com/user-attachments/assets/aa4ad703-e7e7-4620-9032-f5cd4f48ff79"
/>

codex-rs/tui/src/app.rs

@@ -1201,6 +1201,15 @@ impl App {
             }
         }
 
+        let has_non_primary_agent_thread = self
+            .agent_picker_threads
+            .keys()
+            .any(|thread_id| Some(*thread_id) != self.primary_thread_id);
+        if !self.config.features.enabled(Feature::Collab) && !has_non_primary_agent_thread {
+            self.chat_widget.open_multi_agent_enable_prompt();
+            return;
+        }
+
         if self.agent_picker_threads.is_empty() {
             self.chat_widget
                 .add_info_message("No agents available yet.".to_string(), None);
@@ -3601,6 +3610,7 @@ mod tests {
     use crate::history_cell::HistoryCell;
     use crate::history_cell::UserHistoryCell;
     use crate::history_cell::new_session_info;
+    use assert_matches::assert_matches;
     use codex_core::CodexAuth;
     use codex_core::config::ConfigBuilder;
     use codex_core::config::ConfigOverrides;
@@ -3966,6 +3976,51 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn open_agent_picker_prompts_to_enable_multi_agent_when_disabled() -> Result<()> {
+        let (mut app, mut app_event_rx, _op_rx) = make_test_app_with_channels().await;
+
+        app.open_agent_picker().await;
+        app.chat_widget
+            .handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+        assert_matches!(
+            app_event_rx.try_recv(),
+            Ok(AppEvent::UpdateFeatureFlags { updates }) if updates == vec![(Feature::Collab, true)]
+        );
+        let cell = match app_event_rx.try_recv() {
+            Ok(AppEvent::InsertHistoryCell(cell)) => cell,
+            other => panic!("expected InsertHistoryCell event, got {other:?}"),
+        };
+        let rendered = cell
+            .display_lines(120)
+            .into_iter()
+            .map(|line| line.to_string())
+            .collect::<Vec<_>>()
+            .join("\n");
+        assert!(rendered.contains("Multi-agent will be enabled in the next session."));
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn open_agent_picker_allows_existing_agent_threads_when_feature_is_disabled() -> Result<()>
+    {
+        let (mut app, mut app_event_rx, _op_rx) = make_test_app_with_channels().await;
+        let thread_id = ThreadId::new();
+        app.thread_event_channels
+            .insert(thread_id, ThreadEventChannel::new(1));
+
+        app.open_agent_picker().await;
+        app.chat_widget
+            .handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+        assert_matches!(
+            app_event_rx.try_recv(),
+            Ok(AppEvent::SelectAgentThread(selected_thread_id)) if selected_thread_id == thread_id
+        );
+        Ok(())
+    }
+
     #[tokio::test]
     async fn refresh_pending_thread_approvals_only_lists_inactive_threads() {
         let mut app = make_test_app().await;

codex-rs/tui/src/chatwidget.rs

@@ -164,6 +164,10 @@ const PLAN_IMPLEMENTATION_TITLE: &str = "Implement this plan?";
 const PLAN_IMPLEMENTATION_YES: &str = "Yes, implement this plan";
 const PLAN_IMPLEMENTATION_NO: &str = "No, stay in Plan mode";
 const PLAN_IMPLEMENTATION_CODING_MESSAGE: &str = "Implement the plan.";
+const MULTI_AGENT_ENABLE_TITLE: &str = "Enable multi-agent?";
+const MULTI_AGENT_ENABLE_YES: &str = "Yes, enable";
+const MULTI_AGENT_ENABLE_NO: &str = "Not now";
+const MULTI_AGENT_ENABLE_NOTICE: &str = "Multi-agent will be enabled in the next session.";
 const PLAN_MODE_REASONING_SCOPE_TITLE: &str = "Apply reasoning change";
 const PLAN_MODE_REASONING_SCOPE_PLAN_ONLY: &str = "Apply to Plan mode override";
 const PLAN_MODE_REASONING_SCOPE_ALL_MODES: &str = "Apply to global default and Plan mode override";
@@ -1568,6 +1572,41 @@ impl ChatWidget {
         });
     }
 
+    pub(crate) fn open_multi_agent_enable_prompt(&mut self) {
+        let items = vec![
+            SelectionItem {
+                name: MULTI_AGENT_ENABLE_YES.to_string(),
+                description: Some(
+                    "Save the setting now. You will need a new session to use it.".to_string(),
+                ),
+                actions: vec![Box::new(|tx| {
+                    tx.send(AppEvent::UpdateFeatureFlags {
+                        updates: vec![(Feature::Collab, true)],
+                    });
+                    tx.send(AppEvent::InsertHistoryCell(Box::new(
+                        history_cell::new_warning_event(MULTI_AGENT_ENABLE_NOTICE.to_string()),
+                    )));
+                })],
+                dismiss_on_select: true,
+                ..Default::default()
+            },
+            SelectionItem {
+                name: MULTI_AGENT_ENABLE_NO.to_string(),
+                description: Some("Keep multi-agent disabled.".to_string()),
+                dismiss_on_select: true,
+                ..Default::default()
+            },
+        ];
+
+        self.bottom_pane.show_selection_view(SelectionViewParams {
+            title: Some(MULTI_AGENT_ENABLE_TITLE.to_string()),
+            subtitle: Some("Multi-agent is currently disabled in your config.".to_string()),
+            footer_hint: Some(standard_popup_hint_line()),
+            items,
+            ..Default::default()
+        });
+    }
+
     pub(crate) fn set_token_info(&mut self, info: Option<TokenUsageInfo>) {
         match info {
             Some(info) => self.apply_token_info(info),

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__multi_agent_enable_prompt.snap

@@ -0,0 +1,12 @@
+---
+source: tui/src/chatwidget/tests.rs
+assertion_line: 6001
+expression: popup
+---
+  Enable multi-agent?
+  Multi-agent is currently disabled in your config.
+
+› 1. Yes, enable  Save the setting now. You will need a new session to use it.
+  2. Not now      Keep multi-agent disabled.
+
+  Press enter to confirm or esc to go back

codex-rs/tui/src/chatwidget/tests.rs

@@ -5991,6 +5991,35 @@ async fn experimental_popup_shows_js_repl_node_requirement() {
     );
 }
 
+#[tokio::test]
+async fn multi_agent_enable_prompt_snapshot() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(None).await;
+
+    chat.open_multi_agent_enable_prompt();
+
+    let popup = render_bottom_popup(&chat, 80);
+    assert_snapshot!("multi_agent_enable_prompt", popup);
+}
+
+#[tokio::test]
+async fn multi_agent_enable_prompt_updates_feature_and_emits_notice() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
+
+    chat.open_multi_agent_enable_prompt();
+    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
+
+    assert_matches!(
+        rx.try_recv(),
+        Ok(AppEvent::UpdateFeatureFlags { updates }) if updates == vec![(Feature::Collab, true)]
+    );
+    let cell = match rx.try_recv() {
+        Ok(AppEvent::InsertHistoryCell(cell)) => cell,
+        other => panic!("expected InsertHistoryCell event, got {other:?}"),
+    };
+    let rendered = lines_to_single_string(&cell.display_lines(120));
+    assert!(rendered.contains("Multi-agent will be enabled in the next session."));
+}
+
 #[tokio::test]
 async fn model_selection_popup_snapshot() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(Some("gpt-5-codex")).await;