tui: add feature-gated /plan slash command to switch to Plan mode (#10103)

## Summary
Adds a simple `/plan` slash command in the TUI that switches the active
collaboration mode to Plan mode. The command is only available when the
`collaboration_modes` feature is enabled.

## Changes
- Add `plan_mask` helper in `codex-rs/tui/src/collaboration_modes.rs`
- Add `SlashCommand::Plan` metadata in
`codex-rs/tui/src/slash_command.rs`
- Implement and hard-gate `/plan` dispatch in
`codex-rs/tui/src/chatwidget.rs`
- Hide `/plan` when collaboration modes are disabled in
`codex-rs/tui/src/bottom_pane/slash_commands.rs`
- Update command popup tests in
`codex-rs/tui/src/bottom_pane/command_popup.rs`
- Add a focused unit test for `/plan` in
`codex-rs/tui/src/chatwidget/tests.rs`

## Behavior notes
- `/plan` is now a no-op if `Feature::CollaborationModes` is disabled.
- When enabled, `/plan` switches directly to Plan mode without opening
the picker.

## Codex author
`codex resume 019c05da-d7c3-7322-ae2c-3ca38d0ef702`

codex-rs/tui/src/bottom_pane/command_popup.rs

@@ -459,7 +459,7 @@ mod tests {
     #[test]
     fn collab_command_hidden_when_collaboration_modes_disabled() {
         let mut popup = CommandPopup::new(Vec::new(), CommandPopupFlags::default());
-        popup.on_composer_text_change("/coll".to_string());
+        popup.on_composer_text_change("/".to_string());
 
         let cmds: Vec<&str> = popup
             .filtered_items()
@@ -473,6 +473,10 @@ mod tests {
             !cmds.contains(&"collab"),
             "expected '/collab' to be hidden when collaboration modes are disabled, got {cmds:?}"
         );
+        assert!(
+            !cmds.contains(&"plan"),
+            "expected '/plan' to be hidden when collaboration modes are disabled, got {cmds:?}"
+        );
     }
 
     #[test]
@@ -494,6 +498,25 @@ mod tests {
         }
     }
 
+    #[test]
+    fn plan_command_visible_when_collaboration_modes_enabled() {
+        let mut popup = CommandPopup::new(
+            Vec::new(),
+            CommandPopupFlags {
+                collaboration_modes_enabled: true,
+                connectors_enabled: false,
+                personality_command_enabled: true,
+                windows_degraded_sandbox_active: false,
+            },
+        );
+        popup.on_composer_text_change("/plan".to_string());
+
+        match popup.selected_item() {
+            Some(CommandItem::Builtin(cmd)) => assert_eq!(cmd.command(), "plan"),
+            other => panic!("expected plan to be selected for exact match, got {other:?}"),
+        }
+    }
+
     #[test]
     fn personality_command_hidden_when_disabled() {
         let mut popup = CommandPopup::new(

codex-rs/tui/src/bottom_pane/slash_commands.rs

@@ -18,7 +18,10 @@ pub(crate) fn builtins_for_input(
     built_in_slash_commands()
         .into_iter()
         .filter(|(_, cmd)| allow_elevate_sandbox || *cmd != SlashCommand::ElevateSandbox)
-        .filter(|(_, cmd)| collaboration_modes_enabled || *cmd != SlashCommand::Collab)
+        .filter(|(_, cmd)| {
+            collaboration_modes_enabled
+                || !matches!(*cmd, SlashCommand::Collab | SlashCommand::Plan)
+        })
         .filter(|(_, cmd)| connectors_enabled || *cmd != SlashCommand::Apps)
         .filter(|(_, cmd)| personality_command_enabled || *cmd != SlashCommand::Personality)
         .collect()

codex-rs/tui/src/chatwidget.rs

@@ -2668,10 +2668,29 @@ impl ChatWidget {
             SlashCommand::Personality => {
                 self.open_personality_popup();
             }
-            SlashCommand::Collab => {
-                if self.collaboration_modes_enabled() {
-                    self.open_collaboration_modes_popup();
+            SlashCommand::Plan => {
+                if !self.collaboration_modes_enabled() {
+                    self.add_info_message(
+                        "Collaboration modes are disabled.".to_string(),
+                        Some("Enable collaboration modes to use /plan.".to_string()),
+                    );
+                    return;
                 }
+                if let Some(mask) = collaboration_modes::plan_mask(self.models_manager.as_ref()) {
+                    self.set_collaboration_mask(mask);
+                } else {
+                    self.add_info_message("Plan mode unavailable right now.".to_string(), None);
+                }
+            }
+            SlashCommand::Collab => {
+                if !self.collaboration_modes_enabled() {
+                    self.add_info_message(
+                        "Collaboration modes are disabled.".to_string(),
+                        Some("Enable collaboration modes to use /collab.".to_string()),
+                    );
+                    return;
+                }
+                self.open_collaboration_modes_popup();
             }
             SlashCommand::Agent => {
                 self.app_event_tx.send(AppEvent::OpenAgentPicker);
@@ -2844,11 +2863,9 @@ impl ChatWidget {
 
         let trimmed = args.trim();
         match cmd {
-            SlashCommand::Collab => {
+            SlashCommand::Collab | SlashCommand::Plan => {
                 let _ = trimmed;
-                if self.collaboration_modes_enabled() {
-                    self.open_collaboration_modes_popup();
-                }
+                self.dispatch_command(cmd);
             }
             SlashCommand::Review if !trimmed.is_empty() => {
                 self.submit_op(Op::Review {

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

@@ -2267,6 +2267,19 @@ async fn collab_slash_command_opens_picker_and_updates_mode() {
     }
 }
 
+#[tokio::test]
+async fn plan_slash_command_switches_to_plan_mode() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
+    chat.set_feature_enabled(Feature::CollaborationModes, true);
+    let initial = chat.current_collaboration_mode().clone();
+
+    chat.dispatch_command(SlashCommand::Plan);
+
+    assert!(rx.try_recv().is_err(), "plan should not emit an app event");
+    assert_eq!(chat.active_collaboration_mode_kind(), ModeKind::Plan);
+    assert_eq!(chat.current_collaboration_mode(), &initial);
+}
+
 #[tokio::test]
 async fn collaboration_modes_defaults_to_code_on_startup() {
     let codex_home = tempdir().expect("tempdir");

codex-rs/tui/src/collaboration_modes.rs

@@ -59,3 +59,7 @@ pub(crate) fn next_mask(
 pub(crate) fn code_mask(models_manager: &ModelsManager) -> Option<CollaborationModeMask> {
     mask_for_kind(models_manager, ModeKind::Code)
 }
+
+pub(crate) fn plan_mask(models_manager: &ModelsManager) -> Option<CollaborationModeMask> {
+    mask_for_kind(models_manager, ModeKind::Plan)
+}

codex-rs/tui/src/slash_command.rs

@@ -25,6 +25,7 @@ pub enum SlashCommand {
     Fork,
     Init,
     Compact,
+    Plan,
     Collab,
     Agent,
     // Undo,
@@ -63,6 +64,7 @@ impl SlashCommand {
             SlashCommand::Ps => "list background terminals",
             SlashCommand::Model => "choose what model and reasoning effort to use",
             SlashCommand::Personality => "choose a communication style for Codex",
+            SlashCommand::Plan => "switch to Plan mode",
             SlashCommand::Collab => "change collaboration mode (experimental)",
             SlashCommand::Agent => "switch the active agent thread",
             SlashCommand::Approvals => "choose what Codex can do without approval",
@@ -112,6 +114,7 @@ impl SlashCommand {
             | SlashCommand::Exit => true,
             SlashCommand::Rollout => true,
             SlashCommand::TestApproval => true,
+            SlashCommand::Plan => true,
             SlashCommand::Collab => true,
             SlashCommand::Agent => true,
         }