Promote TUI unified mentions in composer to default mentions feature (#27499)

## Summary

This PR promotes Mentions 2.0 (unified TUI mention popup) to stable and
enables it by default.

- Keep `mentions_v2` as a temporary rollback path to the legacy split
popups (`--disable mentions_v2`).
- Add feature-default and snapshot coverage for the default experience.

## Prior work

- [#19068 — Unified mentions in
TUI](https://github.com/openai/codex/pull/19068)
- [#22375 — Use plugin/list to get plugins for
mentions](https://github.com/openai/codex/pull/22375)
- [#23363 — Unified mentions tweaks and rendering
polish](https://github.com/openai/codex/pull/23363)

## Test plan

- Launch Codex without any feature overrides.
- Type `@` in the TUI composer.
- Confirm the unified mentions menu opens and displays filesystem,
plugin, and skill results.

codex-rs/features/src/lib.rs

@@ -191,7 +191,7 @@ pub enum Feature {
     SkillMcpDependencyInstall,
     /// Removed compatibility flag for deleted skill env var dependency prompting.
     SkillEnvVarDependencyPrompt,
-    /// Enable the unified mention popup prototype.
+    /// Enable the unified mention popup used by default in the TUI.
     MentionsV2,
     /// Allow request_user_input in Default collaboration mode.
     DefaultModeRequestUserInput,
@@ -1119,8 +1119,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::MentionsV2,
         key: "mentions_v2",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::Steer,

codex-rs/features/src/tests.rs

@@ -310,9 +310,9 @@ fn auth_elicitation_is_under_development() {
 }
 
 #[test]
-fn mentions_v2_is_under_development_and_disabled_by_default() {
-    assert_eq!(Feature::MentionsV2.stage(), Stage::UnderDevelopment);
-    assert_eq!(Feature::MentionsV2.default_enabled(), false);
+fn mentions_v2_is_stable_and_enabled_by_default() {
+    assert_eq!(Feature::MentionsV2.stage(), Stage::Stable);
+    assert_eq!(Feature::MentionsV2.default_enabled(), true);
     assert_eq!(feature_for_key("mentions_v2"), Some(Feature::MentionsV2));
 }
 

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

@@ -6354,6 +6354,28 @@ mod tests {
         );
     }
 
+    #[test]
+    fn default_unified_mention_popup_snapshot() {
+        snapshot_composer_state(
+            "default_unified_mention_popup",
+            /*enhanced_keys_supported*/ false,
+            |composer| {
+                let features = codex_features::Features::with_defaults();
+                composer
+                    .set_mentions_v2_enabled(features.enabled(codex_features::Feature::MentionsV2));
+                composer.set_text_content("@sa".to_string(), Vec::new(), Vec::new());
+                composer.set_plugin_mentions(Some(vec![PluginCapabilitySummary {
+                    config_name: "sample@test".to_string(),
+                    display_name: "Sample Plugin".to_string(),
+                    description: Some("Plugin with skills and an MCP server".to_string()),
+                    has_skills: true,
+                    mcp_server_names: vec!["sample".to_string()],
+                    app_connector_ids: Vec::new(),
+                }]));
+            },
+        );
+    }
+
     #[test]
     fn mention_popup_type_prefixes_snapshot() {
         snapshot_composer_state_with_width(

codex-rs/tui/src/bottom_pane/mentions_v2/mod.rs

@@ -1,3 +1,8 @@
+//! The unified mention popup used by default in the TUI.
+//!
+//! The `mentions_v2` feature flag remains temporarily as a rollback path: disabling it restores
+//! the legacy split mention and file-search popups.
+
 mod candidate;
 mod filter;
 mod footer;

codex-rs/tui/src/bottom_pane/snapshots/codex_tui__bottom_pane__chat_composer__tests__default_unified_mention_popup.snap

@@ -0,0 +1,13 @@
+---
+source: tui/src/bottom_pane/chat_composer.rs
+expression: terminal.backend()
+---
+"                                                                                                    "
+"› @sa                                                                                               "
+"                                                                                                    "
+"  Sample Plugin  Plugin with skills and an MCP server                                         Plugin"
+"                                                                                                    "
+"                                                                                                    "
+"                                                                                                    "
+"                                                                                                    "
+"  enter insert · esc close · ←/→ switch search modes     [All Results]   Filesystem Only    Plugins "

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

@@ -33,10 +33,17 @@ impl ChatWidget {
     }
 
     pub(crate) fn open_skills_menu(&mut self) {
+        let list_shortcut = if self.config.features.enabled(Feature::MentionsV2) {
+            '@'
+        } else {
+            '$'
+        };
         let items = vec![
             SelectionItem {
                 name: "List skills".to_string(),
-                description: Some("Tip: press $ to open this list directly.".to_string()),
+                description: Some(format!(
+                    "Tip: press {list_shortcut} to open this list directly."
+                )),
                 actions: vec![Box::new(|tx| {
                     tx.send(AppEvent::OpenSkillsList);
                 })],

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

@@ -0,0 +1,11 @@
+---
+source: tui/src/chatwidget/tests/popups_and_settings.rs
+expression: popup
+---
+  Skills
+  Choose an action
+
+› 1. List skills            Tip: press @ to open this list directly.
+  2. Enable/Disable Skills  Enable or disable skills.
+
+  Press enter to confirm or esc to go back

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

@@ -2575,6 +2575,15 @@ async fn personality_selection_popup_snapshot() {
     assert_chatwidget_snapshot!("personality_selection_popup", popup);
 }
 
+#[tokio::test]
+async fn skills_menu_default_mentions_shortcut_snapshot() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.open_skills_menu();
+
+    let popup = render_bottom_popup(&chat, /*width*/ 80);
+    assert_chatwidget_snapshot!("skills_menu_default_mentions_shortcut", popup);
+}
+
 #[tokio::test]
 async fn model_picker_hides_show_in_picker_false_models_from_cache() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(Some("test-visible-model")).await;