add fast mode toggle (#13212)

- add a local Fast mode setting in codex-core (similar to how model id
is currently stored on disk locally)
- send `service_tier=priority` on requests when Fast is enabled
- add `/fast` in the TUI and persist it locally
- feature flag

codex-rs/tui/src/app.rs

@@ -1375,6 +1375,7 @@ impl App {
         // Start a fresh in-memory session while preserving resumability via persisted rollout
         // history.
         let model = self.chat_widget.current_model().to_string();
+        let config = self.fresh_session_config();
         let summary = session_summary(
             self.chat_widget.token_usage(),
             self.chat_widget.thread_id(),
@@ -1385,7 +1386,7 @@ impl App {
             tracing::warn!(error = %err, "failed to close all threads");
         }
         let init = crate::chatwidget::ChatWidgetInit {
-            config: self.config.clone(),
+            config,
             frame_requester: tui.frame_requester(),
             app_event_tx: self.app_event_tx.clone(),
             // New sessions start without prefilled message content.
@@ -1414,6 +1415,12 @@ impl App {
         tui.frame_requester().schedule_frame();
     }
 
+    fn fresh_session_config(&self) -> Config {
+        let mut config = self.config.clone();
+        config.service_tier = self.chat_widget.current_service_tier();
+        config
+    }
+
     async fn drain_active_thread_events(&mut self, tui: &mut tui::Tui) -> Result<()> {
         let Some(mut rx) = self.active_thread_rx.take() else {
             return Ok(());
@@ -2532,6 +2539,7 @@ impl App {
                                         model: None,
                                         effort: None,
                                         summary: None,
+                                        service_tier: None,
                                         collaboration_mode: None,
                                         personality: None,
                                     },
@@ -2554,6 +2562,7 @@ impl App {
                                         model: None,
                                         effort: None,
                                         summary: None,
+                                        service_tier: None,
                                         collaboration_mode: None,
                                         personality: None,
                                     },
@@ -2665,6 +2674,39 @@ impl App {
                     }
                 }
             }
+            AppEvent::PersistServiceTierSelection { service_tier } => {
+                self.refresh_status_line();
+                let profile = self.active_profile.as_deref();
+                match ConfigEditsBuilder::new(&self.config.codex_home)
+                    .with_profile(profile)
+                    .set_service_tier(service_tier)
+                    .apply()
+                    .await
+                {
+                    Ok(()) => {
+                        let status = if service_tier.is_some() { "on" } else { "off" };
+                        let mut message = format!("Fast mode set to {status}");
+                        if let Some(profile) = profile {
+                            message.push_str(" for ");
+                            message.push_str(profile);
+                            message.push_str(" profile");
+                        }
+                        self.chat_widget.add_info_message(message, None);
+                    }
+                    Err(err) => {
+                        tracing::error!(error = %err, "failed to persist fast mode selection");
+                        if let Some(profile) = profile {
+                            self.chat_widget.add_error_message(format!(
+                                "Failed to save Fast mode for profile `{profile}`: {err}"
+                            ));
+                        } else {
+                            self.chat_widget.add_error_message(format!(
+                                "Failed to save default Fast mode: {err}"
+                            ));
+                        }
+                    }
+                }
+            }
             AppEvent::PersistRealtimeAudioDeviceSelection { kind, name } => {
                 let builder = match kind {
                     RealtimeAudioDeviceKind::Microphone => {
@@ -2827,6 +2869,7 @@ impl App {
                                 model: None,
                                 effort: None,
                                 summary: None,
+                                service_tier: None,
                                 collaboration_mode: None,
                                 personality: None,
                             }));
@@ -4916,6 +4959,20 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn fresh_session_config_uses_current_service_tier() {
+        let mut app = make_test_app().await;
+        app.chat_widget
+            .set_service_tier(Some(codex_protocol::config_types::ServiceTier::Fast));
+
+        let config = app.fresh_session_config();
+
+        assert_eq!(
+            config.service_tier,
+            Some(codex_protocol::config_types::ServiceTier::Fast)
+        );
+    }
+
     #[tokio::test]
     async fn backtrack_selection_with_duplicate_history_targets_unique_turn() {
         let (mut app, _app_event_rx, mut op_rx) = make_test_app_with_channels().await;