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/chatwidget.rs

@@ -81,6 +81,7 @@ use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::CollaborationModeMask;
 use codex_protocol::config_types::ModeKind;
 use codex_protocol::config_types::Personality;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Settings;
 #[cfg(target_os = "windows")]
 use codex_protocol::config_types::WindowsSandboxLevel;
@@ -1156,6 +1157,7 @@ impl ChatWidget {
             mask.reasoning_effort = Some(event.reasoning_effort);
         }
         self.refresh_model_display();
+        self.sync_fast_command_enabled();
         self.sync_personality_command_enabled();
         let startup_tooltip_override = self.startup_tooltip_override.take();
         let session_info_cell = history_cell::new_session_info(
@@ -2960,6 +2962,7 @@ impl ChatWidget {
             .bottom_pane
             .set_status_line_enabled(!widget.configured_status_line_items().is_empty());
         widget.bottom_pane.set_collaboration_modes_enabled(true);
+        widget.sync_fast_command_enabled();
         widget.sync_personality_command_enabled();
         widget
             .bottom_pane
@@ -3139,6 +3142,7 @@ impl ChatWidget {
             .bottom_pane
             .set_status_line_enabled(!widget.configured_status_line_items().is_empty());
         widget.bottom_pane.set_collaboration_modes_enabled(true);
+        widget.sync_fast_command_enabled();
         widget.sync_personality_command_enabled();
         widget
             .bottom_pane
@@ -3307,6 +3311,7 @@ impl ChatWidget {
             .bottom_pane
             .set_status_line_enabled(!widget.configured_status_line_items().is_empty());
         widget.bottom_pane.set_collaboration_modes_enabled(true);
+        widget.sync_fast_command_enabled();
         widget.sync_personality_command_enabled();
         widget
             .bottom_pane
@@ -3605,6 +3610,14 @@ impl ChatWidget {
             SlashCommand::Model => {
                 self.open_model_popup();
             }
+            SlashCommand::Fast => {
+                let next_tier = if self.config.service_tier.is_some() {
+                    None
+                } else {
+                    Some(ServiceTier::Fast)
+                };
+                self.set_service_tier_selection(next_tier);
+            }
             SlashCommand::Realtime => {
                 if !self.realtime_conversation_enabled() {
                     return;
@@ -3884,6 +3897,27 @@ impl ChatWidget {
 
         let trimmed = args.trim();
         match cmd {
+            SlashCommand::Fast => {
+                if trimmed.is_empty() {
+                    self.dispatch_command(cmd);
+                    return;
+                }
+                match trimmed.to_ascii_lowercase().as_str() {
+                    "on" => self.set_service_tier_selection(Some(ServiceTier::Fast)),
+                    "off" => self.set_service_tier_selection(None),
+                    "status" => {
+                        let status = if self.config.service_tier.is_some() {
+                            "on"
+                        } else {
+                            "off"
+                        };
+                        self.add_info_message(format!("Fast mode is {status}."), None);
+                    }
+                    _ => {
+                        self.add_error_message("Usage: /fast [on|off|status]".to_string());
+                    }
+                }
+            }
             SlashCommand::Rename if !trimmed.is_empty() => {
                 self.otel_manager.counter("codex.thread.rename", 1, &[]);
                 let Some((prepared_args, _prepared_elements)) =
@@ -4222,6 +4256,7 @@ impl ChatWidget {
             .personality
             .filter(|_| self.config.features.enabled(Feature::Personality))
             .filter(|_| self.current_model_supports_personality());
+        let service_tier = self.fast_mode_enabled().then_some(self.config.service_tier);
         let op = Op::UserTurn {
             items,
             cwd: self.config.cwd.clone(),
@@ -4230,6 +4265,7 @@ impl ChatWidget {
             model: effective_mode.model().to_string(),
             effort: effective_mode.reasoning_effort(),
             summary: None,
+            service_tier,
             final_output_json_schema: None,
             collaboration_mode,
             personality,
@@ -5210,6 +5246,7 @@ impl ChatWidget {
                 model: Some(switch_model_for_events.clone()),
                 effort: Some(Some(default_effort)),
                 summary: None,
+                service_tier: None,
                 collaboration_mode: None,
                 personality: None,
             }));
@@ -5329,6 +5366,7 @@ impl ChatWidget {
                         model: None,
                         effort: None,
                         summary: None,
+                        service_tier: None,
                         collaboration_mode: None,
                         windows_sandbox_level: None,
                         personality: Some(personality),
@@ -6242,6 +6280,7 @@ impl ChatWidget {
                 model: None,
                 effort: None,
                 summary: None,
+                service_tier: None,
                 collaboration_mode: None,
                 personality: None,
             }));
@@ -6777,6 +6816,9 @@ impl ChatWidget {
                 self.reset_realtime_conversation_state();
             }
         }
+        if feature == Feature::FastMode {
+            self.sync_fast_command_enabled();
+        }
         if feature == Feature::Personality {
             self.sync_personality_command_enabled();
         }
@@ -6859,6 +6901,19 @@ impl ChatWidget {
         self.config.personality = Some(personality);
     }
 
+    /// Set Fast mode in the widget's config copy.
+    pub(crate) fn set_service_tier(&mut self, service_tier: Option<ServiceTier>) {
+        self.config.service_tier = service_tier;
+    }
+
+    pub(crate) fn current_service_tier(&self) -> Option<ServiceTier> {
+        self.config.service_tier
+    }
+
+    fn fast_mode_enabled(&self) -> bool {
+        self.config.features.enabled(Feature::FastMode)
+    }
+
     pub(crate) fn set_realtime_audio_device(
         &mut self,
         kind: RealtimeAudioDeviceKind,
@@ -6888,6 +6943,25 @@ impl ChatWidget {
         self.refresh_model_display();
     }
 
+    fn set_service_tier_selection(&mut self, service_tier: Option<ServiceTier>) {
+        self.set_service_tier(service_tier);
+        self.app_event_tx
+            .send(AppEvent::CodexOp(Op::OverrideTurnContext {
+                cwd: None,
+                approval_policy: None,
+                sandbox_policy: None,
+                windows_sandbox_level: None,
+                model: None,
+                effort: None,
+                summary: None,
+                service_tier: Some(service_tier),
+                collaboration_mode: None,
+                personality: None,
+            }));
+        self.app_event_tx
+            .send(AppEvent::PersistServiceTierSelection { service_tier });
+    }
+
     pub(crate) fn current_model(&self) -> &str {
         if !self.collaboration_modes_enabled() {
             return self.current_collaboration_mode.model();
@@ -6914,6 +6988,11 @@ impl ChatWidget {
             .unwrap_or_else(|| "System default".to_string())
     }
 
+    fn sync_fast_command_enabled(&mut self) {
+        self.bottom_pane
+            .set_fast_command_enabled(self.fast_mode_enabled());
+    }
+
     fn sync_personality_command_enabled(&mut self) {
         self.bottom_pane
             .set_personality_command_enabled(self.config.features.enabled(Feature::Personality));