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/tests.rs

@@ -38,6 +38,7 @@ use codex_protocol::account::PlanType;
 use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::ModeKind;
 use codex_protocol::config_types::Personality;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Settings;
 use codex_protocol::items::AgentMessageContent;
 use codex_protocol::items::AgentMessageItem;
@@ -6519,6 +6520,61 @@ async fn disabled_slash_command_while_task_running_snapshot() {
     assert_snapshot!(blob);
 }
 
+#[tokio::test]
+async fn fast_slash_command_updates_and_persists_local_service_tier() {
+    let (mut chat, mut rx, mut op_rx) = make_chatwidget_manual(Some("gpt-5.3-codex")).await;
+    chat.set_feature_enabled(Feature::FastMode, true);
+
+    chat.dispatch_command(SlashCommand::Fast);
+
+    let events = std::iter::from_fn(|| rx.try_recv().ok()).collect::<Vec<_>>();
+    assert!(
+        events.iter().any(|event| matches!(
+            event,
+            AppEvent::CodexOp(Op::OverrideTurnContext {
+                service_tier: Some(Some(ServiceTier::Fast)),
+                ..
+            })
+        )),
+        "expected fast-mode override app event; events: {events:?}"
+    );
+    assert!(
+        events.iter().any(|event| matches!(
+            event,
+            AppEvent::PersistServiceTierSelection {
+                service_tier: Some(ServiceTier::Fast),
+            }
+        )),
+        "expected fast-mode persistence app event; events: {events:?}"
+    );
+
+    assert_matches!(op_rx.try_recv(), Err(TryRecvError::Empty));
+}
+
+#[tokio::test]
+async fn user_turn_carries_service_tier_after_fast_toggle() {
+    let (mut chat, mut rx, mut op_rx) = make_chatwidget_manual(Some("gpt-5.3-codex")).await;
+    chat.thread_id = Some(ThreadId::new());
+    set_chatgpt_auth(&mut chat);
+    chat.set_feature_enabled(Feature::FastMode, true);
+
+    chat.dispatch_command(SlashCommand::Fast);
+
+    let _events = std::iter::from_fn(|| rx.try_recv().ok()).collect::<Vec<_>>();
+
+    chat.bottom_pane
+        .set_composer_text("hello".to_string(), Vec::new(), Vec::new());
+    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
+
+    match next_submit_op(&mut op_rx) {
+        Op::UserTurn {
+            service_tier: Some(Some(ServiceTier::Fast)),
+            ..
+        } => {}
+        other => panic!("expected Op::UserTurn with fast service tier, got {other:?}"),
+    }
+}
+
 #[tokio::test]
 async fn approvals_popup_shows_disabled_presets() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(None).await;