Default Fast service tier for eligible ChatGPT plans (#19053)

## Why

Enterprise and business-like ChatGPT plans should get Codex's Fast
service tier by default when the user or caller has not made an explicit
service-tier choice. At the same time, callers need a durable way to
choose standard routing without adding a new persisted `standard`
service tier value. This keeps existing config compatibility while
letting core own the managed default policy.

## What changed

- Resolve the effective service tier in core at session creation:
explicit `fast` or `flex` wins, explicit null/clear or
`[notice].fast_default_opt_out = true` resolves to standard routing, and
otherwise eligible ChatGPT plans resolve to Fast when FastMode is
enabled.
- Add `[notice].fast_default_opt_out` as the persisted opt-out marker
for managed Fast defaults.
- Treat app-server/TUI `service_tier: null` as an explicit
standard/clear choice by preserving that intent through config loading.
- Update TUI rendering to use core's effective service tier for startup
and status surfaces while still keeping `config.service_tier` as the
explicit configured choice.
- Update `/fast off` to clear `service_tier`, persist the opt-out
marker, and send explicit standard for subsequent turns.

## Verification

- Added unit coverage for config override/notice handling, service-tier
resolution, runtime null clearing, and `/fast off` turn propagation.
- `cargo build -p codex-cli`

Full test suite was not run locally per author request.

codex-rs/tui/src/app/event_dispatch.rs

@@ -1029,14 +1029,24 @@ impl App {
             AppEvent::PersistServiceTierSelection { service_tier } => {
                 self.refresh_status_line();
                 let profile = self.active_profile.as_deref();
-                match ConfigEditsBuilder::new(&self.config.codex_home)
+                self.config.service_tier = service_tier;
+                let mut edits = ConfigEditsBuilder::new(&self.config.codex_home)
                     .with_profile(profile)
-                    .set_service_tier(service_tier)
-                    .apply()
-                    .await
-                {
+                    .set_service_tier(service_tier);
+                if service_tier.is_none() {
+                    self.config.notices.fast_default_opt_out = Some(true);
+                    edits = edits.set_fast_default_opt_out(/*opted_out*/ true);
+                }
+                match edits.apply().await {
                     Ok(()) => {
-                        let status = if service_tier.is_some() { "on" } else { "off" };
+                        let status = if matches!(
+                            service_tier,
+                            Some(codex_protocol::config_types::ServiceTier::Fast)
+                        ) {
+                            "on"
+                        } else {
+                            "off"
+                        };
                         let mut message = format!("Fast mode set to {status}");
                         if let Some(profile) = profile {
                             message.push_str(" for ");

codex-rs/tui/src/app/session_lifecycle.rs

@@ -622,7 +622,8 @@ impl App {
 
     pub(super) fn fresh_session_config(&self) -> Config {
         let mut config = self.config.clone();
-        config.service_tier = self.chat_widget.current_service_tier();
+        config.service_tier = self.chat_widget.configured_service_tier();
+        config.notices.fast_default_opt_out = self.chat_widget.fast_default_opt_out();
         config
     }
     pub(super) async fn resume_target_session(

codex-rs/tui/src/chatwidget.rs

@@ -776,6 +776,8 @@ pub(crate) struct ChatWidget {
     /// where the overlay may briefly treat new tail content as already cached.
     active_cell_revision: u64,
     config: Config,
+    /// Runtime value resolved by core. `config.service_tier` remains the explicit user choice.
+    effective_service_tier: Option<ServiceTier>,
     /// The unmasked collaboration mode settings (always Default mode).
     ///
     /// Masks are applied on top of this base mode to derive the effective mode.
@@ -2120,6 +2122,7 @@ impl ChatWidget {
         self.current_rollout_path = event.rollout_path.clone();
         self.current_cwd = Some(event.cwd.to_path_buf());
         self.config.cwd = event.cwd.clone();
+        self.effective_service_tier = event.service_tier;
         if let Err(err) = self
             .config
             .permissions
@@ -5106,6 +5109,7 @@ impl ChatWidget {
         let active_cell = Some(Self::placeholder_session_header_cell(&config));
 
         let current_cwd = Some(config.cwd.to_path_buf());
+        let effective_service_tier = config.service_tier;
         let queued_message_edit_binding = queued_message_edit_binding_for_terminal(terminal_info());
         let mut widget = Self {
             app_event_tx: app_event_tx.clone(),
@@ -5124,6 +5128,7 @@ impl ChatWidget {
             active_cell,
             active_cell_revision: 0,
             config,
+            effective_service_tier,
             skills_all: Vec::new(),
             skills_initial_state: None,
             current_collaboration_mode,
@@ -5942,7 +5947,11 @@ impl ChatWidget {
             .personality
             .filter(|_| self.config.features.enabled(Feature::Personality))
             .filter(|_| self.current_model_supports_personality());
-        let service_tier = Some(self.config.service_tier);
+        let service_tier = match self.config.service_tier {
+            Some(service_tier) => Some(Some(service_tier)),
+            None if self.config.notices.fast_default_opt_out == Some(true) => Some(None),
+            None => None,
+        };
         let op = AppCommand::user_turn(
             items,
             self.config.cwd.to_path_buf(),
@@ -9850,12 +9859,21 @@ impl ChatWidget {
     /// 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;
+        self.effective_service_tier = service_tier;
     }
 
     pub(crate) fn current_service_tier(&self) -> Option<ServiceTier> {
+        self.effective_service_tier
+    }
+
+    pub(crate) fn configured_service_tier(&self) -> Option<ServiceTier> {
         self.config.service_tier
     }
 
+    pub(crate) fn fast_default_opt_out(&self) -> Option<bool> {
+        self.config.notices.fast_default_opt_out
+    }
+
     pub(crate) fn status_account_display(&self) -> Option<&StatusAccountDisplay> {
         self.status_account_display.as_ref()
     }
@@ -9932,6 +9950,9 @@ impl ChatWidget {
     }
 
     fn set_service_tier_selection(&mut self, service_tier: Option<ServiceTier>) {
+        if service_tier.is_none() {
+            self.config.notices.fast_default_opt_out = Some(true);
+        }
         self.set_service_tier(service_tier);
         self.app_event_tx.send(AppEvent::CodexOp(
             AppCommand::override_turn_context(

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

@@ -172,7 +172,7 @@ impl ChatWidget {
                 self.open_model_popup();
             }
             SlashCommand::Fast => {
-                let next_tier = if matches!(self.config.service_tier, Some(ServiceTier::Fast)) {
+                let next_tier = if matches!(self.current_service_tier(), Some(ServiceTier::Fast)) {
                     None
                 } else {
                     Some(ServiceTier::Fast)
@@ -527,12 +527,12 @@ impl ChatWidget {
                     "on" => self.set_service_tier_selection(Some(ServiceTier::Fast)),
                     "off" => self.set_service_tier_selection(/*service_tier*/ None),
                     "status" => {
-                        let status = if matches!(self.config.service_tier, Some(ServiceTier::Fast))
-                        {
-                            "on"
-                        } else {
-                            "off"
-                        };
+                        let status =
+                            if matches!(self.current_service_tier(), Some(ServiceTier::Fast)) {
+                                "on"
+                            } else {
+                                "off"
+                            };
                         self.add_info_message(
                             format!("Fast mode is {status}."),
                             /*hint*/ None,

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

@@ -482,7 +482,7 @@ impl ChatWidget {
             )),
             StatusLineItem::SessionId => self.thread_id.map(|id| id.to_string()),
             StatusLineItem::FastMode => Some(
-                if matches!(self.config.service_tier, Some(ServiceTier::Fast)) {
+                if matches!(self.current_service_tier(), Some(ServiceTier::Fast)) {
                     "Fast on".to_string()
                 } else {
                     "Fast off".to_string()
@@ -603,7 +603,7 @@ impl ChatWidget {
     fn model_with_reasoning_display_name(&self) -> String {
         let label = Self::status_line_reasoning_effort_label(self.effective_reasoning_effort());
         let fast_label =
-            if self.should_show_fast_status(self.current_model(), self.config.service_tier) {
+            if self.should_show_fast_status(self.current_model(), self.current_service_tier()) {
                 " fast"
             } else {
                 ""

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

@@ -181,6 +181,7 @@ pub(super) async fn make_chatwidget_manual(
     };
     let current_collaboration_mode = base_mode;
     let active_collaboration_mask = collaboration_modes::default_mask(model_catalog.as_ref());
+    let effective_service_tier = cfg.service_tier;
     let mut widget = ChatWidget {
         app_event_tx,
         codex_op_target: super::CodexOpTarget::Direct(op_tx),
@@ -188,6 +189,7 @@ pub(super) async fn make_chatwidget_manual(
         active_cell: None,
         active_cell_revision: 0,
         config: cfg,
+        effective_service_tier,
         current_collaboration_mode,
         active_collaboration_mask,
         has_chatgpt_account: false,

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

@@ -1550,7 +1550,7 @@ async fn queued_fast_slash_applies_before_next_queued_message() {
 }
 
 #[tokio::test]
-async fn user_turn_clears_service_tier_after_fast_is_turned_off() {
+async fn user_turn_sends_standard_override_after_fast_is_turned_off() {
     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);
@@ -1560,7 +1560,24 @@ async fn user_turn_clears_service_tier_after_fast_is_turned_off() {
     let _events = std::iter::from_fn(|| rx.try_recv().ok()).collect::<Vec<_>>();
 
     chat.dispatch_command_with_args(SlashCommand::Fast, "off".to_string(), Vec::new());
-    let _events = std::iter::from_fn(|| rx.try_recv().ok()).collect::<Vec<_>>();
+    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(None),
+                ..
+            })
+        )),
+        "expected fast-mode off override app event; events: {events:?}"
+    );
+    assert!(
+        events.iter().any(|event| matches!(
+            event,
+            AppEvent::PersistServiceTierSelection { service_tier: None }
+        )),
+        "expected fast-mode opt-out persistence app event; events: {events:?}"
+    );
 
     chat.bottom_pane
         .set_composer_text("hello".to_string(), Vec::new(), Vec::new());
@@ -1571,7 +1588,7 @@ async fn user_turn_clears_service_tier_after_fast_is_turned_off() {
             service_tier: Some(None),
             ..
         } => {}
-        other => panic!("expected Op::UserTurn to clear service tier, got {other:?}"),
+        other => panic!("expected Op::UserTurn with standard service tier override, got {other:?}"),
     }
 }
 

codex-rs/tui/src/history_cell.rs

@@ -50,7 +50,6 @@ use codex_config::types::McpServerTransportConfig;
 use codex_mcp::qualified_mcp_tool_name_prefix;
 use codex_otel::RuntimeMetricsSummary;
 use codex_protocol::account::PlanType;
-use codex_protocol::config_types::ServiceTier;
 #[cfg(test)]
 use codex_protocol::mcp::Resource;
 #[cfg(test)]
@@ -1241,12 +1240,7 @@ pub(crate) fn new_session_info(
     } else {
         if config.show_tooltips
             && let Some(tooltips) = tooltip_override
-                .or_else(|| {
-                    tooltips::get_tooltip(
-                        auth_plan,
-                        matches!(config.service_tier, Some(ServiceTier::Fast)),
-                    )
-                })
+                .or_else(|| tooltips::get_tooltip(auth_plan, show_fast_status))
                 .map(|tip| TooltipHistoryCell::new(tip, &config.cwd))
         {
             parts.push(Box::new(tooltips));