Feat: add model reroute notification (#12001)

### Summary
Builiding off
5c75aa7b89 (diff-058ae8f109a8b84b4b79bbfa45f522c2233b9d9e139696044ae374d50b6196e0),
we have created a `model/rerouted` notification that captures the event
so that consumers can render as expected. Keep the `EventMsg::Warning`
path in core so that this does not affect TUI rendering.

`model/rerouted` is meant to be generic to account for future usage
including capacity planning etc.

codex-rs/app-server/src/bespoke_event_handling.rs

@@ -41,6 +41,7 @@ use codex_app_server_protocol::JSONRPCErrorError;
 use codex_app_server_protocol::McpToolCallError;
 use codex_app_server_protocol::McpToolCallResult;
 use codex_app_server_protocol::McpToolCallStatus;
+use codex_app_server_protocol::ModelReroutedNotification;
 use codex_app_server_protocol::PatchApplyStatus;
 use codex_app_server_protocol::PatchChangeKind as V2PatchChangeKind;
 use codex_app_server_protocol::PlanDeltaNotification;
@@ -68,7 +69,6 @@ use codex_app_server_protocol::TurnInterruptResponse;
 use codex_app_server_protocol::TurnPlanStep;
 use codex_app_server_protocol::TurnPlanUpdatedNotification;
 use codex_app_server_protocol::TurnStatus;
-use codex_app_server_protocol::UserInput as V2UserInput;
 use codex_app_server_protocol::build_turns_from_rollout_items;
 use codex_core::CodexThread;
 use codex_core::parse_command::shlex_join;
@@ -96,8 +96,6 @@ use codex_protocol::request_user_input::RequestUserInputAnswer as CoreRequestUse
 use codex_protocol::request_user_input::RequestUserInputResponse as CoreRequestUserInputResponse;
 use std::collections::HashMap;
 use std::convert::TryFrom;
-use std::hash::Hash;
-use std::hash::Hasher;
 use std::path::PathBuf;
 use std::sync::Arc;
 use tokio::sync::Mutex;
@@ -125,32 +123,18 @@ pub(crate) async fn apply_bespoke_event_handling(
         EventMsg::TurnComplete(_ev) => {
             handle_turn_complete(conversation_id, event_turn_id, &outgoing, &thread_state).await;
         }
-        EventMsg::Warning(warning_event) => {
-            if matches!(api_version, ApiVersion::V2)
-                && is_safety_check_downgrade_warning(&warning_event.message)
-            {
-                let item = ThreadItem::UserMessage {
-                    id: warning_item_id(&event_turn_id, &warning_event.message),
-                    content: vec![V2UserInput::Text {
-                        text: format!("Warning: {}", warning_event.message),
-                        text_elements: Vec::new(),
-                    }],
-                };
-                let started = ItemStartedNotification {
+        EventMsg::Warning(_warning_event) => {}
+        EventMsg::ModelReroute(event) => {
+            if let ApiVersion::V2 = api_version {
+                let notification = ModelReroutedNotification {
                     thread_id: conversation_id.to_string(),
                     turn_id: event_turn_id.clone(),
-                    item: item.clone(),
+                    from_model: event.from_model,
+                    to_model: event.to_model,
+                    reason: event.reason.into(),
                 };
                 outgoing
-                    .send_server_notification(ServerNotification::ItemStarted(started))
-                    .await;
-                let completed = ItemCompletedNotification {
-                    thread_id: conversation_id.to_string(),
-                    turn_id: event_turn_id.clone(),
-                    item,
-                };
-                outgoing
-                    .send_server_notification(ServerNotification::ItemCompleted(completed))
+                    .send_server_notification(ServerNotification::ModelRerouted(notification))
                     .await;
             }
         }
@@ -1318,18 +1302,6 @@ async fn complete_command_execution_item(
         .await;
 }
 
-fn is_safety_check_downgrade_warning(message: &str) -> bool {
-    message.contains("Your account was flagged for potentially high-risk cyber activity")
-        && message.contains("apply for trusted access: https://chatgpt.com/cyber")
-}
-
-fn warning_item_id(turn_id: &str, message: &str) -> String {
-    let mut hasher = std::collections::hash_map::DefaultHasher::new();
-    message.hash(&mut hasher);
-    let digest = hasher.finish();
-    format!("{turn_id}-warning-{digest:x}")
-}
-
 async fn maybe_emit_raw_response_item_completed(
     api_version: ApiVersion,
     conversation_id: ThreadId,
@@ -2060,18 +2032,6 @@ mod tests {
         assert_eq!(item, expected);
     }
 
-    #[test]
-    fn safety_check_downgrade_warning_detection_matches_expected_message() {
-        let warning = "Your account was flagged for potentially high-risk cyber activity and this request was routed to gpt-5.2 as a fallback. To regain access to gpt-5.3-codex, apply for trusted access: https://chatgpt.com/cyber\nLearn more: https://developers.openai.com/codex/concepts/cyber-safety";
-        assert!(is_safety_check_downgrade_warning(warning));
-    }
-
-    #[test]
-    fn safety_check_downgrade_warning_detection_ignores_other_warnings() {
-        let warning = "Model metadata for `mock-model` not found. Defaulting to fallback metadata; this can degrade performance and cause issues.";
-        assert!(!is_safety_check_downgrade_warning(warning));
-    }
-
     #[tokio::test]
     async fn test_handle_error_records_message() -> Result<()> {
         let conversation_id = ThreadId::new();

codex-rs/app-server/src/outgoing_message.rs

@@ -399,6 +399,8 @@ mod tests {
     use codex_app_server_protocol::AuthMode;
     use codex_app_server_protocol::ConfigWarningNotification;
     use codex_app_server_protocol::LoginChatGptCompleteNotification;
+    use codex_app_server_protocol::ModelRerouteReason;
+    use codex_app_server_protocol::ModelReroutedNotification;
     use codex_app_server_protocol::RateLimitSnapshot;
     use codex_app_server_protocol::RateLimitWindow;
     use codex_protocol::ThreadId;
@@ -546,6 +548,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn verify_model_rerouted_notification_serialization() {
+        let notification = ServerNotification::ModelRerouted(ModelReroutedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            from_model: "gpt-5.3-codex".to_string(),
+            to_model: "gpt-5.2".to_string(),
+            reason: ModelRerouteReason::HighRiskCyberActivity,
+        });
+
+        let jsonrpc_notification = OutgoingMessage::AppServerNotification(notification);
+        assert_eq!(
+            json!({
+                "method": "model/rerouted",
+                "params": {
+                    "threadId": "thread-1",
+                    "turnId": "turn-1",
+                    "fromModel": "gpt-5.3-codex",
+                    "toModel": "gpt-5.2",
+                    "reason": "highRiskCyberActivity",
+                },
+            }),
+            serde_json::to_value(jsonrpc_notification)
+                .expect("ensure the notification serializes correctly"),
+            "ensure the notification serializes correctly"
+        );
+    }
+
     #[tokio::test]
     async fn send_response_routes_to_target_connection() {
         let (tx, mut rx) = mpsc::channel::<OutgoingEnvelope>(4);