Rebase resume model switch on hydrated history

codex-rs/core/src/codex.rs

@@ -3482,12 +3482,34 @@ mod handlers {
             if let Some(previous_model) = sess.previous_model().await
                 && let Some(previous_context_item) = previous_context_item.as_ref()
                 && previous_model != previous_context_item.model
-                && !update_items
-                    .iter()
-                    .any(Session::is_model_switch_developer_message)
             {
-                // Apply resume/fork model hydration only to model-switch diffing so it does not
-                // suppress other updates (for example personality changes).
+                // Rebase model-switch diffing on resume/fork model hydration so model-switch
+                // updates reflect rollout history while other diffs (for example personality) still
+                // use the real previous turn context.
+                let model_switch_insert_index = update_items
+                    .iter()
+                    .position(Session::is_model_switch_developer_message)
+                    .or_else(|| {
+                        update_items.iter().position(|item| {
+                            let codex_protocol::models::ResponseItem::Message {
+                                role,
+                                content,
+                                ..
+                            } = item
+                            else {
+                                return false;
+                            };
+                            role == "developer"
+                                && content.iter().any(|content_item| {
+                                    matches!(
+                                        content_item,
+                                        codex_protocol::models::ContentItem::InputText { text } if text.starts_with("<personality_spec>")
+                                    )
+                                })
+                        })
+                    });
+                update_items.retain(|item| !Session::is_model_switch_developer_message(item));
+
                 let mut previous_context_item_for_model_switch = previous_context_item.clone();
                 previous_context_item_for_model_switch.model = previous_model;
                 if let Some(model_switch_item) =
@@ -3496,7 +3518,11 @@ mod handlers {
                         &current_context_item,
                     )
                 {
-                    update_items.push(model_switch_item);
+                    if let Some(index) = model_switch_insert_index {
+                        update_items.insert(index, model_switch_item);
+                    } else {
+                        update_items.push(model_switch_item);
+                    }
                 }
             }
             if !update_items.is_empty() {

codex-rs/core/tests/suite/resume.rs

@@ -472,3 +472,98 @@ async fn resume_model_hydration_does_not_suppress_personality_update() -> Result
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn resume_override_matching_rollout_model_skips_model_switch_update() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let mut builder = test_codex().with_config(|config| {
+        config.model = Some("gpt-5.2".to_string());
+    });
+    let initial = builder.build(&server).await?;
+    let codex = Arc::clone(&initial.codex);
+    let home = initial.home.clone();
+    let rollout_path = initial
+        .session_configured
+        .rollout_path
+        .clone()
+        .expect("rollout path");
+
+    let initial_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-initial"),
+            ev_assistant_message("msg-1", "Completed first turn"),
+            ev_completed("resp-initial"),
+        ]),
+    )
+    .await;
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "record initial rollout model".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&codex, |event| matches!(event, EventMsg::TurnComplete(_))).await;
+    let _ = initial_mock.single_request();
+
+    let resumed_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-resume"),
+            ev_assistant_message("msg-2", "Resumed turn"),
+            ev_completed("resp-resume"),
+        ]),
+    )
+    .await;
+
+    let mut resume_builder = test_codex().with_config(|config| {
+        config.model = Some("gpt-5.2-codex".to_string());
+    });
+    let resumed = resume_builder.resume(&server, home, rollout_path).await?;
+    resumed
+        .codex
+        .submit(Op::OverrideTurnContext {
+            cwd: None,
+            approval_policy: None,
+            sandbox_policy: None,
+            windows_sandbox_level: None,
+            model: Some("gpt-5.2".to_string()),
+            effort: None,
+            summary: None,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+    resumed
+        .codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "first turn after override to rollout model".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&resumed.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let request = resumed_mock.single_request();
+    let developer_texts = request.message_input_texts("developer");
+    let model_switch_count = developer_texts
+        .iter()
+        .filter(|text| text.contains("<model_switch>"))
+        .count();
+    assert_eq!(
+        model_switch_count, 0,
+        "did not expect model switch update when override matches rollout model"
+    );
+
+    Ok(())
+}