fix(core,app-server) resume with different model (#10719)

## Summary
When resuming with a different model, we should also append a developer
message with the model instructions

## Testing
- [x] Added unit tests

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

@@ -9,6 +9,7 @@ use core_test_support::responses::ev_completed;
 use core_test_support::responses::ev_reasoning_item;
 use core_test_support::responses::ev_response_created;
 use core_test_support::responses::mount_sse_once;
+use core_test_support::responses::mount_sse_sequence;
 use core_test_support::responses::sse;
 use core_test_support::responses::start_mock_server;
 use core_test_support::skip_if_no_network;
@@ -182,12 +183,22 @@ async fn resume_switches_models_preserves_base_instructions() -> Result<()> {
         .unwrap_or_default()
         .to_string();
 
-    let resumed_sse = sse(vec![
-        ev_response_created("resp-resume"),
-        ev_assistant_message("msg-2", "Resumed turn"),
-        ev_completed("resp-resume"),
-    ]);
-    let resumed_mock = mount_sse_once(&server, resumed_sse).await;
+    let resumed_mock = mount_sse_sequence(
+        &server,
+        vec![
+            sse(vec![
+                ev_response_created("resp-resume-1"),
+                ev_assistant_message("msg-2", "Resumed turn"),
+                ev_completed("resp-resume-1"),
+            ]),
+            sse(vec![
+                ev_response_created("resp-resume-2"),
+                ev_assistant_message("msg-3", "Second resumed turn"),
+                ev_completed("resp-resume-2"),
+            ]),
+        ],
+    )
+    .await;
 
     let mut resume_builder = test_codex().with_config(|config| {
         config.model = Some("gpt-5.2-codex".to_string());
@@ -208,13 +219,139 @@ async fn resume_switches_models_preserves_base_instructions() -> Result<()> {
     })
     .await;
 
-    let resumed_body = resumed_mock.single_request().body_json();
-    let resumed_instructions = resumed_body
-        .get("instructions")
-        .and_then(|v| v.as_str())
-        .unwrap_or_default()
-        .to_string();
-    assert_eq!(resumed_instructions, initial_instructions);
+    resumed
+        .codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "Second turn after resume".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&resumed.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let requests = resumed_mock.requests();
+    assert_eq!(requests.len(), 2, "expected two resumed requests");
+
+    let first_resumed = &requests[0];
+    assert_eq!(first_resumed.instructions_text(), initial_instructions);
+    let first_developer_texts = first_resumed.message_input_texts("developer");
+    let first_model_switch_count = first_developer_texts
+        .iter()
+        .filter(|text| text.contains("<model_switch>"))
+        .count();
+    assert!(
+        first_model_switch_count >= 1,
+        "expected model switch message on first post-resume turn"
+    );
+
+    let second_resumed = &requests[1];
+    assert_eq!(second_resumed.instructions_text(), initial_instructions);
+    let second_developer_texts = second_resumed.message_input_texts("developer");
+    let second_model_switch_count = second_developer_texts
+        .iter()
+        .filter(|text| text.contains("<model_switch>"))
+        .count();
+    assert_eq!(
+        second_model_switch_count, 1,
+        "did not expect duplicate model switch message after first post-resume turn"
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn resume_model_switch_is_not_duplicated_after_pre_turn_override() -> 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 instructions".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-5.1-codex-max".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".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, 1);
 
     Ok(())
 }