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

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

## Testing
- [x] Added unit tests

Author: dylan-hurd-oai

codex-rs/app-server/tests/suite/send_message.rs

@@ -1,5 +1,7 @@
 use anyhow::Result;
 use app_test_support::McpProcess;
+use app_test_support::create_fake_rollout;
+use app_test_support::rollout_path;
 use app_test_support::to_response;
 use codex_app_server_protocol::AddConversationListenerParams;
 use codex_app_server_protocol::AddConversationSubscriptionResponse;
@@ -9,18 +11,25 @@ use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::NewConversationParams;
 use codex_app_server_protocol::NewConversationResponse;
 use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::ResumeConversationParams;
+use codex_app_server_protocol::ResumeConversationResponse;
 use codex_app_server_protocol::SendUserMessageParams;
 use codex_app_server_protocol::SendUserMessageResponse;
 use codex_execpolicy::Policy;
 use codex_protocol::ThreadId;
+use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::DeveloperInstructions;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::RawResponseItemEvent;
+use codex_protocol::protocol::RolloutItem;
+use codex_protocol::protocol::RolloutLine;
 use codex_protocol::protocol::SandboxPolicy;
+use codex_protocol::protocol::TurnContextItem;
 use core_test_support::responses;
 use pretty_assertions::assert_eq;
+use std::io::Write;
 use std::path::Path;
 use std::path::PathBuf;
 use tempfile::TempDir;
@@ -263,6 +272,114 @@ async fn test_send_message_session_not_found() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn resume_with_model_mismatch_appends_model_switch_once() -> Result<()> {
+    let server = responses::start_mock_server().await;
+    let response_mock = responses::mount_sse_sequence(
+        &server,
+        vec![
+            responses::sse(vec![
+                responses::ev_response_created("resp-1"),
+                responses::ev_assistant_message("msg-1", "Done"),
+                responses::ev_completed("resp-1"),
+            ]),
+            responses::sse(vec![
+                responses::ev_response_created("resp-2"),
+                responses::ev_assistant_message("msg-2", "Done again"),
+                responses::ev_completed("resp-2"),
+            ]),
+        ],
+    )
+    .await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let filename_ts = "2025-01-02T12-00-00";
+    let meta_rfc3339 = "2025-01-02T12:00:00Z";
+    let preview = "Resume me";
+    let conversation_id = create_fake_rollout(
+        codex_home.path(),
+        filename_ts,
+        meta_rfc3339,
+        preview,
+        Some("mock_provider"),
+        None,
+    )?;
+    let rollout_path = rollout_path(codex_home.path(), filename_ts, &conversation_id);
+    append_rollout_turn_context(&rollout_path, meta_rfc3339, "previous-model")?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let resume_id = mcp
+        .send_resume_conversation_request(ResumeConversationParams {
+            path: Some(rollout_path.clone()),
+            conversation_id: None,
+            history: None,
+            overrides: Some(NewConversationParams {
+                model: Some("gpt-5.2-codex".to_string()),
+                ..Default::default()
+            }),
+        })
+        .await?;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("sessionConfigured"),
+    )
+    .await??;
+    let resume_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(resume_id)),
+    )
+    .await??;
+    let ResumeConversationResponse {
+        conversation_id, ..
+    } = to_response::<ResumeConversationResponse>(resume_resp)?;
+
+    let add_listener_id = mcp
+        .send_add_conversation_listener_request(AddConversationListenerParams {
+            conversation_id,
+            experimental_raw_events: false,
+        })
+        .await?;
+    let add_listener_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(add_listener_id)),
+    )
+    .await??;
+    let AddConversationSubscriptionResponse { subscription_id: _ } =
+        to_response::<_>(add_listener_resp)?;
+
+    send_message("hello after resume", conversation_id, &mut mcp).await?;
+    send_message("second turn", conversation_id, &mut mcp).await?;
+
+    let requests = response_mock.requests();
+    assert_eq!(requests.len(), 2, "expected two model requests");
+
+    let first_developer_texts = requests[0].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, got {first_developer_texts:?}"
+    );
+
+    let second_developer_texts = requests[1].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 on second post-resume turn, got {second_developer_texts:?}"
+    );
+
+    Ok(())
+}
+
 // ---------------------------------------------------------------------------
 // Helpers
 // ---------------------------------------------------------------------------
@@ -438,3 +555,28 @@ fn content_texts(content: &[ContentItem]) -> Vec<&str> {
         })
         .collect()
 }
+
+fn append_rollout_turn_context(path: &Path, timestamp: &str, model: &str) -> std::io::Result<()> {
+    let line = RolloutLine {
+        timestamp: timestamp.to_string(),
+        item: RolloutItem::TurnContext(TurnContextItem {
+            cwd: PathBuf::from("/"),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::DangerFullAccess,
+            model: model.to_string(),
+            personality: None,
+            collaboration_mode: None,
+            effort: None,
+            summary: ReasoningSummary::Auto,
+            user_instructions: None,
+            developer_instructions: None,
+            final_output_json_schema: None,
+            truncation_policy: None,
+        }),
+    };
+    let serialized = serde_json::to_string(&line).map_err(std::io::Error::other)?;
+    std::fs::OpenOptions::new()
+        .append(true)
+        .open(path)?
+        .write_all(format!("{serialized}\n").as_bytes())
+}

codex-rs/core/src/codex.rs

@@ -1176,32 +1176,26 @@ impl Session {
                 {
                     let mut state = self.state.lock().await;
                     state.initial_context_seeded = false;
+                    state.pending_resume_previous_model = None;
                 }
 
                 // If resuming, warn when the last recorded model differs from the current one.
-                if let Some(prev) = rollout_items.iter().rev().find_map(|it| {
-                    if let RolloutItem::TurnContext(ctx) = it {
-                        Some(ctx.model.as_str())
-                    } else {
-                        None
-                    }
-                }) {
-                    let curr = turn_context.model_info.slug.as_str();
-                    if prev != curr {
-                        warn!(
-                            "resuming session with different model: previous={prev}, current={curr}"
-                        );
-                        self.send_event(
-                            &turn_context,
-                            EventMsg::Warning(WarningEvent {
-                                message: format!(
-                                    "This session was recorded with model `{prev}` but is resuming with `{curr}`. \
+                let curr = turn_context.model_info.slug.as_str();
+                if let Some(prev) = Self::last_model_name(&rollout_items, curr) {
+                    warn!("resuming session with different model: previous={prev}, current={curr}");
+                    self.send_event(
+                        &turn_context,
+                        EventMsg::Warning(WarningEvent {
+                            message: format!(
+                                "This session was recorded with model `{prev}` but is resuming with `{curr}`. \
                          Consider switching back to `{prev}` as it may affect Codex performance."
-                                ),
-                            }),
-                        )
-                            .await;
-                    }
+                            ),
+                        }),
+                    )
+                    .await;
+
+                    let mut state = self.state.lock().await;
+                    state.pending_resume_previous_model = Some(prev.to_string());
                 }
 
                 // Always add response items to conversation history
@@ -1260,13 +1254,33 @@ impl Session {
         }
     }
 
+    fn last_model_name<'a>(rollout_items: &'a [RolloutItem], current: &str) -> Option<&'a str> {
+        let previous = rollout_items.iter().rev().find_map(|it| {
+            if let RolloutItem::TurnContext(ctx) = it {
+                Some(ctx.model.as_str())
+            } else {
+                None
+            }
+        })?;
+        if previous == current {
+            None
+        } else {
+            Some(previous)
+        }
+    }
+
     fn last_token_info_from_rollout(rollout_items: &[RolloutItem]) -> Option<TokenUsageInfo> {
         rollout_items.iter().rev().find_map(|item| match item {
             RolloutItem::EventMsg(EventMsg::TokenCount(ev)) => ev.info.clone(),
             _ => None,
         })
     }
 
+    async fn take_pending_resume_previous_model(&self) -> Option<String> {
+        let mut state = self.state.lock().await;
+        state.pending_resume_previous_model.take()
+    }
+
     pub(crate) async fn update_settings(
         &self,
         updates: SessionSettingsUpdate,
@@ -1504,10 +1518,12 @@ impl Session {
     fn build_model_instructions_update_item(
         &self,
         previous: Option<&Arc<TurnContext>>,
+        resumed_model: Option<&str>,
         next: &TurnContext,
     ) -> Option<ResponseItem> {
-        let prev = previous?;
-        if prev.model_info.slug == next.model_info.slug {
+        let previous_model =
+            resumed_model.or_else(|| previous.map(|prev| prev.model_info.slug.as_str()))?;
+        if previous_model == next.model_info.slug {
             return None;
         }
 
@@ -1522,6 +1538,7 @@ impl Session {
     fn build_settings_update_items(
         &self,
         previous_context: Option<&Arc<TurnContext>>,
+        resumed_model: Option<&str>,
         current_context: &TurnContext,
     ) -> Vec<ResponseItem> {
         let mut update_items = Vec::new();
@@ -1540,9 +1557,11 @@ impl Session {
         {
             update_items.push(collaboration_mode_item);
         }
-        if let Some(model_instructions_item) =
-            self.build_model_instructions_update_item(previous_context, current_context)
-        {
+        if let Some(model_instructions_item) = self.build_model_instructions_update_item(
+            previous_context,
+            resumed_model,
+            current_context,
+        ) {
             update_items.push(model_instructions_item);
         }
         if let Some(personality_item) =
@@ -2819,8 +2838,12 @@ mod handlers {
         // Attempt to inject input into current task
         if let Err(items) = sess.inject_input(items).await {
             sess.seed_initial_context_if_needed(&current_context).await;
-            let update_items =
-                sess.build_settings_update_items(previous_context.as_ref(), &current_context);
+            let resumed_model = sess.take_pending_resume_previous_model().await;
+            let update_items = sess.build_settings_update_items(
+                previous_context.as_ref(),
+                resumed_model.as_deref(),
+                &current_context,
+            );
             if !update_items.is_empty() {
                 sess.record_conversation_items(&current_context, &update_items)
                     .await;

codex-rs/core/src/state/session.rs

@@ -24,6 +24,8 @@ pub(crate) struct SessionState {
     /// TODO(owen): This is a temporary solution to avoid updating a thread's updated_at
     /// timestamp when resuming a session. Remove this once SQLite is in place.
     pub(crate) initial_context_seeded: bool,
+    /// Previous rollout model for one-shot model-switch handling on first turn after resume.
+    pub(crate) pending_resume_previous_model: Option<String>,
 }
 
 impl SessionState {
@@ -38,6 +40,7 @@ impl SessionState {
             dependency_env: HashMap::new(),
             mcp_dependency_prompted: HashSet::new(),
             initial_context_seeded: false,
+            pending_resume_previous_model: None,
         }
     }