Sync subagent multi-agent mode with root

codex-rs/core/src/agent/control.rs

@@ -17,6 +17,7 @@ use crate::thread_rollout_truncation::truncate_rollout_to_last_n_fork_turns;
 use codex_protocol::AgentPath;
 use codex_protocol::SessionId;
 use codex_protocol::ThreadId;
+use codex_protocol::config_types::MultiAgentMode;
 use codex_protocol::error::CodexErr;
 use codex_protocol::error::Result as CodexResult;
 use codex_protocol::models::ContentItem;
@@ -121,6 +122,12 @@ impl AgentControl {
         self.session_id
     }
 
+    pub(crate) async fn root_multi_agent_mode(&self) -> CodexResult<Option<MultiAgentMode>> {
+        let state = self.upgrade()?;
+        let root_thread = state.get_thread(ThreadId::from(self.session_id)).await?;
+        Ok(root_thread.config_snapshot().await.multi_agent_mode)
+    }
+
     /// Send rich user input items to an existing agent thread.
     pub(crate) async fn send_input(
         &self,

codex-rs/core/src/agent/control_tests.rs

@@ -830,9 +830,9 @@ async fn spawn_thread_subagent_inherits_parent_selected_multi_agent_mode_without
         })
         .await
         .expect("update parent multi-agent mode");
+    let control = parent_thread.codex.session.services.agent_control.clone();
 
-    let child_thread_id = harness
-        .control
+    let child_thread_id = control
         .spawn_agent(
             harness.config.clone(),
             text_input("child task"),
@@ -858,6 +858,27 @@ async fn spawn_thread_subagent_inherits_parent_selected_multi_agent_mode_without
         child_snapshot.multi_agent_mode,
         Some(MultiAgentMode::Proactive)
     );
+
+    parent_thread
+        .codex
+        .session
+        .update_settings(crate::session::SessionSettingsUpdate {
+            multi_agent_mode: Some(MultiAgentMode::ExplicitRequestOnly),
+            ..Default::default()
+        })
+        .await
+        .expect("update parent multi-agent mode after child spawn");
+    let child_thread = harness
+        .manager
+        .get_thread(child_thread_id)
+        .await
+        .expect("child thread should remain registered");
+    let child_turn = child_thread.codex.session.new_default_turn().await;
+
+    assert_eq!(
+        child_turn.multi_agent_mode,
+        Some(MultiAgentMode::ExplicitRequestOnly)
+    );
 }
 
 #[tokio::test]
@@ -2254,7 +2275,7 @@ async fn spawn_thread_subagent_uses_role_specific_nickname_candidates() {
 }
 
 #[tokio::test]
-async fn resume_thread_subagent_restores_stored_metadata_and_effective_multi_agent_mode() {
+async fn resumed_thread_subagent_uses_root_multi_agent_mode_for_its_next_turn() {
     let (home, mut config) = test_config().await;
     config
         .features
@@ -2277,11 +2298,11 @@ async fn resume_thread_subagent_restores_stored_metadata_and_effective_multi_age
         control,
     };
     let (parent_thread_id, parent_thread) = harness.start_thread().await;
+    let control = parent_thread.codex.session.services.agent_control.clone();
     let agent_path = AgentPath::from_string("/root/explorer".to_string())
         .expect("test agent path should be valid");
 
-    let child_thread_id = harness
-        .control
+    let child_thread_id = control
         .spawn_agent(
             harness.config.clone(),
             text_input("hello child"),
@@ -2333,8 +2354,7 @@ async fn resume_thread_subagent_restores_stored_metadata_and_effective_multi_age
         })
         .await
         .expect("change parent multi-agent mode before child resume");
-    let mut status_rx = harness
-        .control
+    let mut status_rx = control
         .subscribe_status(child_thread_id)
         .await
         .expect("status subscription should succeed");
@@ -2375,14 +2395,12 @@ async fn resume_thread_subagent_restores_stored_metadata_and_effective_multi_age
     .await
     .expect("child thread metadata should be persisted to sqlite before shutdown");
 
-    let _ = harness
-        .control
+    let _ = control
         .shutdown_live_agent(child_thread_id)
         .await
         .expect("child shutdown should submit");
 
-    let resumed_thread_id = harness
-        .control
+    let resumed_thread_id = control
         .resume_agent_from_rollout(
             harness.config.clone(),
             child_thread_id,
@@ -2425,9 +2443,18 @@ async fn resume_thread_subagent_restores_stored_metadata_and_effective_multi_age
         resumed_snapshot.multi_agent_mode,
         Some(MultiAgentMode::Proactive)
     );
+    let resumed_thread = harness
+        .manager
+        .get_thread(resumed_thread_id)
+        .await
+        .expect("resumed child thread should remain registered");
+    let resumed_turn = resumed_thread.codex.session.new_default_turn().await;
+    assert_eq!(
+        resumed_turn.multi_agent_mode,
+        Some(MultiAgentMode::ExplicitRequestOnly)
+    );
 
-    let _ = harness
-        .control
+    let _ = control
         .shutdown_live_agent(resumed_thread_id)
         .await
         .expect("resumed child shutdown should submit");

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

@@ -746,10 +746,20 @@ impl Session {
     async fn new_turn_context_from_configuration(
         &self,
         sub_id: String,
-        session_configuration: SessionConfiguration,
+        mut session_configuration: SessionConfiguration,
         final_output_json_schema: Option<Option<Value>>,
         multi_agent_runtime: TurnMultiAgentRuntime,
     ) -> Arc<TurnContext> {
+        // Thread-spawn subagents follow the root's current selection. If the root is unavailable,
+        // retain the subagent's persisted selection so standalone resume still has a fallback.
+        if matches!(
+            session_configuration.session_source,
+            SessionSource::SubAgent(SubAgentSource::ThreadSpawn { .. })
+        ) && let Ok(root_multi_agent_mode) =
+            self.services.agent_control.root_multi_agent_mode().await
+        {
+            session_configuration.multi_agent_mode = root_multi_agent_mode;
+        }
         let turn_environments = self.services.turn_environments.snapshot().await;
         let primary_turn_environment = turn_environments.primary().cloned();
         // TODO(anp): Migrate per-turn config and legacy TurnContext cwd consumers to PathUri so

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

@@ -7,8 +7,10 @@ use codex_protocol::protocol::Op;
 use codex_protocol::protocol::ThreadSettingsOverrides;
 use codex_protocol::user_input::UserInput;
 use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_function_call;
 use core_test_support::responses::ev_response_created;
 use core_test_support::responses::mount_sse_once;
+use core_test_support::responses::mount_sse_once_match;
 use core_test_support::responses::mount_sse_sequence;
 use core_test_support::responses::sse;
 use core_test_support::responses::start_mock_server;
@@ -17,6 +19,10 @@ use core_test_support::test_codex::test_codex;
 use core_test_support::wait_for_event;
 use pretty_assertions::assert_eq;
 use serde_json::Value;
+use serde_json::json;
+use std::time::Duration;
+
+const SPAWN_CALL_ID: &str = "spawn-call-1";
 
 const NO_SPAWN_TEXT: &str = "Do not spawn sub-agents unless the user explicitly asks for sub-agents, delegation, or parallel agent work.";
 const PROACTIVE_TEXT: &str = "Proactive multi-agent delegation is active.";
@@ -59,6 +65,10 @@ async fn submit_turn(
     Ok(())
 }
 
+fn body_contains(req: &wiremock::Request, text: &str) -> bool {
+    String::from_utf8_lossy(&req.body).contains(text)
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn multi_agent_mode_is_sticky_and_emits_only_on_change() -> Result<()> {
     skip_if_no_network!(Ok(()));
@@ -132,6 +142,118 @@ async fn multi_agent_mode_is_sticky_and_emits_only_on_change() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn existing_subagent_uses_root_multi_agent_mode_on_its_next_turn() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let root_prompt = "spawn a child";
+    let child_prompt = "child initial task";
+    let child_followup = "child followup task";
+    let spawn_args = serde_json::to_string(&json!({
+        "message": child_prompt,
+        "task_name": "worker",
+    }))?;
+    mount_sse_once_match(
+        &server,
+        move |req: &wiremock::Request| body_contains(req, root_prompt),
+        sse(vec![
+            ev_response_created("root-spawn-response"),
+            ev_function_call(SPAWN_CALL_ID, "spawn_agent", &spawn_args),
+            ev_completed("root-spawn-response"),
+        ]),
+    )
+    .await;
+    mount_sse_once_match(
+        &server,
+        move |req: &wiremock::Request| {
+            body_contains(req, child_prompt) && !body_contains(req, SPAWN_CALL_ID)
+        },
+        sse(vec![
+            ev_response_created("child-initial-response"),
+            ev_completed("child-initial-response"),
+        ]),
+    )
+    .await;
+    mount_sse_once_match(
+        &server,
+        |req: &wiremock::Request| body_contains(req, SPAWN_CALL_ID),
+        sse(vec![
+            ev_response_created("root-followup-response"),
+            ev_completed("root-followup-response"),
+        ]),
+    )
+    .await;
+    let child_followup_response = mount_sse_once_match(
+        &server,
+        move |req: &wiremock::Request| body_contains(req, child_followup),
+        sse(vec![
+            ev_response_created("child-followup-response"),
+            ev_completed("child-followup-response"),
+        ]),
+    )
+    .await;
+
+    let test = test_codex()
+        .with_config(|config| {
+            config
+                .features
+                .enable(Feature::Collab)
+                .expect("test config should allow feature update");
+            config
+                .features
+                .enable(Feature::MultiAgentV2)
+                .expect("test config should allow feature update");
+            config
+                .features
+                .enable(Feature::MultiAgentMode)
+                .expect("test config should allow feature update");
+            config
+                .features
+                .disable(Feature::EnableRequestCompression)
+                .expect("test config should allow feature update");
+        })
+        .build(&server)
+        .await?;
+    let mut created_threads = test.thread_manager.subscribe_thread_created();
+
+    submit_turn(&test.codex, root_prompt, /*mode*/ None).await?;
+    let child_thread_id =
+        tokio::time::timeout(Duration::from_secs(10), created_threads.recv()).await??;
+    let child_thread = test.thread_manager.get_thread(child_thread_id).await?;
+    wait_for_event(&child_thread, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+    test.codex
+        .submit(Op::ThreadSettings {
+            thread_settings: ThreadSettingsOverrides {
+                multi_agent_mode: Some(MultiAgentMode::Proactive),
+                ..Default::default()
+            },
+        })
+        .await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::ThreadSettingsApplied(_))
+    })
+    .await;
+
+    submit_turn(&child_thread, child_followup, /*mode*/ None).await?;
+
+    let input = child_followup_response.single_request().input();
+    let texts = developer_texts(&input);
+    assert_eq!(
+        (
+            count_containing(&texts, MULTI_AGENT_MODE_OPEN_TAG),
+            count_containing(&texts, NO_SPAWN_TEXT),
+            count_containing(&texts, PROACTIVE_TEXT),
+        ),
+        (2, 1, 1)
+    );
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn multi_agent_mode_feature_uses_explicit_mode_when_disabled() -> Result<()> {
     skip_if_no_network!(Ok(()));