feat: tasks can't be assigned to root agent (#16424)

codex-rs/core/src/tools/handlers/multi_agents_tests.rs

@@ -709,6 +709,86 @@ async fn multi_agent_v2_send_message_accepts_root_target_from_child() {
     }));
 }
 
+#[tokio::test]
+async fn multi_agent_v2_assign_task_rejects_root_target_from_child() {
+    let (mut session, mut turn) = make_session_and_context().await;
+    let manager = thread_manager();
+    let root = manager
+        .start_thread((*turn.config).clone())
+        .await
+        .expect("root thread should start");
+    session.services.agent_control = manager.agent_control();
+    session.conversation_id = root.thread_id;
+    let mut config = (*turn.config).clone();
+    config
+        .features
+        .enable(Feature::MultiAgentV2)
+        .expect("test config should allow feature update");
+    turn.config = Arc::new(config);
+
+    let child_path = AgentPath::try_from("/root/worker").expect("agent path");
+    let child_thread_id = session
+        .services
+        .agent_control
+        .spawn_agent_with_metadata(
+            (*turn.config).clone(),
+            vec![UserInput::Text {
+                text: "inspect this repo".to_string(),
+                text_elements: Vec::new(),
+            }]
+            .into(),
+            Some(SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
+                parent_thread_id: root.thread_id,
+                depth: 1,
+                agent_path: Some(child_path.clone()),
+                agent_nickname: None,
+                agent_role: None,
+            })),
+            crate::agent::control::SpawnAgentOptions::default(),
+        )
+        .await
+        .expect("worker spawn should succeed")
+        .thread_id;
+    session.conversation_id = child_thread_id;
+    turn.session_source = SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
+        parent_thread_id: root.thread_id,
+        depth: 1,
+        agent_path: Some(child_path),
+        agent_nickname: None,
+        agent_role: None,
+    });
+
+    let err = AssignTaskHandlerV2
+        .handle(invocation(
+            Arc::new(session),
+            Arc::new(turn),
+            "assign_task",
+            function_payload(json!({
+                "target": "/root",
+                "message": "run this",
+                "interrupt": true
+            })),
+        ))
+        .await
+        .expect_err("assign_task should reject the root target");
+
+    assert_eq!(
+        err,
+        FunctionCallError::RespondToModel("Tasks can't be assigned to the root agent".to_string())
+    );
+    let root_ops = manager
+        .captured_ops()
+        .into_iter()
+        .filter_map(|(id, op)| (id == root.thread_id).then_some(op))
+        .collect::<Vec<_>>();
+    assert!(!root_ops.iter().any(|op| matches!(op, Op::Interrupt)));
+    assert!(
+        !root_ops
+            .iter()
+            .any(|op| matches!(op, Op::InterAgentCommunication { .. }))
+    );
+}
+
 #[tokio::test]
 async fn multi_agent_v2_list_agents_returns_completed_status_and_last_task_message() {
     let (mut session, mut turn) = make_session_and_context().await;

codex-rs/core/src/tools/handlers/multi_agents_v2/message_tool.rs

@@ -6,7 +6,7 @@
 use super::*;
 use codex_protocol::protocol::InterAgentCommunication;
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub(crate) enum MessageDeliveryMode {
     QueueOnly,
     TriggerTurn,
@@ -118,6 +118,16 @@ async fn handle_message_submission(
         .agent_control
         .get_agent_metadata(receiver_thread_id)
         .unwrap_or_default();
+    if mode == MessageDeliveryMode::TriggerTurn
+        && receiver_agent
+            .agent_path
+            .as_ref()
+            .is_some_and(AgentPath::is_root)
+    {
+        return Err(FunctionCallError::RespondToModel(
+            "Tasks can't be assigned to the root agent".to_string(),
+        ));
+    }
     if interrupt {
         session
             .services

codex-rs/tools/src/agent_tool.rs

@@ -179,7 +179,7 @@ pub fn create_assign_task_tool() -> ToolSpec {
 
     ToolSpec::Function(ResponsesApiTool {
         name: "assign_task".to_string(),
-        description: "Add a message to an existing agent and trigger a turn in the target. Use interrupt=true to redirect work immediately. In MultiAgentV2, this tool currently supports text content only."
+        description: "Add a message to an existing non-root agent and trigger a turn in the target. Use interrupt=true to redirect work immediately. In MultiAgentV2, this tool currently supports text content only."
             .to_string(),
         strict: false,
         defer_loading: None,