feat: emit warning when sub agent is done

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

@@ -0,0 +1,57 @@
+use std::sync::Arc;
+
+use codex_protocol::ThreadId;
+
+use crate::agent::AgentStatus;
+use crate::agent::status::is_final;
+use crate::codex::Session;
+use crate::codex::TurnContext;
+
+/// Subscribe to a spawned sub-agent and warn the model once it reaches a final status.
+pub(crate) fn spawn_collab_completion_warning_watcher(
+    session: Arc<Session>,
+    turn_context: Arc<TurnContext>,
+    agent_id: ThreadId,
+) {
+    tokio::spawn(async move {
+        if let Some(status) = wait_for_final_status(session.as_ref(), agent_id).await {
+            let message = completion_warning_message(agent_id, &status);
+            session.record_model_warning(message, &turn_context).await;
+        }
+    });
+}
+
+async fn wait_for_final_status(session: &Session, agent_id: ThreadId) -> Option<AgentStatus> {
+    let mut status_rx = match session
+        .services
+        .agent_control
+        .subscribe_status(agent_id)
+        .await
+    {
+        Ok(rx) => rx,
+        Err(_) => {
+            let status = session.services.agent_control.get_status(agent_id).await;
+            return is_final(&status).then_some(status);
+        }
+    };
+
+    let mut status = status_rx.borrow().clone();
+    if is_final(&status) {
+        return Some(status);
+    }
+
+    loop {
+        if status_rx.changed().await.is_err() {
+            let latest = session.services.agent_control.get_status(agent_id).await;
+            return is_final(&latest).then_some(latest);
+        }
+        status = status_rx.borrow().clone();
+        if is_final(&status) {
+            return Some(status);
+        }
+    }
+}
+
+fn completion_warning_message(agent_id: ThreadId, status: &AgentStatus) -> String {
+    format!("Sub-agent {agent_id} finished with status {status:?}. Use wait to collect the result.")
+}

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

@@ -1,9 +1,11 @@
+mod collab_completion_warning;
 pub(crate) mod control;
 mod guards;
 pub(crate) mod role;
 pub(crate) mod status;
 
 pub(crate) use codex_protocol::protocol::AgentStatus;
+pub(crate) use collab_completion_warning::spawn_collab_completion_warning_watcher;
 pub(crate) use control::AgentControl;
 pub(crate) use guards::MAX_THREAD_SPAWN_DEPTH;
 pub(crate) use guards::exceeds_thread_spawn_depth_limit;

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

@@ -168,6 +168,11 @@ mod spawn {
             )
             .await;
         let new_thread_id = result?;
+        crate::agent::spawn_collab_completion_warning_watcher(
+            session.clone(),
+            turn.clone(),
+            new_thread_id,
+        );
 
         let content = serde_json::to_string(&SpawnAgentResult {
             agent_id: new_thread_id.to_string(),