core: omit empty multi-agent mode hints

codex-rs/core/src/context/multi_agent_mode_instructions.rs

@@ -12,8 +12,15 @@ pub(crate) struct MultiAgentModeInstructions {
 }
 
 impl MultiAgentModeInstructions {
-    pub(crate) fn new(multi_agent_mode: MultiAgentMode) -> Self {
-        Self { multi_agent_mode }
+    pub(crate) fn from_mode(multi_agent_mode: MultiAgentMode) -> Option<Self> {
+        if matches!(
+            &multi_agent_mode,
+            MultiAgentMode::Custom(hint_text) if hint_text.is_empty()
+        ) {
+            return None;
+        }
+
+        Some(Self { multi_agent_mode })
     }
 }
 

codex-rs/core/src/context_manager/updates.rs

@@ -95,9 +95,11 @@ fn build_multi_agent_mode_update_item(
     }
 
     match effective_multi_agent_mode {
-        Some(multi_agent_mode) => Some(MultiAgentModeInstructions::new(multi_agent_mode).render()),
+        Some(multi_agent_mode) => MultiAgentModeInstructions::from_mode(multi_agent_mode)
+            .map(|instructions| instructions.render()),
         None if previous.multi_agent_mode == Some(MultiAgentMode::Proactive) => {
-            Some(MultiAgentModeInstructions::new(MultiAgentMode::ExplicitRequestOnly).render())
+            MultiAgentModeInstructions::from_mode(MultiAgentMode::ExplicitRequestOnly)
+                .map(|instructions| instructions.render())
         }
         None => None,
     }

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

@@ -3428,10 +3428,10 @@ impl Session {
         {
             items.push(usage_hint_message);
         }
-        if let Some(multi_agent_mode) = multi_agents::effective_multi_agent_mode(turn_context) {
-            items.push(ContextualUserFragment::into(
-                MultiAgentModeInstructions::new(multi_agent_mode),
-            ));
+        if let Some(multi_agent_mode) = multi_agents::effective_multi_agent_mode(turn_context)
+            && let Some(instructions) = MultiAgentModeInstructions::from_mode(multi_agent_mode)
+        {
+            items.push(ContextualUserFragment::into(instructions));
         }
         if let Some(contextual_user_message) =
             crate::context_manager::updates::build_contextual_user_message(contextual_user_sections)

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

@@ -193,7 +193,7 @@ async fn configured_mode_hint_uses_custom_mode_across_reasoning_efforts() -> Res
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn empty_configured_mode_hint_suppresses_builtin_text() -> Result<()> {
+async fn empty_configured_mode_hint_emits_no_mode_message() -> Result<()> {
     skip_if_no_network!(Ok(()));
 
     let server = start_mock_server().await;
@@ -220,7 +220,62 @@ async fn empty_configured_mode_hint_suppresses_builtin_text() -> Result<()> {
             count_containing(&texts, NO_SPAWN_TEXT),
             count_containing(&texts, PROACTIVE_TEXT),
         ),
-        (1, 0, 0)
+        (0, 0, 0)
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn changing_configured_mode_hint_to_empty_emits_no_update() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let responses = mount_sse_sequence(
+        &server,
+        (1..=2)
+            .map(|index| {
+                sse(vec![
+                    ev_response_created(&format!("resp-{index}")),
+                    ev_completed(&format!("resp-{index}")),
+                ])
+            })
+            .collect(),
+    )
+    .await;
+    let initial = test_codex()
+        .with_config(configure_custom_mode_hint)
+        .build(&server)
+        .await?;
+    let home = initial.home.clone();
+    let rollout_path = initial
+        .session_configured
+        .rollout_path
+        .clone()
+        .expect("rollout path");
+
+    submit_turn(&initial.codex, "before resume", /*effort*/ None).await?;
+    drop(initial);
+
+    let mut resume_builder = test_codex().with_config(|config| {
+        configure_multi_agent_v2(config);
+        config.multi_agent_v2.multi_agent_mode_hint_text = Some(String::new());
+    });
+    let resumed = resume_builder.resume(&server, home, rollout_path).await?;
+    submit_turn(&resumed.codex, "after resume", /*effort*/ None).await?;
+
+    let requests = responses.requests();
+    let first_input = requests[0].input();
+    let first_texts = developer_texts(&first_input);
+    let resumed_input = requests[1].input();
+    let resumed_texts = developer_texts(&resumed_input);
+    assert_eq!(
+        (
+            count_containing(&first_texts, MULTI_AGENT_MODE_OPEN_TAG),
+            count_containing(&resumed_texts, MULTI_AGENT_MODE_OPEN_TAG),
+            count_containing(&resumed_texts, CUSTOM_MODE_HINT_TEXT),
+        ),
+        (1, 1, 1)
     );
 
     Ok(())