diff --git a/codex-rs/app-server/tests/suite/v2/thread_fork.rs b/codex-rs/app-server/tests/suite/v2/thread_fork.rs
index f623738742..6023a3ef2e 100644
--- a/codex-rs/app-server/tests/suite/v2/thread_fork.rs
+++ b/codex-rs/app-server/tests/suite/v2/thread_fork.rs
@@ -338,17 +338,22 @@ async fn thread_fork_honors_explicit_null_thread_instructions() -> Result<()> {
             expect_instructions,
             "unexpected instructions field in payload: {payload:?}"
         );
+        let developer_texts = request.message_input_texts("developer");
         if index == 2 {
             assert_eq!(
                 payload.get("instructions"),
                 Some(&Value::String(String::new()))
             );
+            assert!(
+                developer_texts.iter().any(|text| text.is_empty()),
+                "expected explicit empty developerInstructions to produce an empty developer instruction message: {developer_texts:?}"
+            );
+        } else {
+            assert!(
+                developer_texts.iter().all(|text| !text.is_empty()),
+                "did not expect empty developer instruction messages: {developer_texts:?}"
+            );
         }
-        let developer_texts = request.message_input_texts("developer");
-        assert!(
-            developer_texts.iter().all(|text| !text.is_empty()),
-            "did not expect empty developer instruction messages: {developer_texts:?}"
-        );
     }
 
     Ok(())
diff --git a/codex-rs/app-server/tests/suite/v2/turn_start.rs b/codex-rs/app-server/tests/suite/v2/turn_start.rs
index 193a76ccf3..b09b78d3bc 100644
--- a/codex-rs/app-server/tests/suite/v2/turn_start.rs
+++ b/codex-rs/app-server/tests/suite/v2/turn_start.rs
@@ -252,17 +252,22 @@ async fn turn_start_honors_explicit_null_thread_instructions() -> Result<()> {
             expect_instructions,
             "unexpected instructions field in payload: {payload:?}"
         );
+        let developer_texts = request.message_input_texts("developer");
         if index == 2 {
             assert_eq!(
                 payload.get("instructions"),
                 Some(&Value::String(String::new()))
             );
+            assert!(
+                developer_texts.iter().any(|text| text.is_empty()),
+                "expected explicit empty developerInstructions to produce an empty developer instruction message: {developer_texts:?}"
+            );
+        } else {
+            assert!(
+                developer_texts.iter().all(|text| !text.is_empty()),
+                "did not expect empty developer instruction messages: {developer_texts:?}"
+            );
         }
-        let developer_texts = request.message_input_texts("developer");
-        assert!(
-            developer_texts.iter().all(|text| !text.is_empty()),
-            "did not expect empty developer instruction messages: {developer_texts:?}"
-        );
     }
 
     Ok(())
diff --git a/codex-rs/core/src/codex.rs b/codex-rs/core/src/codex.rs
index 11bfade15c..f7484ba677 100644
--- a/codex-rs/core/src/codex.rs
+++ b/codex-rs/core/src/codex.rs
@@ -3664,7 +3664,6 @@ impl Session {
         // stays isolated as its own top-level developer message for guardian subagents.
         if !separate_guardian_developer_message
             && let Some(developer_instructions) = turn_context.developer_instructions.as_deref()
-            && !developer_instructions.is_empty()
         {
             developer_sections.push(developer_instructions.to_string());
         }
@@ -3779,7 +3778,6 @@ impl Session {
         // subagent sees a distinct, easy-to-audit instruction block.
         if separate_guardian_developer_message
             && let Some(developer_instructions) = turn_context.developer_instructions.as_deref()
-            && !developer_instructions.is_empty()
             && let Some(guardian_developer_message) =
                 crate::context_manager::updates::build_developer_update_item(vec![
                     developer_instructions.to_string(),