Deduplicate forked subagent developer instructions

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

@@ -396,10 +396,9 @@ impl AgentControl {
             forked_rollout_items =
                 truncate_rollout_to_last_n_fork_turns(&forked_rollout_items, *last_n_turns);
         }
-        // MultiAgentV2 root/subagent usage hints are injected as standalone developer
-        // messages at thread start. When forking history, drop hints from the parent
-        // so the child gets a fresh hint that matches its own session source/config.
-        let multi_agent_v2_usage_hint_texts_to_filter: Vec<String> =
+        // Standalone developer items that the child reinjects at startup should not
+        // survive the inherited forked history, or they appear twice in the child prompt.
+        let mut forked_developer_texts_to_filter: Vec<String> =
             if let Some(parent_thread) = parent_thread.as_ref() {
                 parent_thread
                     .codex
@@ -417,11 +416,16 @@ impl AgentControl {
             } else {
                 Vec::new()
             };
+        if let Some(developer_instructions) = config.developer_instructions.as_deref()
+            && !developer_instructions.is_empty()
+        {
+            forked_developer_texts_to_filter.push(developer_instructions.to_string());
+        }
         forked_rollout_items.retain(|item| {
             if let RolloutItem::ResponseItem(ResponseItem::Message { role, content, .. }) = item
                 && role == "developer"
                 && let [ContentItem::InputText { text }] = content.as_slice()
-                && multi_agent_v2_usage_hint_texts_to_filter
+                && forked_developer_texts_to_filter
                     .iter()
                     .any(|usage_hint_text| usage_hint_text == text)
             {

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

@@ -614,6 +614,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
         Some("Child root guidance.".to_string());
     child_config.multi_agent_v2.subagent_usage_hint_text =
         Some("Child subagent guidance.".to_string());
+    child_config.developer_instructions = Some("Parent developer instructions.".to_string());
     let new_thread = harness
         .manager
         .start_thread(parent_config.clone())
@@ -655,6 +656,14 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                     }],
                     phase: None,
                 },
+                ResponseItem::Message {
+                    id: None,
+                    role: "developer".to_string(),
+                    content: vec![ContentItem::InputText {
+                        text: "Parent developer instructions.".to_string(),
+                    }],
+                    phase: None,
+                },
                 assistant_message("parent commentary", Some(MessagePhase::Commentary)),
                 assistant_message("parent final answer", Some(MessagePhase::FinalAnswer)),
                 assistant_message("parent unknown phase", /*phase*/ None),
@@ -726,6 +735,15 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
         &expected_history,
         "forked child history should keep only parent user messages and assistant final answers"
     );
+    let child_rollout_path = child_thread
+        .rollout_path()
+        .expect("forked child rollout path");
+    let child_rollout = std::fs::read_to_string(&child_rollout_path)
+        .expect("forked child rollout should be readable");
+    assert!(
+        !child_rollout.contains("Parent developer instructions."),
+        "forked child rollout should not retain developer instructions that will be reinjected"
+    );
 
     let expected = (
         child_thread_id,