diff --git a/codex-rs/core/src/context/hook_additional_context.rs b/codex-rs/core/src/context/hook_additional_context.rs
index 01c95a6524..5ef6218aa0 100644
--- a/codex-rs/core/src/context/hook_additional_context.rs
+++ b/codex-rs/core/src/context/hook_additional_context.rs
@@ -13,8 +13,8 @@ impl HookAdditionalContext {
 
 impl ContextualUserFragment for HookAdditionalContext {
     const ROLE: &'static str = "developer";
-    const START_MARKER: &'static str = "";
-    const END_MARKER: &'static str = "";
+    const START_MARKER: &'static str = "<hook_context>";
+    const END_MARKER: &'static str = "</hook_context>";
 
     fn body(&self) -> String {
         self.text.clone()
diff --git a/codex-rs/core/src/context_manager/history_tests.rs b/codex-rs/core/src/context_manager/history_tests.rs
index 74f4d29bfb..4cafbeee93 100644
--- a/codex-rs/core/src/context_manager/history_tests.rs
+++ b/codex-rs/core/src/context_manager/history_tests.rs
@@ -902,6 +902,7 @@ fn drop_last_n_user_turns_trims_context_updates_above_rolled_back_turn() {
         user_input_text_msg("turn 1 user"),
         assistant_msg("turn 1 assistant"),
         developer_msg("Generated images are saved to /tmp as /tmp/image-1.png by default."),
+        developer_msg("<hook_context>ROLLED_BACK_HOOK_CONTEXT</hook_context>"),
         developer_msg("<collaboration_mode>ROLLED_BACK_DEV_INSTRUCTIONS</collaboration_mode>"),
         user_input_text_msg(
             "<environment_context><cwd>PRETURN_CONTEXT_DIFF_CWD</cwd></environment_context>",
diff --git a/codex-rs/core/src/event_mapping.rs b/codex-rs/core/src/event_mapping.rs
index e7c79e6dd2..3fabe854ac 100644
--- a/codex-rs/core/src/event_mapping.rs
+++ b/codex-rs/core/src/event_mapping.rs
@@ -30,6 +30,7 @@ const CONTEXTUAL_DEVELOPER_PREFIXES: &[&str] = &[
     COLLABORATION_MODE_OPEN_TAG,
     REALTIME_CONVERSATION_OPEN_TAG,
     "<personality_spec>",
+    "<hook_context>",
 ];
 
 pub(crate) fn is_contextual_user_message_content(message: &[ContentItem]) -> bool {
diff --git a/codex-rs/core/src/hook_runtime.rs b/codex-rs/core/src/hook_runtime.rs
index 3d72e4a7c6..c0d1364ea0 100644
--- a/codex-rs/core/src/hook_runtime.rs
+++ b/codex-rs/core/src/hook_runtime.rs
@@ -33,6 +33,7 @@ use serde_json::Value;
 
 use crate::context::ContextualUserFragment;
 use crate::context::HookAdditionalContext;
+use crate::context_manager::updates::build_developer_update_item;
 use crate::event_mapping::parse_turn_item;
 use crate::session::session::Session;
 use crate::session::turn_context::TurnContext;
@@ -452,11 +453,13 @@ pub(crate) async fn record_additional_contexts(
 }
 
 fn additional_context_messages(additional_contexts: Vec<String>) -> Vec<ResponseItem> {
-    additional_contexts
+    let sections = additional_contexts
         .into_iter()
         .map(HookAdditionalContext::new)
-        .map(ContextualUserFragment::into)
-        .collect()
+        .map(|context| context.render())
+        .collect();
+
+    build_developer_update_item(sections).into_iter().collect()
 }
 
 async fn emit_hook_started_events(
@@ -616,36 +619,39 @@ mod tests {
     use codex_utils_absolute_path::test_support::test_path_buf;
 
     #[test]
-    fn additional_context_messages_stay_separate_and_ordered() {
+    fn additional_context_messages_merge_and_preserve_order() {
         let messages = additional_context_messages(vec![
             "first tide note".to_string(),
             "second tide note".to_string(),
         ]);
 
-        assert_eq!(messages.len(), 2);
+        assert_eq!(messages.len(), 1);
         assert_eq!(
             messages
                 .iter()
                 .map(|message| match message {
                     codex_protocol::models::ResponseItem::Message { role, content, .. } => {
-                        let text = content
+                        let texts = content
                             .iter()
                             .map(|item| match item {
-                                ContentItem::InputText { text } => text.as_str(),
+                                ContentItem::InputText { text } => text.clone(),
                                 ContentItem::InputImage { .. } | ContentItem::OutputText { .. } => {
                                     panic!("expected input text content, got {item:?}")
                                 }
                             })
-                            .collect::<String>();
-                        (role.as_str(), text)
+                            .collect::<Vec<_>>();
+                        (role.as_str(), texts)
                     }
                     other => panic!("expected developer message, got {other:?}"),
                 })
                 .collect::<Vec<_>>(),
-            vec![
-                ("developer", "first tide note".to_string()),
-                ("developer", "second tide note".to_string()),
-            ],
+            vec![(
+                "developer",
+                vec![
+                    "<hook_context>first tide note</hook_context>".to_string(),
+                    "<hook_context>second tide note</hook_context>".to_string(),
+                ],
+            )],
         );
     }
 
diff --git a/codex-rs/core/tests/suite/hooks.rs b/codex-rs/core/tests/suite/hooks.rs
index 635aeaf488..821fbc94e4 100644
--- a/codex-rs/core/tests/suite/hooks.rs
+++ b/codex-rs/core/tests/suite/hooks.rs
@@ -180,6 +180,50 @@ else:
     Ok(())
 }
 
+fn write_parallel_session_start_hooks(home: &Path, contexts: &[&str]) -> Result<()> {
+    let hook_entries = contexts
+        .iter()
+        .enumerate()
+        .map(|(index, context)| {
+            let script_path = home.join(format!("session_start_hook_{index}.py"));
+            let context_json =
+                serde_json::to_string(context).context("serialize session start context")?;
+            let script = format!(
+                r#"import json
+
+print(json.dumps({{
+    "hookSpecificOutput": {{
+        "hookEventName": "SessionStart",
+        "additionalContext": {context_json}
+    }}
+}}))
+"#
+            );
+            fs::write(&script_path, script).with_context(|| {
+                format!(
+                    "write session start hook script fixture at {}",
+                    script_path.display()
+                )
+            })?;
+            Ok(serde_json::json!({
+                "type": "command",
+                "command": format!("python3 {}", script_path.display()),
+            }))
+        })
+        .collect::<Result<Vec<_>>>()?;
+
+    let hooks = serde_json::json!({
+        "hooks": {
+            "SessionStart": [{
+                "hooks": hook_entries,
+            }]
+        }
+    });
+
+    fs::write(home.join("hooks.json"), hooks.to_string()).context("write hooks.json")?;
+    Ok(())
+}
+
 fn write_user_prompt_submit_hook(
     home: &Path,
     blocked_prompt: &str,
@@ -1064,6 +1108,56 @@ async fn session_start_hook_spills_large_additional_context() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn parallel_session_start_additional_contexts_share_one_developer_message() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let response = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-1"),
+            ev_assistant_message("msg-1", "merged hook context observed"),
+            ev_completed("resp-1"),
+        ]),
+    )
+    .await;
+    let contexts = ["first tide note", "second tide note"];
+
+    let mut builder = test_codex()
+        .with_pre_build_hook(move |home| {
+            if let Err(error) = write_parallel_session_start_hooks(home, &contexts) {
+                panic!("failed to write parallel session start hook fixtures: {error}");
+            }
+        })
+        .with_config(trust_discovered_hooks);
+    let test = builder.build(&server).await?;
+
+    test.submit_turn("hello").await?;
+
+    let request = response.single_request();
+    let merged_developer_messages = request
+        .input()
+        .iter()
+        .filter(|item| item.get("role").and_then(Value::as_str) == Some("developer"))
+        .filter(|item| {
+            let joined = item["content"]
+                .as_array()
+                .into_iter()
+                .flatten()
+                .filter_map(|content| content.get("text").and_then(Value::as_str))
+                .collect::<Vec<_>>()
+                .join("\n");
+            contexts
+                .iter()
+                .all(|context| joined.contains(&format!("<hook_context>{context}</hook_context>")))
+        })
+        .count();
+    assert_eq!(merged_developer_messages, 1);
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn stop_hook_spills_large_continuation_prompt() -> Result<()> {
     skip_if_no_network!(Ok(()));