diff --git a/codex-rs/core/src/context_manager/history.rs b/codex-rs/core/src/context_manager/history.rs
index b6b3ceae5a..670931c6b1 100644
--- a/codex-rs/core/src/context_manager/history.rs
+++ b/codex-rs/core/src/context_manager/history.rs
@@ -1,16 +1,12 @@
 use crate::codex::TurnContext;
 use crate::context_manager::normalize;
-use crate::instructions::SkillInstructions;
-use crate::instructions::UserInstructions;
-use crate::session_prefix::is_session_prefix;
+use crate::session_prefix::is_contextual_user_message;
 use crate::truncate::TruncationPolicy;
 use crate::truncate::approx_token_count;
 use crate::truncate::approx_tokens_from_byte_count_i64;
 use crate::truncate::truncate_function_output_items_with_policy;
 use crate::truncate::truncate_text;
-use crate::user_shell_command::is_user_shell_command_text;
 use codex_protocol::models::BaseInstructions;
-use codex_protocol::models::ContentItem;
 use codex_protocol::models::FunctionCallOutputBody;
 use codex_protocol::models::FunctionCallOutputContentItem;
 use codex_protocol::models::FunctionCallOutputPayload;
@@ -448,29 +444,7 @@ pub(crate) fn is_user_turn_boundary(item: &ResponseItem) -> bool {
         return false;
     }
 
-    if UserInstructions::is_user_instructions(content)
-        || SkillInstructions::is_skill_instructions(content)
-    {
-        return false;
-    }
-
-    for content_item in content {
-        match content_item {
-            ContentItem::InputText { text } => {
-                if is_session_prefix(text) || is_user_shell_command_text(text) {
-                    return false;
-                }
-            }
-            ContentItem::OutputText { text } => {
-                if is_session_prefix(text) {
-                    return false;
-                }
-            }
-            ContentItem::InputImage { .. } => {}
-        }
-    }
-
-    true
+    !is_contextual_user_message(content)
 }
 
 fn user_message_positions(items: &[ResponseItem]) -> Vec<usize> {
diff --git a/codex-rs/core/src/context_manager/history_tests.rs b/codex-rs/core/src/context_manager/history_tests.rs
index 28c157b1e2..516472bda6 100644
--- a/codex-rs/core/src/context_manager/history_tests.rs
+++ b/codex-rs/core/src/context_manager/history_tests.rs
@@ -561,6 +561,8 @@ fn drop_last_n_user_turns_preserves_prefix() {
 
 #[test]
 fn drop_last_n_user_turns_ignores_session_prefix_user_messages() {
+    let context_update =
+        "<context_update>\n<environment_context>delta</environment_context>\n</context_update>";
     let items = vec![
         user_input_text_msg("<environment_context>ctx</environment_context>"),
         user_input_text_msg("<user_instructions>do the thing</user_instructions>"),
@@ -571,6 +573,7 @@ fn drop_last_n_user_turns_ignores_session_prefix_user_messages() {
             "<skill>\n<name>demo</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>",
         ),
         user_input_text_msg("<user_shell_command>echo 42</user_shell_command>"),
+        user_input_text_msg(context_update),
         user_input_text_msg("turn 1 user"),
         assistant_msg("turn 1 assistant"),
         user_input_text_msg("turn 2 user"),
@@ -591,6 +594,7 @@ fn drop_last_n_user_turns_ignores_session_prefix_user_messages() {
             "<skill>\n<name>demo</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>",
         ),
         user_input_text_msg("<user_shell_command>echo 42</user_shell_command>"),
+        user_input_text_msg(context_update),
         user_input_text_msg("turn 1 user"),
         assistant_msg("turn 1 assistant"),
     ];
@@ -610,6 +614,7 @@ fn drop_last_n_user_turns_ignores_session_prefix_user_messages() {
             "<skill>\n<name>demo</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>",
         ),
         user_input_text_msg("<user_shell_command>echo 42</user_shell_command>"),
+        user_input_text_msg(context_update),
     ];
 
     let mut history = create_history_with_items(vec![
@@ -622,6 +627,7 @@ fn drop_last_n_user_turns_ignores_session_prefix_user_messages() {
             "<skill>\n<name>demo</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>",
         ),
         user_input_text_msg("<user_shell_command>echo 42</user_shell_command>"),
+        user_input_text_msg(context_update),
         user_input_text_msg("turn 1 user"),
         assistant_msg("turn 1 assistant"),
         user_input_text_msg("turn 2 user"),
@@ -640,6 +646,7 @@ fn drop_last_n_user_turns_ignores_session_prefix_user_messages() {
             "<skill>\n<name>demo</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>",
         ),
         user_input_text_msg("<user_shell_command>echo 42</user_shell_command>"),
+        user_input_text_msg(context_update),
         user_input_text_msg("turn 1 user"),
         assistant_msg("turn 1 assistant"),
         user_input_text_msg("turn 2 user"),
diff --git a/codex-rs/core/src/event_mapping.rs b/codex-rs/core/src/event_mapping.rs
index 16f7e1c47a..83fc6bfd73 100644
--- a/codex-rs/core/src/event_mapping.rs
+++ b/codex-rs/core/src/event_mapping.rs
@@ -18,16 +18,11 @@ use codex_protocol::user_input::UserInput;
 use tracing::warn;
 use uuid::Uuid;
 
-use crate::instructions::SkillInstructions;
-use crate::instructions::UserInstructions;
-use crate::session_prefix::is_session_prefix;
-use crate::user_shell_command::is_user_shell_command_text;
+use crate::session_prefix::is_contextual_user_message;
 use crate::web_search::web_search_action_detail;
 
 fn parse_user_message(message: &[ContentItem]) -> Option<UserMessageItem> {
-    if UserInstructions::is_user_instructions(message)
-        || SkillInstructions::is_skill_instructions(message)
-    {
+    if is_contextual_user_message(message) {
         return None;
     }
 
@@ -44,9 +39,6 @@ fn parse_user_message(message: &[ContentItem]) -> Option<UserMessageItem> {
                 {
                     continue;
                 }
-                if is_session_prefix(text) || is_user_shell_command_text(text) {
-                    return None;
-                }
                 content.push(UserInput::Text {
                     text: text.clone(),
                     // Model input content does not carry UI element ranges.
@@ -59,9 +51,6 @@ fn parse_user_message(message: &[ContentItem]) -> Option<UserMessageItem> {
                 });
             }
             ContentItem::OutputText { text } => {
-                if is_session_prefix(text) {
-                    return None;
-                }
                 warn!("Output text in user message: {}", text);
             }
         }
@@ -339,7 +328,16 @@ mod tests {
                     text: "<user_shell_command>echo 42</user_shell_command>".to_string(),
                 }],
                 end_turn: None,
-            phase: None,
+                phase: None,
+            },
+            ResponseItem::Message {
+                id: None,
+                role: "user".to_string(),
+                content: vec![ContentItem::InputText {
+                    text: "<context_update>\n<environment_context>test_text</environment_context>\n</context_update>".to_string(),
+                }],
+                end_turn: None,
+                phase: None,
             },
         ];
 
diff --git a/codex-rs/core/src/rollout/truncation.rs b/codex-rs/core/src/rollout/truncation.rs
index c50eacc48b..9179e9bf66 100644
--- a/codex-rs/core/src/rollout/truncation.rs
+++ b/codex-rs/core/src/rollout/truncation.rs
@@ -194,6 +194,10 @@ mod tests {
     async fn ignores_session_prefix_messages_when_truncating_rollout_from_start() {
         let (session, turn_context) = make_session_and_context().await;
         let mut items = session.build_initial_context(&turn_context).await;
+        items.push(user_msg(
+            "<context_update>\n<environment_context>delta</environment_context>\n</context_update>",
+        ));
+        let prefix_and_context_count = items.len();
         items.push(user_msg("feature request"));
         items.push(assistant_msg("ack"));
         items.push(user_msg("second question"));
@@ -206,12 +210,12 @@ mod tests {
             .collect();
 
         let truncated = truncate_rollout_before_nth_user_message_from_start(&rollout_items, 1);
-        let expected: Vec<RolloutItem> = vec![
-            RolloutItem::ResponseItem(items[0].clone()),
-            RolloutItem::ResponseItem(items[1].clone()),
-            RolloutItem::ResponseItem(items[2].clone()),
-            RolloutItem::ResponseItem(items[3].clone()),
-        ];
+        let expected: Vec<RolloutItem> = items
+            .iter()
+            .take(prefix_and_context_count + 2)
+            .cloned()
+            .map(RolloutItem::ResponseItem)
+            .collect();
 
         assert_eq!(
             serde_json::to_value(&truncated).unwrap(),
diff --git a/codex-rs/core/src/session_prefix.rs b/codex-rs/core/src/session_prefix.rs
index 99283082b6..f4ac423ffb 100644
--- a/codex-rs/core/src/session_prefix.rs
+++ b/codex-rs/core/src/session_prefix.rs
@@ -1,3 +1,9 @@
+use codex_protocol::models::ContentItem;
+
+use crate::instructions::SkillInstructions;
+use crate::instructions::UserInstructions;
+use crate::user_shell_command::is_user_shell_command_text;
+
 /// Helpers for identifying model-visible "session prefix" messages.
 ///
 /// A session prefix is a user-role message that carries configuration or state needed by
@@ -6,10 +12,99 @@
 /// boundaries.
 pub(crate) const ENVIRONMENT_CONTEXT_OPEN_TAG: &str = "<environment_context>";
 pub(crate) const TURN_ABORTED_OPEN_TAG: &str = "<turn_aborted>";
+pub(crate) const CONTEXT_UPDATE_OPEN_TAG: &str = "<context_update>";
 
 /// Returns true if `text` starts with a session prefix marker (case-insensitive).
 pub(crate) fn is_session_prefix(text: &str) -> bool {
     let trimmed = text.trim_start();
     let lowered = trimmed.to_ascii_lowercase();
-    lowered.starts_with(ENVIRONMENT_CONTEXT_OPEN_TAG) || lowered.starts_with(TURN_ABORTED_OPEN_TAG)
+    lowered.starts_with(ENVIRONMENT_CONTEXT_OPEN_TAG)
+        || lowered.starts_with(TURN_ABORTED_OPEN_TAG)
+        || lowered.starts_with(CONTEXT_UPDATE_OPEN_TAG)
+}
+
+pub(crate) fn is_contextual_user_message(content: &[ContentItem]) -> bool {
+    if UserInstructions::is_user_instructions(content)
+        || SkillInstructions::is_skill_instructions(content)
+    {
+        return true;
+    }
+
+    content.iter().any(|content_item| match content_item {
+        ContentItem::InputText { text } => {
+            is_session_prefix(text) || is_user_shell_command_text(text)
+        }
+        ContentItem::OutputText { text } => is_session_prefix(text),
+        ContentItem::InputImage { .. } => false,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use codex_protocol::models::ContentItem;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn recognizes_context_update_session_prefix() {
+        assert!(is_session_prefix(
+            "<context_update>\nfoo\n</context_update>"
+        ));
+        assert!(is_session_prefix(
+            "   <context_update>\nfoo\n</context_update>"
+        ));
+        assert!(is_session_prefix(
+            "<CONTEXT_UPDATE>\nfoo\n</CONTEXT_UPDATE>"
+        ));
+    }
+
+    #[test]
+    fn recognizes_legacy_session_prefixes() {
+        assert!(is_session_prefix(
+            "<environment_context>foo</environment_context>"
+        ));
+        assert!(is_session_prefix("<turn_aborted>foo</turn_aborted>"));
+    }
+
+    #[test]
+    fn does_not_treat_plain_text_as_session_prefix() {
+        assert_eq!(is_session_prefix("normal user message"), false);
+    }
+
+    #[test]
+    fn contextual_user_message_detects_context_markers_and_wrappers() {
+        let shell = [ContentItem::InputText {
+            text: "<user_shell_command>echo hi</user_shell_command>".to_string(),
+        }];
+        let context_update = [ContentItem::InputText {
+            text: "<context_update>\nfoo\n</context_update>".to_string(),
+        }];
+        let user_instructions = [ContentItem::InputText {
+            text: "# AGENTS.md instructions for test\n\n<INSTRUCTIONS>\nfoo\n</INSTRUCTIONS>"
+                .to_string(),
+        }];
+        let skill = [ContentItem::InputText {
+            text: "<skill>\n<name>demo</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>"
+                .to_string(),
+        }];
+
+        assert!(is_contextual_user_message(&shell));
+        assert!(is_contextual_user_message(&context_update));
+        assert!(is_contextual_user_message(&user_instructions));
+        assert!(is_contextual_user_message(&skill));
+    }
+
+    #[test]
+    fn contextual_user_message_keeps_real_user_content() {
+        let real_message = [
+            ContentItem::InputText {
+                text: "normal user message".to_string(),
+            },
+            ContentItem::InputImage {
+                image_url: "https://example.com/img.png".to_string(),
+            },
+        ];
+
+        assert_eq!(is_contextual_user_message(&real_message), false);
+    }
 }