Narrow rollback developer context trimming

Co-authored-by: Codex <noreply@openai.com>

Author: charley-oai

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

@@ -1,5 +1,6 @@
 use crate::codex::TurnContext;
 use crate::context_manager::normalize;
+use crate::event_mapping::is_contextual_dev_message_content;
 use crate::event_mapping::is_contextual_user_message_content;
 use crate::truncate::TruncationPolicy;
 use crate::truncate::approx_bytes_for_tokens;
@@ -650,7 +651,8 @@ fn is_pre_turn_context_update_item(item: &ResponseItem) -> bool {
         return false;
     };
 
-    role == "developer" || (role == "user" && is_contextual_user_message_content(content))
+    (role == "developer" && is_contextual_dev_message_content(content))
+        || (role == "user" && is_contextual_user_message_content(content))
 }
 
 fn user_message_positions(items: &[ResponseItem]) -> Vec<usize> {

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

@@ -869,6 +869,7 @@ fn drop_last_n_user_turns_trims_context_updates_above_rolled_back_turn() {
         assistant_msg("session prefix item"),
         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("<collaboration_mode>ROLLED_BACK_DEV_INSTRUCTIONS</collaboration_mode>"),
         user_input_text_msg(
             "<environment_context><cwd>PRETURN_CONTEXT_DIFF_CWD</cwd></environment_context>",
@@ -887,6 +888,7 @@ fn drop_last_n_user_turns_trims_context_updates_above_rolled_back_turn() {
             assistant_msg("session prefix item"),
             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."),
         ]
     );
 }

codex-rs/core/src/event_mapping.rs

@@ -14,6 +14,8 @@ use codex_protocol::models::is_image_close_tag_text;
 use codex_protocol::models::is_image_open_tag_text;
 use codex_protocol::models::is_local_image_close_tag_text;
 use codex_protocol::models::is_local_image_open_tag_text;
+use codex_protocol::protocol::COLLABORATION_MODE_OPEN_TAG;
+use codex_protocol::protocol::REALTIME_CONVERSATION_OPEN_TAG;
 use codex_protocol::user_input::UserInput;
 use tracing::warn;
 use uuid::Uuid;
@@ -22,10 +24,35 @@ use crate::contextual_user_message::is_contextual_user_fragment;
 use crate::contextual_user_message::parse_visible_hook_prompt_message;
 use crate::web_search::web_search_action_detail;
 
+const CONTEXTUAL_DEVELOPER_PREFIXES: &[&str] = &[
+    "<permissions instructions>",
+    "<model_switch>",
+    COLLABORATION_MODE_OPEN_TAG,
+    REALTIME_CONVERSATION_OPEN_TAG,
+    "<personality_spec>",
+];
+
 pub(crate) fn is_contextual_user_message_content(message: &[ContentItem]) -> bool {
     message.iter().any(is_contextual_user_fragment)
 }
 
+pub(crate) fn is_contextual_dev_message_content(message: &[ContentItem]) -> bool {
+    message.iter().any(is_contextual_dev_fragment)
+}
+
+fn is_contextual_dev_fragment(content_item: &ContentItem) -> bool {
+    let ContentItem::InputText { text } = content_item else {
+        return false;
+    };
+
+    let trimmed = text.trim_start();
+    CONTEXTUAL_DEVELOPER_PREFIXES.iter().any(|prefix| {
+        trimmed
+            .get(..prefix.len())
+            .is_some_and(|candidate| candidate.eq_ignore_ascii_case(prefix))
+    })
+}
+
 fn parse_user_message(message: &[ContentItem]) -> Option<UserMessageItem> {
     if is_contextual_user_message_content(message) {
         return None;