core: simplify compaction tail merge helper

Rename the append-only compaction merge helper, switch it to a boolean success signal, and document the exact concurrency assumption.

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

codex-rs/core/src/compact.rs

@@ -212,13 +212,11 @@ async fn run_compact_task_inner(
         .collect();
     new_history.extend(ghost_snapshots);
     let latest_history_snapshot = sess.clone_history().await;
-    if merge_appended_history_items(
+    if !append_concurrent_history_tail_if_append_only(
         &mut new_history,
         history_items,
         latest_history_snapshot.raw_items(),
-    )
-    .is_none()
-    {
+    ) {
         warn!(
             turn_id = %turn_context.sub_id,
             "session history changed non-append-only during compaction; skipping concurrent tail merge"
@@ -284,21 +282,28 @@ pub(crate) fn is_summary_message(message: &str) -> bool {
     message.starts_with(format!("{SUMMARY_PREFIX}\n").as_str())
 }
 
-pub(crate) fn merge_appended_history_items(
+/// Appends items added after `base_history` only when `latest_history` still
+/// preserves `base_history` as an exact prefix.
+///
+/// Returns `true` when no concurrent history change occurred or when the newer
+/// history differs only by append-only tail growth, in which case that tail is
+/// appended to `new_history`. Returns `false` when concurrent history mutation
+/// rewrote or removed earlier items, since this helper cannot safely merge that
+/// shape.
+pub(crate) fn append_concurrent_history_tail_if_append_only(
     new_history: &mut Vec<ResponseItem>,
     base_history: &[ResponseItem],
     latest_history: &[ResponseItem],
-) -> Option<usize> {
+) -> bool {
     if latest_history == base_history {
-        return Some(0);
+        return true;
     }
     if !latest_history.starts_with(base_history) {
-        return None;
+        return false;
     }
 
-    let appended_count = latest_history.len() - base_history.len();
     new_history.extend_from_slice(&latest_history[base_history.len()..]);
-    Some(appended_count)
+    true
 }
 
 /// Inserts canonical initial context into compacted replacement history at the

codex-rs/core/src/compact_remote.rs

@@ -6,8 +6,8 @@ use crate::codex::Session;
 use crate::codex::TurnContext;
 use crate::codex::built_tools;
 use crate::compact::InitialContextInjection;
+use crate::compact::append_concurrent_history_tail_if_append_only;
 use crate::compact::insert_initial_context_before_last_real_user_or_summary;
-use crate::compact::merge_appended_history_items;
 use crate::context_manager::ContextManager;
 use crate::context_manager::TotalTokenUsageBreakdown;
 use crate::context_manager::estimate_response_item_model_visible_bytes;
@@ -152,13 +152,11 @@ async fn run_remote_compact_task_inner_impl(
         new_history.extend(ghost_snapshots);
     }
     let latest_history_snapshot = sess.clone_history().await;
-    if merge_appended_history_items(
+    if !append_concurrent_history_tail_if_append_only(
         &mut new_history,
         history_snapshot.raw_items(),
         latest_history_snapshot.raw_items(),
-    )
-    .is_none()
-    {
+    ) {
         warn!(
             turn_id = %turn_context.sub_id,
             "session history changed non-append-only during remote compaction; skipping concurrent tail merge"

codex-rs/core/src/compact_tests.rs

@@ -186,7 +186,7 @@ fn build_token_limited_compacted_history_appends_summary_message() {
 }
 
 #[test]
-fn merge_appended_history_items_appends_concurrent_tail() {
+fn append_concurrent_history_tail_if_append_only_appends_concurrent_tail() {
     let base_history = vec![ResponseItem::Message {
         id: None,
         role: "user".to_string(),
@@ -215,10 +215,13 @@ fn merge_appended_history_items_appends_concurrent_tail() {
         phase: None,
     }];
 
-    let merged =
-        merge_appended_history_items(&mut compacted_history, &base_history, &latest_history);
+    let merged = append_concurrent_history_tail_if_append_only(
+        &mut compacted_history,
+        &base_history,
+        &latest_history,
+    );
 
-    assert_eq!(merged, Some(1));
+    assert!(merged);
     assert_eq!(
         compacted_history,
         vec![
@@ -237,7 +240,7 @@ fn merge_appended_history_items_appends_concurrent_tail() {
 }
 
 #[test]
-fn merge_appended_history_items_rejects_non_append_only_changes() {
+fn append_concurrent_history_tail_if_append_only_rejects_non_append_only_changes() {
     let base_history = vec![ResponseItem::Message {
         id: None,
         role: "user".to_string(),
@@ -266,10 +269,13 @@ fn merge_appended_history_items_rejects_non_append_only_changes() {
         phase: None,
     }];
 
-    let merged =
-        merge_appended_history_items(&mut compacted_history, &base_history, &latest_history);
+    let merged = append_concurrent_history_tail_if_append_only(
+        &mut compacted_history,
+        &base_history,
+        &latest_history,
+    );
 
-    assert_eq!(merged, None);
+    assert!(!merged);
     assert_eq!(
         compacted_history,
         vec![ResponseItem::Message {