[codex-core] Preserve ghost snapshots in compaction fallback [ci changed_files]

Keep same-turn ghost snapshots when pre-turn inline compaction downgrades to the legacy client-side path so undo state survives compatibility fallback.

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

codex-rs/core/src/codex.rs

@@ -5990,6 +5990,20 @@ struct PreTurnInlineCompactionState {
     turn_context_item: TurnContextItem,
 }
 
+fn collect_new_ghost_snapshots_since(
+    history_before_turn: &[ResponseItem],
+    current_history: &[ResponseItem],
+) -> Vec<ResponseItem> {
+    current_history
+        .iter()
+        .filter(|item| {
+            matches!(item, ResponseItem::GhostSnapshot { .. })
+                && !history_before_turn.contains(item)
+        })
+        .cloned()
+        .collect()
+}
+
 fn record_compaction_metric(
     sess: &Session,
     mode: &'static str,
@@ -6127,8 +6141,17 @@ async fn downgrade_known_inline_compaction_error(
             let Some(preturn_state) = preturn_state else {
                 return Ok(false);
             };
+            // Preserve same-turn ghost snapshots that may have completed after
+            // the pre-turn baseline was captured so `/undo` still works after
+            // we downgrade to the legacy compaction path.
+            let current_history = sess.clone_history().await;
+            let mut restored_history = preturn_state.history_before_turn.clone();
+            restored_history.extend(collect_new_ghost_snapshots_since(
+                &preturn_state.history_before_turn,
+                current_history.raw_items(),
+            ));
             sess.replace_history(
-                preturn_state.history_before_turn.clone(),
+                restored_history,
                 preturn_state.reference_context_before_turn.clone(),
             )
             .await;

codex-rs/core/src/codex_tests.rs

@@ -55,6 +55,7 @@ use crate::tools::registry::ToolHandler;
 use crate::tools::router::ToolCallSource;
 use crate::turn_diff_tracker::TurnDiffTracker;
 use codex_app_server_protocol::AppInfo;
+use codex_git::GhostCommit;
 use codex_otel::TelemetryAuthMode;
 use codex_protocol::models::BaseInstructions;
 use codex_protocol::models::ContentItem;
@@ -136,6 +137,12 @@ fn skill_message(text: &str) -> ResponseItem {
     }
 }
 
+fn ghost_snapshot(id: &str) -> ResponseItem {
+    ResponseItem::GhostSnapshot {
+        ghost_commit: GhostCommit::new(id.to_string(), None, Vec::new(), Vec::new()),
+    }
+}
+
 fn developer_input_texts(items: &[ResponseItem]) -> Vec<&str> {
     items
         .iter()
@@ -253,6 +260,24 @@ fn assistant_message_stream_parsers_seed_plan_parser_across_added_and_delta_boun
     assert!(tail.plan_segments.is_empty());
 }
 
+#[test]
+fn collect_new_ghost_snapshots_since_returns_only_snapshots_added_after_turn_start() {
+    let prior_snapshot = ghost_snapshot("ghost-before");
+    let same_turn_snapshot = ghost_snapshot("ghost-during");
+    let history_before_turn = vec![user_message("earlier"), prior_snapshot.clone()];
+    let current_history = vec![
+        user_message("earlier"),
+        prior_snapshot,
+        user_message("current turn"),
+        assistant_message("in progress"),
+        same_turn_snapshot.clone(),
+    ];
+
+    let new_snapshots = collect_new_ghost_snapshots_since(&history_before_turn, &current_history);
+
+    assert_eq!(new_snapshots, vec![same_turn_snapshot]);
+}
+
 fn make_mcp_tool(
     server_name: &str,
     tool_name: &str,