[core] Fix empty-history inline summary replacement [ci changed_files]

Handle repeated inline compactions on turns that started from empty history by stripping leading compaction items after prefix calculation, and add regression coverage for the fresh-session case.

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

codex-rs/core/src/codex.rs

@@ -5951,21 +5951,21 @@ fn build_server_side_compaction_replacement_history(
     history_before_turn: &[ResponseItem],
     current_history: &[ResponseItem],
 ) -> Vec<ResponseItem> {
-    let current_turn_items =
-        if let Some(current_turn_items) = current_history.strip_prefix(history_before_turn) {
-            current_turn_items
-        } else if matches!(
-            current_history.first(),
-            Some(ResponseItem::Compaction { .. })
-        ) {
-            let first_non_compaction = current_history
-                .iter()
-                .position(|item| !matches!(item, ResponseItem::Compaction { .. }))
-                .unwrap_or(current_history.len());
-            &current_history[first_non_compaction..]
-        } else {
-            current_history
-        };
+    let current_turn_items = current_history
+        .strip_prefix(history_before_turn)
+        .unwrap_or(current_history);
+    let current_turn_items = if matches!(
+        current_turn_items.first(),
+        Some(ResponseItem::Compaction { .. })
+    ) {
+        let first_non_compaction = current_turn_items
+            .iter()
+            .position(|item| !matches!(item, ResponseItem::Compaction { .. }))
+            .unwrap_or(current_turn_items.len());
+        &current_turn_items[first_non_compaction..]
+    } else {
+        current_turn_items
+    };
     let mut replacement_history = vec![compaction_item];
     replacement_history.extend(
         current_turn_items

codex-rs/core/src/codex_tests.rs

@@ -341,6 +341,45 @@ fn build_server_side_compaction_replacement_history_replaces_prior_same_turn_sum
     );
 }
 
+#[test]
+fn build_server_side_compaction_replacement_history_replaces_prior_summary_with_empty_history() {
+    let same_turn_snapshot = ghost_snapshot("ghost-during");
+    let history_before_turn = Vec::new();
+    let current_turn_user = user_message("current turn");
+    let current_turn_tool_output = ResponseItem::FunctionCallOutput {
+        call_id: "call-1".to_string(),
+        output: FunctionCallOutputPayload::from_text("tool result".to_string()),
+    };
+    let prior_compaction = ResponseItem::Compaction {
+        encrypted_content: "INLINE_SUMMARY_1".to_string(),
+    };
+    let new_compaction = ResponseItem::Compaction {
+        encrypted_content: "INLINE_SUMMARY_2".to_string(),
+    };
+    let current_history = vec![
+        prior_compaction,
+        current_turn_user.clone(),
+        current_turn_tool_output.clone(),
+        same_turn_snapshot.clone(),
+    ];
+
+    let replacement_history = build_server_side_compaction_replacement_history(
+        new_compaction.clone(),
+        &history_before_turn,
+        &current_history,
+    );
+
+    assert_eq!(
+        replacement_history,
+        vec![
+            new_compaction,
+            current_turn_user,
+            current_turn_tool_output,
+            same_turn_snapshot,
+        ]
+    );
+}
+
 fn make_mcp_tool(
     server_name: &str,
     tool_name: &str,