From b029cc2d01d2e5bd334fa46b1d7bdb4596a69dd7 Mon Sep 17 00:00:00 2001
From: Charles Cunningham <ccunningham@openai.com>
Date: Sun, 15 Feb 2026 17:56:37 -0800
Subject: [PATCH] Reinject context for summary-only compacted history

---
 codex-rs/core/src/compact.rs                  | 56 +++++++++++++------
 ..._history_reinject_summary_only_shapes.snap |  9 +++
 2 files changed, 48 insertions(+), 17 deletions(-)
 create mode 100644 codex-rs/core/src/snapshots/codex_core__compact__tests__process_compacted_history_reinject_summary_only_shapes.snap

diff --git a/codex-rs/core/src/compact.rs b/codex-rs/core/src/compact.rs
index 6223287ee2..f7f395d8b6 100644
--- a/codex-rs/core/src/compact.rs
+++ b/codex-rs/core/src/compact.rs
@@ -389,15 +389,27 @@ pub(crate) fn process_compacted_history(
 
     match turn_context_reinjection {
         TurnContextReinjection::ReinjectAboveLastRealUser => {
-            // Insert immediately above the last real user message so turn context applies to that
-            // user input rather than an earlier turn.
-            if let Some(insertion_index) = compacted_history
+            // Prefer inserting immediately above the last real user message so turn context
+            // applies to that user input rather than an earlier turn. If compaction output is
+            // summary-only, insert before the first summary user message to keep canonical context
+            // present for the next sampling request.
+            let insertion_index = if let Some(last_real_user_index) = compacted_history
                 .iter()
                 .rposition(is_non_summary_user_message)
             {
-                compacted_history
-                    .splice(insertion_index..insertion_index, initial_context.to_vec());
-            }
+                last_real_user_index
+            } else if let Some(first_summary_index) = compacted_history.iter().position(|item| {
+                matches!(
+                    crate::event_mapping::parse_turn_item(item),
+                    Some(TurnItem::UserMessage(user_message))
+                        if is_summary_message(&user_message.message())
+                )
+            }) {
+                first_summary_index
+            } else {
+                compacted_history.len()
+            };
+            compacted_history.splice(insertion_index..insertion_index, initial_context.to_vec());
         }
         TurnContextReinjection::Skip => {}
     }
@@ -560,7 +572,12 @@ async fn drain_to_completed(
 mod tests {
 
     use super::*;
+    use core_test_support::context_snapshot;
+    use core_test_support::context_snapshot::ContextSnapshotOptions;
+    use core_test_support::context_snapshot::ContextSnapshotRenderMode;
+    use insta::assert_snapshot;
     use pretty_assertions::assert_eq;
+    use serde_json::Value;
 
     #[test]
     fn content_items_to_text_joins_non_empty_segments() {
@@ -1534,7 +1551,7 @@ keep me updated
     }
 
     #[test]
-    fn process_compacted_history_reinject_noops_without_real_user_message() {
+    fn process_compacted_history_reinjects_context_when_compaction_output_is_summary_only() {
         let compacted_history = vec![ResponseItem::Message {
             id: None,
             role: "user".to_string(),
@@ -1559,16 +1576,21 @@ keep me updated
             &initial_context,
             TurnContextReinjection::ReinjectAboveLastRealUser,
         );
-        let expected = vec![ResponseItem::Message {
-            id: None,
-            role: "user".to_string(),
-            content: vec![ContentItem::InputText {
-                text: format!("{SUMMARY_PREFIX}\nsummary text"),
-            }],
-            end_turn: None,
-            phase: None,
-        }];
-        assert_eq!(refreshed, expected);
+        let refreshed_value =
+            serde_json::to_value(&refreshed).expect("serialize refreshed history");
+        let Value::Array(refreshed_items) = refreshed_value else {
+            panic!("expected refreshed history to serialize as array");
+        };
+
+        assert_snapshot!(
+            "process_compacted_history_reinject_summary_only_shapes",
+            context_snapshot::format_labeled_items_snapshot(
+                "When compaction output contains only a summary user message, canonical context is still reinserted before the summary.",
+                &[("Refreshed History Layout", refreshed_items.as_slice())],
+                &ContextSnapshotOptions::default()
+                    .render_mode(ContextSnapshotRenderMode::KindWithTextPrefix { max_chars: 64 }),
+            )
+        );
     }
 
     #[test]
diff --git a/codex-rs/core/src/snapshots/codex_core__compact__tests__process_compacted_history_reinject_summary_only_shapes.snap b/codex-rs/core/src/snapshots/codex_core__compact__tests__process_compacted_history_reinject_summary_only_shapes.snap
new file mode 100644
index 0000000000..aba7c96a6d
--- /dev/null
+++ b/codex-rs/core/src/snapshots/codex_core__compact__tests__process_compacted_history_reinject_summary_only_shapes.snap
@@ -0,0 +1,9 @@
+---
+source: core/src/compact.rs
+expression: "context_snapshot::format_labeled_items_snapshot(\"When compaction output contains only a summary user message, canonical context is still reinserted before the summary.\",\n&[(\"Refreshed History Layout\", refreshed_items.as_slice())],\n&ContextSnapshotOptions::default().render_mode(ContextSnapshotRenderMode::KindWithTextPrefix\n{ max_chars: 64 }),)"
+---
+Scenario: When compaction output contains only a summary user message, canonical context is still reinserted before the summary.
+
+## Refreshed History Layout
+00:message/developer:fresh permissions
+01:message/user:<COMPACTION_SUMMARY>\nsummary text