fix: stabilize compaction summary timestamps

Author: Ariasu123

pion/session/manager.py

@@ -227,7 +227,15 @@ def build_context(self) -> list[Message]:
                 selected.append(entry)
         selected.extend(path[compaction_idx + 1 :])
 
-        messages: list[Message] = [UserMessage(content=compaction.summary or "")]
+        # The summary is a deterministic projection of the persisted
+        # compaction entry. Reusing its timestamp prevents repeated context
+        # builds (and reloads) from producing different message snapshots.
+        messages: list[Message] = [
+            UserMessage(
+                content=compaction.summary or "",
+                timestamp=compaction.timestamp,
+            )
+        ]
         messages.extend(
             entry.message
             for entry in selected

tests/test_session.py

@@ -81,6 +81,18 @@ def test_append_and_persistence_round_trip(tmp_path: Path) -> None:
     assert len(path.read_text(encoding="utf-8").strip().split("\n")) == 5
 
 
+def test_compaction_summary_timestamp_is_stable() -> None:
+    manager = SessionManager()
+    manager.append_message(_user("before"))
+    compaction_id = manager.append_compaction("stable summary")
+
+    first = manager.build_context()[0]
+    second = manager.build_context()[0]
+
+    assert first.timestamp == manager.get_entry(compaction_id).timestamp
+    assert second.timestamp == first.timestamp
+
+
 def test_in_memory_session_writes_nothing(tmp_path: Path) -> None:
     manager = SessionManager()
     manager.append_message(_user("hello"))