fix(session): preserve history during idle compaction (#5167)

tests/agent/test_consolidator.py

@@ -586,7 +586,7 @@ class TestConsolidatorTokenBudget:
 
 
 class TestCompactIdleSession:
-    """Tests for Consolidator.compact_idle_session — lock-protected idle truncation."""
+    """Idle compaction tests."""
 
     @pytest.fixture
     def real_consolidator(self, store, mock_provider):
@@ -602,11 +602,9 @@ class TestCompactIdleSession:
         )
 
     @pytest.mark.asyncio
-    async def test_archives_prefix_keeps_suffix(
+    async def test_archives_prefix_preserves_messages_and_hides_prefix(
         self, real_consolidator, mock_provider, runtime
     ):
-        """20 user/assistant turns → compact with max_suffix=8 → messages ≤ 8,
-        last_consolidated=0, _last_summary stored."""
         mock_provider.chat_with_retry.return_value = MagicMock(
             content="Summary of old conversation.", finish_reason="stop"
         )
@@ -624,9 +622,15 @@ class TestCompactIdleSession:
         )
         assert result == "Summary of old conversation."
 
+        sessions.invalidate("cli:test")
         reloaded = sessions.get_or_create("cli:test")
-        assert len(reloaded.messages) <= 8
-        assert reloaded.last_consolidated == 0
+        assert len(reloaded.messages) == 40
+        assert reloaded.messages[0]["content"] == "user msg 0"
+        assert reloaded.last_consolidated == 32
+        visible = reloaded.get_history(max_messages=40)
+        assert len(visible) == 8
+        assert visible[0]["content"] == "user msg 16"
+        assert visible[-1]["content"] == "assistant msg 19"
         meta = reloaded.metadata.get("_last_summary")
         assert meta is not None
         assert meta["text"] == "Summary of old conversation."
@@ -665,9 +669,7 @@ class TestCompactIdleSession:
     async def test_raw_dumps_only_dropped_messages_on_llm_failure(
         self, real_consolidator, mock_provider, store, runtime
     ):
-        """Summarizing over the full tail must not widen what gets raw-dumped on
-        LLM failure: the breadcrumb should contain only the removed prefix, not
-        the retained suffix that stays live in the session. Regression for #4264."""
+        """Extra summary context must not enter raw fallback. Regression for #4264."""
         mock_provider.chat_with_retry.side_effect = RuntimeError("LLM unavailable")
         sessions = real_consolidator.sessions
         session = sessions.get_or_create("cli:rawdrop")
@@ -684,8 +686,11 @@ class TestCompactIdleSession:
 
         raw = "\n".join(e["content"] for e in store.read_unprocessed_history(since_cursor=0))
         assert "[RAW]" in raw
-        assert "user msg 0" in raw  # removed prefix is the breadcrumb
-        assert "RETAINED_SUFFIX_marker" not in raw  # retained suffix not dumped
+        assert "user msg 0" in raw
+        assert "RETAINED_SUFFIX_marker" not in raw
+        reloaded = sessions.get_or_create("cli:rawdrop")
+        assert len(reloaded.messages) == 38
+        assert reloaded.messages[-1]["content"] == "RETAINED_SUFFIX_marker"
 
     @pytest.mark.asyncio
     async def test_idle_compact_writes_session_key_to_history(
@@ -757,10 +762,9 @@ class TestCompactIdleSession:
         assert "_last_summary" not in reloaded.metadata
 
     @pytest.mark.asyncio
-    async def test_llm_failure_still_truncates(
+    async def test_llm_failure_preserves_history_but_advances_replay_boundary(
         self, real_consolidator, mock_provider, store, runtime
     ):
-        """LLM raises RuntimeError → raw_archive fires, session still truncated, returns None."""
         mock_provider.chat_with_retry.side_effect = RuntimeError("LLM unavailable")
         sessions = real_consolidator.sessions
         session = sessions.get_or_create("cli:fail")
@@ -778,9 +782,16 @@ class TestCompactIdleSession:
         entries = store.read_unprocessed_history(since_cursor=0)
         assert any("[RAW]" in e["content"] for e in entries)
 
-        # Session should still be truncated
         reloaded = sessions.get_or_create("cli:fail")
-        assert len(reloaded.messages) <= 4
+        assert len(reloaded.messages) == 20
+        assert reloaded.messages[0]["content"] == "u0"
+        assert reloaded.last_consolidated == 16
+        assert [m["content"] for m in reloaded.get_history(max_messages=20)] == [
+            "u8",
+            "a8",
+            "u9",
+            "a9",
+        ]
 
     @pytest.mark.asyncio
     async def test_respects_last_consolidated(
@@ -802,6 +813,9 @@ class TestCompactIdleSession:
             "cli:offset", runtime=runtime, max_suffix=4
         )
         assert result == "Tail summary."
+        reloaded = sessions.get_or_create("cli:offset")
+        assert len(reloaded.messages) == 60
+        assert reloaded.last_consolidated == 56
 
         # Verify only the unconsolidated tail was processed:
         # 10 unconsolidated messages (50-59), keep suffix of 4 → archive 6
@@ -812,14 +826,12 @@ class TestCompactIdleSession:
         assert "u25" in user_content or "a25" in user_content
 
     @pytest.mark.asyncio
-    async def test_non_contiguous_suffix_archives_actual_dropped_messages(
+    async def test_extended_suffix_archives_only_hidden_prefix(
         self,
         real_consolidator,
         mock_provider,
         runtime,
     ):
-        """Assistant-only tails extend back to the latest user turn, so archive
-        the actual dropped messages rather than a computed prefix."""
         mock_provider.chat_with_retry.return_value = MagicMock(
             content="Tail summary.", finish_reason="stop"
         )
@@ -837,7 +849,9 @@ class TestCompactIdleSession:
         assert result == "Tail summary."
 
         reloaded = sessions.get_or_create("cli:noncontiguous")
-        assert [m["content"] for m in reloaded.messages] == [
+        assert len(reloaded.messages) == 25
+        assert reloaded.last_consolidated == 14
+        assert [m["content"] for m in reloaded.get_history(max_messages=25)] == [
             "user-14",
             "assistant-00",
             "assistant-01",
@@ -987,23 +1001,21 @@ class TestConsolidatorSessionRefresh:
         # Simulate: background consolidation captures old reference
         old_ref = session
 
-        # AutoCompact runs first and truncates to 8
         await consolidator.compact_idle_session(
             "cli:test",
             runtime=runtime,
             max_suffix=8,
         )
 
-        # Background consolidation runs with stale reference —
-        # should detect the session was replaced and not undo the compact.
         await consolidator.maybe_consolidate_by_tokens(
             old_ref,
             runtime=runtime,
         )
 
         session_after = sessions.get_or_create("cli:test")
-        # Messages should still be truncated (not restored to 40)
-        assert len(session_after.messages) <= 8
+        assert len(session_after.messages) == 40
+        assert session_after.last_consolidated == 32
+        assert len(session_after.get_history(max_messages=40)) == 8
 
 
 class TestRawArchiveTruncation: