fix(memory): add session-refresh guard to maybe_consolidate_by_tokens

When background consolidation runs with a stale session reference (captured
before AutoCompact replaced the session via compact_idle_session), it could
operate on outdated data. Now, after acquiring the per-session lock, the
method refreshes its session reference from SessionManager.get_or_create().
If the session was replaced, it swaps in the fresh reference before doing
any consolidation work.

This prevents a race where AutoCompact truncates an idle session while a
background maybe_consolidate_by_tokens call is in flight with the old
session object.

nanobot/agent/memory.py

@@ -683,6 +683,11 @@ class Consolidator:
 
         lock = self.get_lock(session.key)
         async with lock:
+            # Refresh session reference: AutoCompact may have replaced it.
+            fresh = self.sessions.get_or_create(session.key)
+            if fresh is not session:
+                session = fresh
+
             budget = self._input_token_budget
             target = int(budget * self.consolidation_ratio)
             last_summary = await self._consolidate_replay_overflow(

tests/agent/test_consolidator.py

@@ -28,6 +28,12 @@ def mock_provider():
 def consolidator(store, mock_provider):
     sessions = MagicMock()
     sessions.save = MagicMock()
+    # When maybe_consolidate_by_tokens refreshes the session reference via
+    # get_or_create(session.key), it should get back the same object the test
+    # passed in.  Store sessions by key so the lookup is transparent.
+    _session_cache: dict[str, MagicMock] = {}
+    sessions.get_or_create = MagicMock(side_effect=lambda key: _session_cache.get(key, MagicMock()))
+    sessions._session_cache = _session_cache
     return Consolidator(
         store=store,
         provider=mock_provider,
@@ -117,6 +123,7 @@ class TestConsolidatorTokenBudget:
         session.last_consolidated = 0
         session.messages = [{"role": "user", "content": "hi"}]
         session.key = "test:key"
+        consolidator.sessions._session_cache[session.key] = session
         consolidator.estimate_session_prompt_tokens = MagicMock(return_value=(100, "tiktoken"))
         consolidator.archive = AsyncMock(return_value=True)
         await consolidator.maybe_consolidate_by_tokens(session)
@@ -152,6 +159,7 @@ class TestConsolidatorTokenBudget:
             session.add_message("user", f"u{i}")
             session.add_message("assistant", f"a{i}")
 
+        consolidator.sessions._session_cache[session.key] = session
         consolidator.estimate_session_prompt_tokens = MagicMock(return_value=(100, "tiktoken"))
         consolidator.archive = AsyncMock(return_value="old conversation summary")
 
@@ -184,6 +192,7 @@ class TestConsolidatorTokenBudget:
         session.add_message("tool", "tool result", tool_call_id="call-1", name="x")
         session.add_message("assistant", "final answer")
 
+        consolidator.sessions._session_cache[session.key] = session
         consolidator.estimate_session_prompt_tokens = MagicMock(return_value=(100, "tiktoken"))
         consolidator.archive = AsyncMock(return_value="tool turn summary")
 
@@ -210,6 +219,7 @@ class TestConsolidatorTokenBudget:
             }
             for i in range(70)
         ]
+        consolidator.sessions._session_cache[session.key] = session
         consolidator.estimate_session_prompt_tokens = MagicMock(
             side_effect=[(1200, "tiktoken"), (400, "tiktoken")]
         )
@@ -238,6 +248,7 @@ class TestConsolidatorTokenBudget:
             for i in range(70)
         ]
         session.metadata = {}
+        consolidator.sessions._session_cache[session.key] = session
         consolidator.estimate_session_prompt_tokens = MagicMock(
             side_effect=[(1200, "tiktoken"), (400, "tiktoken")]
         )
@@ -263,6 +274,7 @@ class TestConsolidatorTokenBudget:
             for i in range(70)
         ]
         session.metadata = {}
+        consolidator.sessions._session_cache[session.key] = session
         # Keep estimates high so the loop would otherwise run multiple rounds.
         consolidator.estimate_session_prompt_tokens = MagicMock(
             return_value=(1200, "tiktoken")
@@ -287,6 +299,7 @@ class TestConsolidatorTokenBudget:
             }
             for i in range(70)
         ]
+        consolidator.sessions._session_cache[session.key] = session
         consolidator.estimate_session_prompt_tokens = MagicMock(
             side_effect=[(1200, "tiktoken"), (400, "tiktoken")]
         )
@@ -461,6 +474,57 @@ class TestCompactIdleSession:
         assert not lock.locked()
 
 
+class TestConsolidatorSessionRefresh:
+    """Background consolidation must detect stale session references."""
+
+    @pytest.mark.asyncio
+    async def test_reloads_stale_session_after_compact(self, tmp_path):
+        """After compact_idle_session replaces the session, a concurrent
+        maybe_consolidate_by_tokens with the old reference should use the
+        fresh session from cache instead of overwriting."""
+        from nanobot.agent.memory import Consolidator, MemoryStore
+        from nanobot.session.manager import SessionManager
+
+        store = MemoryStore(tmp_path)
+        provider = MagicMock()
+        provider.chat_with_retry = AsyncMock(
+            return_value=MagicMock(content="summary", finish_reason="stop")
+        )
+        provider.generation.max_tokens = 4096
+        provider.estimate_prompt_tokens = MagicMock(return_value=(10, "test"))
+        sessions = SessionManager(tmp_path)
+        consolidator = Consolidator(
+            store=store,
+            provider=provider,
+            model="test-model",
+            sessions=sessions,
+            context_window_tokens=128_000,
+            build_messages=MagicMock(return_value=[]),
+            get_tool_definitions=MagicMock(return_value=[]),
+        )
+
+        # Populate session with many messages
+        session = sessions.get_or_create("cli:test")
+        for i in range(20):
+            session.add_message("user", f"u{i}")
+            session.add_message("assistant", f"a{i}")
+        sessions.save(session)
+
+        # Simulate: background consolidation captures old reference
+        old_ref = session
+
+        # AutoCompact runs first and truncates to 8
+        await consolidator.compact_idle_session("cli:test", 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)
+
+        session_after = sessions.get_or_create("cli:test")
+        # Messages should still be truncated (not restored to 40)
+        assert len(session_after.messages) <= 8
+
+
 class TestRawArchiveTruncation:
     """raw_archive() must cap entry size to avoid bloating history.jsonl."""