fix: simplify session recency activity tracking

maintainer edit: remove the _last_compacted_at maintenance state, gate idle compaction on whether a session still has a removable tail, and sort WebUI sessions by the latest visible transcript activity.

tests/agent/test_auto_compact.py

@@ -8,7 +8,6 @@ from unittest.mock import AsyncMock, MagicMock
 import pytest
 
 from nanobot.agent.loop import AgentLoop
-from nanobot.agent.memory import LAST_COMPACTED_AT_META
 from nanobot.bus.events import InboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.command import CommandContext
@@ -89,11 +88,9 @@ def _make_fake_compact(
     async def _fake_compact(key: str, max_suffix: int = 8) -> str:
         state["count"] += 1
         session = loop.sessions.get_or_create(key)
-        compacted_at = datetime.now().isoformat()
 
         tail = list(session.messages[session.last_consolidated:])
         if not tail:
-            session.metadata[LAST_COMPACTED_AT_META] = compacted_at
             loop.sessions.save(session)
             return ""
 
@@ -113,7 +110,6 @@ def _make_fake_compact(
         archive_msgs = result.dropped[result.already_consolidated_count:]
 
         if not archive_msgs and not kept:
-            session.metadata[LAST_COMPACTED_AT_META] = compacted_at
             loop.sessions.save(session)
             return ""
 
@@ -134,7 +130,6 @@ def _make_fake_compact(
 
         session.messages = kept
         session.last_consolidated = 0
-        session.metadata[LAST_COMPACTED_AT_META] = compacted_at
         loop.sessions.save(session)
         return s
 
@@ -1023,27 +1018,28 @@ class TestProactiveAutoCompact:
         await self._run_check_expired(loop)
         assert _fake_compact.state["count"] == 1
 
-        # Second tick: should NOT re-schedule (maintenance timestamp is fresh)
+        # Second tick: should NOT re-schedule because the session has no removable tail.
         await self._run_check_expired(loop)
         assert _fake_compact.state["count"] == 1  # Still 1, not re-scheduled
         await loop.close_mcp()
 
     @pytest.mark.asyncio
-    async def test_empty_skip_records_compaction_prevents_reschedule(self, tmp_path):
-        """Empty session skip records maintenance, preventing immediate re-scheduling."""
+    async def test_empty_session_does_not_schedule_idle_compact(self, tmp_path):
+        """Empty expired sessions have no removable tail and should not schedule."""
         loop = _make_loop(tmp_path, session_ttl_minutes=15)
         session = loop.sessions.get_or_create("cli:test")
         session.updated_at = datetime.now() - timedelta(minutes=20)
         loop.sessions.save(session)
 
-        loop.consolidator.compact_idle_session = _make_fake_compact(loop)
+        _fake_compact = _make_fake_compact(loop)
+        loop.consolidator.compact_idle_session = _fake_compact
 
-        # First tick: skips (no messages), records compaction metadata
         await self._run_check_expired(loop)
+        assert _fake_compact.state["count"] == 0
         assert "cli:test" not in loop.auto_compact._summaries
 
-        # Second tick: should NOT re-schedule because maintenance metadata is fresh
         await self._run_check_expired(loop)
+        assert _fake_compact.state["count"] == 0
         assert "cli:test" not in loop.auto_compact._summaries
         await loop.close_mcp()
 

tests/agent/test_autocompact_unit.py

@@ -6,7 +6,6 @@ from unittest.mock import AsyncMock, MagicMock
 import pytest
 
 from nanobot.agent.autocompact import AutoCompact
-from nanobot.agent.memory import LAST_COMPACTED_AT_META
 from nanobot.session.manager import Session, SessionManager
 
 
@@ -201,8 +200,11 @@ class TestCheckExpired:
         """Expired session should trigger schedule_background."""
         ac = _make_autocompact(ttl=15)
         mock_sm = MagicMock(spec=SessionManager)
-        old_ts = (datetime.now() - timedelta(minutes=20)).isoformat()
-        mock_sm.list_sessions.return_value = [{"key": "cli:old", "updated_at": old_ts}]
+        old_dt = datetime.now() - timedelta(minutes=20)
+        session = _make_session("cli:old", updated_at=old_dt)
+        _add_turns(session, 5)
+        mock_sm.list_sessions.return_value = [{"key": "cli:old", "updated_at": old_dt.isoformat()}]
+        mock_sm.get_or_create.return_value = session
         ac.sessions = mock_sm
 
         scheduled = []
@@ -274,17 +276,17 @@ class TestCheckExpired:
         scheduler.assert_not_called()
         assert "dream:20260602-155256" not in ac._archiving
 
-    def test_already_compacted_session_skips(self):
-        """Expired session already maintained after last activity should not be re-scheduled."""
+    def test_already_trimmed_session_skips(self):
+        """Expired session with no removable tail should not be re-scheduled."""
         ac = _make_autocompact(ttl=15)
         mock_sm = MagicMock(spec=SessionManager)
         last_active = datetime(2026, 1, 1, 10, 0, 0)
+        session = _make_session("cli:done", updated_at=last_active)
+        _add_turns(session, 2)
         mock_sm.list_sessions.return_value = [
             {"key": "cli:done", "updated_at": last_active.isoformat()},
         ]
-        mock_sm.read_session_metadata.return_value = {
-            "metadata": {LAST_COMPACTED_AT_META: datetime(2026, 1, 1, 10, 30, 0).isoformat()},
-        }
+        mock_sm.get_or_create.return_value = session
         ac.sessions = mock_sm
 
         scheduler = MagicMock()

tests/agent/test_consolidator.py

@@ -6,7 +6,6 @@ import pytest
 
 from nanobot.agent.memory import (
     _ARCHIVE_SUMMARY_MAX_CHARS,
-    LAST_COMPACTED_AT_META,
     Consolidator,
     MemoryStore,
 )
@@ -448,7 +447,6 @@ class TestCompactIdleSession:
         assert meta is not None
         assert meta["text"] == "Summary of old conversation."
         assert "last_active" in meta
-        assert LAST_COMPACTED_AT_META in reloaded.metadata
         assert reloaded.updated_at == old_ts
 
     @pytest.mark.asyncio
@@ -523,10 +521,10 @@ class TestCompactIdleSession:
         assert entries[0]["session_key"] == "cli:test"
 
     @pytest.mark.asyncio
-    async def test_empty_session_records_compaction_without_refreshing_timestamp(
+    async def test_empty_session_does_not_refresh_timestamp(
         self, real_consolidator
     ):
-        """Empty session with old updated_at records maintenance separately."""
+        """Empty session with old updated_at does not look active after compaction."""
         from datetime import datetime, timedelta
 
         sessions = real_consolidator.sessions
@@ -540,7 +538,7 @@ class TestCompactIdleSession:
 
         reloaded = sessions.get_or_create("cli:empty")
         assert reloaded.updated_at == old_ts
-        assert LAST_COMPACTED_AT_META in reloaded.metadata
+        assert reloaded.metadata == {}
 
     @pytest.mark.asyncio
     async def test_nothing_summary_not_stored(self, real_consolidator, mock_provider):