diff --git a/navi/api/routes/sessions.py b/navi/api/routes/sessions.py
index 77d9dcc..d3fb32c 100644
--- a/navi/api/routes/sessions.py
+++ b/navi/api/routes/sessions.py
@@ -94,6 +94,8 @@
     for session in sessions:
         if not session.messages:
             continue
+        if session.user_id is None:
+            continue  # skip legacy sessions — no multi-user memory for unowned sessions
         if session.last_active >= cutoff:
             continue  # still active
         extracted_at = await memory.get_extracted_at(session.id)
diff --git a/navi/memory/_facts.py b/navi/memory/_facts.py
index e94a847..b75bea8 100644
--- a/navi/memory/_facts.py
+++ b/navi/memory/_facts.py
@@ -88,9 +88,6 @@
                 )
 
     async def search_facts(self, query: str, user_id: str | None = None, limit: int = 15) -> list[dict]:
-        user_clause = "AND user_id IS NULL" if user_id is None else "AND user_id = $3"
-        user_param = () if user_id is None else (user_id,)
-
         # 1. Try vector search if pgvector + embedding backend are available
         if self._embedding_backend and await self._has_pgvector():
             query_embedding = await self._generate_embedding(query)
diff --git a/navi/memory/_summary.py b/navi/memory/_summary.py
index 716306a..467ab0a 100644
--- a/navi/memory/_summary.py
+++ b/navi/memory/_summary.py
@@ -1,5 +1,6 @@
 """Conversation summary persistence — single-row table."""
 
+import zlib
 from datetime import datetime, timezone
 
 
@@ -12,7 +13,8 @@
     if user_id is None:
         return 1
     # Deterministic, non-overlapping with legacy id=1
-    return (abs(hash(user_id)) % 2147483647) + 2
+    # zlib.crc32 is stable across process restarts (unlike Python hash()).
+    return (abs(zlib.crc32(user_id.encode())) % 2147483646) + 2
 
 
 class SummaryMixin:
diff --git a/navi/memory/extractor.py b/navi/memory/extractor.py
index bebcb80..f4d135f 100644
--- a/navi/memory/extractor.py
+++ b/navi/memory/extractor.py
@@ -75,14 +75,19 @@
     """
     Extract facts from a session and update the memory summary.
     Safe to call multiple times — already-extracted sessions produce no duplicates.
+    Legacy sessions (user_id=None) are skipped — no multi-user memory for unowned sessions.
     """
+    user_id = getattr(session, "user_id", None)
+    if user_id is None:
+        return
+
     facts_added = await _extract_facts(session, llm, model, memory_store)
     log.info("memory.extracted", session_id=session.id, facts_added=facts_added)
 
     await memory_store.mark_session_extracted(session.id)
 
     if facts_added > 0:
-        await _regenerate_summary(llm, model, memory_store, user_id=session.user_id)
+        await _regenerate_summary(llm, model, memory_store, user_id=user_id)
 
 
 _MAX_TOOL_RESULT_LEN = 500
diff --git a/tests/unit/memory/test_extractor.py b/tests/unit/memory/test_extractor.py
index 78caa73..dcb0b82 100644
--- a/tests/unit/memory/test_extractor.py
+++ b/tests/unit/memory/test_extractor.py
@@ -115,7 +115,7 @@
         conn = FakeConnection()
         conn.enqueue("OK")  # mark_session_extracted
         store = make_store_with_pool(conn)
-        session = FakeSession([Message(role="user", content="hi")])
+        session = FakeSession([Message(role="user", content="hi")], user_id="user-1")
         await extract_and_update(session, backend, "test-model", store)
         assert any("session_memory_state" in c[1] for c in conn.calls)
 
@@ -134,7 +134,7 @@
         ])
         conn.enqueue("OK")  # set_summary
         store = make_store_with_pool(conn)
-        session = FakeSession([Message(role="user", content="My name is Eugene")])
+        session = FakeSession([Message(role="user", content="My name is Eugene")], user_id="user-1")
         await extract_and_update(session, backend, "test-model", store)
         assert any("memory_summary" in c[1] for c in conn.calls)
 
@@ -143,7 +143,16 @@
         conn = FakeConnection()
         conn.enqueue("OK")  # mark_session_extracted
         store = make_store_with_pool(conn)
-        session = FakeSession([Message(role="user", content="hi")])
+        session = FakeSession([Message(role="user", content="hi")], user_id="user-1")
         await extract_and_update(session, backend, "test-model", store)
         # Should NOT call get_all_facts or set_summary
         assert not any("memory_summary" in c[1] for c in conn.calls)
+
+    async def test_skips_legacy_sessions(self):
+        backend = FakeLLMBackend(responses=["[]"])
+        conn = FakeConnection()
+        store = make_store_with_pool(conn)
+        session = FakeSession([Message(role="user", content="hi")], user_id=None)
+        await extract_and_update(session, backend, "test-model", store)
+        # No DB calls for legacy (user_id=None) sessions
+        assert len(conn.calls) == 0