diff --git a/navi/memory/_facts.py b/navi/memory/_facts.py
index 2677782..e94a847 100644
--- a/navi/memory/_facts.py
+++ b/navi/memory/_facts.py
@@ -99,17 +99,30 @@
                 try:
                     pool = await self._get_pool()
                     async with pool.acquire() as conn:
-                        rows = await conn.fetch(
-                            """SELECT id, category, key, value, updated_at,
-                                      source, confidence, expires_at, source_context,
-                                      embedding <=> $1::vector AS distance
-                               FROM memory_facts
-                               WHERE (expires_at IS NULL OR expires_at > now())
-                                 AND (user_id IS NOT DISTINCT FROM $3)
-                               ORDER BY embedding <=> $1::vector
-                               LIMIT $2""",
-                            vec_str, limit, *user_param,
-                        )
+                        if user_id is None:
+                            rows = await conn.fetch(
+                                """SELECT id, category, key, value, updated_at,
+                                          source, confidence, expires_at, source_context,
+                                          embedding <=> $1::vector AS distance
+                                   FROM memory_facts
+                                   WHERE (expires_at IS NULL OR expires_at > now())
+                                     AND user_id IS NULL
+                                   ORDER BY embedding <=> $1::vector
+                                   LIMIT $2""",
+                                vec_str, limit,
+                            )
+                        else:
+                            rows = await conn.fetch(
+                                """SELECT id, category, key, value, updated_at,
+                                          source, confidence, expires_at, source_context,
+                                          embedding <=> $1::vector AS distance
+                                   FROM memory_facts
+                                   WHERE (expires_at IS NULL OR expires_at > now())
+                                     AND user_id = $3
+                                   ORDER BY embedding <=> $1::vector
+                                   LIMIT $2""",
+                                vec_str, limit, user_id,
+                            )
                         results = [
                             _row_to_dict(r)
                             for r in rows
diff --git a/navi/memory/_summary.py b/navi/memory/_summary.py
index d6baec4..716306a 100644
--- a/navi/memory/_summary.py
+++ b/navi/memory/_summary.py
@@ -3,6 +3,18 @@
 from datetime import datetime, timezone
 
 
+def _summary_id(user_id: str | None) -> int:
+    """Return a deterministic primary key for the user's summary row.
+
+    Legacy single-user mode uses id=1.  Multi-user mode generates a unique
+    id per user_id so the PRIMARY KEY constraint on `id` is never violated.
+    """
+    if user_id is None:
+        return 1
+    # Deterministic, non-overlapping with legacy id=1
+    return (abs(hash(user_id)) % 2147483647) + 2
+
+
 class SummaryMixin:
     """Summary storage operations.
 
@@ -12,19 +24,27 @@
 
     async def get_summary(self, user_id: str | None = None) -> str | None:
         pool = await self._get_pool()
+        sid = _summary_id(user_id)
         async with pool.acquire() as conn:
             if user_id is None:
-                return await conn.fetchval("SELECT content FROM memory_summary WHERE id=1 AND user_id IS NULL")
-            return await conn.fetchval("SELECT content FROM memory_summary WHERE id=1 AND user_id = $1", user_id)
+                return await conn.fetchval(
+                    "SELECT content FROM memory_summary WHERE id=$1 AND user_id IS NULL",
+                    sid,
+                )
+            return await conn.fetchval(
+                "SELECT content FROM memory_summary WHERE id=$1 AND user_id = $2",
+                sid, user_id,
+            )
 
     async def set_summary(self, content: str, user_id: str | None = None) -> None:
         now = datetime.now(timezone.utc)
+        sid = _summary_id(user_id)
         pool = await self._get_pool()
         async with pool.acquire() as conn:
             await conn.execute(
-                """INSERT INTO memory_summary (id, user_id, content, generated_at) VALUES (1, $1, $2, $3)
-                   ON CONFLICT(id, user_id) DO UPDATE SET
+                """INSERT INTO memory_summary (id, user_id, content, generated_at) VALUES ($1, $2, $3, $4)
+                   ON CONFLICT(id) DO UPDATE SET
                        content      = EXCLUDED.content,
                        generated_at = EXCLUDED.generated_at""",
-                user_id, content, now,
+                sid, user_id, content, now,
             )