diff --git a/mem0/vector_stores/upstash_vector.py b/mem0/vector_stores/upstash_vector.py
index 3dbefc7ae..49c2bda79 100644
--- a/mem0/vector_stores/upstash_vector.py
+++ b/mem0/vector_stores/upstash_vector.py
@@ -132,11 +132,10 @@ class UpstashVector(VectorStoreBase):
                     "top_k": top_k,
                     "filter": filters_str or "",
                     "include_metadata": True,
-                    "namespace": self.collection_name,
                 }
                 for v in vectors
             ]
-            responses = self.client.query_many(queries=queries)
+            responses = self.client.query_many(queries=queries, namespace=self.collection_name)
             # flatten
             response = [res for res_list in responses for res in res_list]
 
diff --git a/tests/vector_stores/test_upstash_vector.py b/tests/vector_stores/test_upstash_vector.py
index b64357047..070e78bd1 100644
--- a/tests/vector_stores/test_upstash_vector.py
+++ b/tests/vector_stores/test_upstash_vector.py
@@ -69,11 +69,11 @@ def test_search_vectors(upstash_instance, mock_index):
             {
                 "vector": vectors[0],
                 "top_k": 2,
-                "namespace": "ns",
                 "include_metadata": True,
                 "filter": 'age = 30 AND name = "John"',
             }
-        ]
+        ],
+        namespace="ns",
     )
 
     assert len(results) == 2
@@ -311,11 +311,11 @@ def test_search_vectors_empty_filters(upstash_instance):
             {
                 "vector": vectors[0],
                 "top_k": 1,
-                "namespace": "ns",
                 "include_metadata": True,
                 "filter": "",
             }
-        ]
+        ],
+        namespace="ns",
     )
 
     assert len(results) == 1
@@ -350,3 +350,46 @@ def test_insert_vectors_no_ids(upstash_instance):
         ],
         namespace="ns",
     )
+
+
+def test_search_vectors_multi_query_namespace_at_top_level(upstash_instance):
+    """Regression test for #4207.
+
+    The Upstash client's `query_many` takes `namespace` as a top-level kwarg
+    and the per-query `QueryRequest` TypedDict has no `namespace` field. If we
+    pass `namespace` inside each query dict it is silently dropped (multi-query
+    branch) or causes `TypeError: got multiple values for keyword argument
+    'namespace'` (single-query branch, where `query_many` internally calls
+    `self.query(**query, namespace=namespace)`). This test locks in that
+    `namespace` is forwarded only at the top level and that multi-query
+    results are flattened across all per-query response lists.
+    """
+    mock_results = [
+        [QueryResult(id="id1", score=0.9, vector=None, metadata={"name": "vector1"}, data=None)],
+        [
+            QueryResult(id="id2", score=0.8, vector=None, metadata={"name": "vector2"}, data=None),
+            QueryResult(id="id3", score=0.7, vector=None, metadata={"name": "vector3"}, data=None),
+        ],
+    ]
+    upstash_instance.client.query_many.return_value = mock_results
+
+    vectors = [[0.1, 0.2, 0.3], [0.4, 0.5, 0.6]]
+    results = upstash_instance.search(query="hello world", vectors=vectors, top_k=5, filters=None)
+
+    upstash_instance.client.query_many.assert_called_once_with(
+        queries=[
+            {"vector": vectors[0], "top_k": 5, "include_metadata": True, "filter": ""},
+            {"vector": vectors[1], "top_k": 5, "include_metadata": True, "filter": ""},
+        ],
+        namespace="ns",
+    )
+
+    # No per-query dict should carry a `namespace` key.
+    sent_queries = upstash_instance.client.query_many.call_args.kwargs["queries"]
+    for q in sent_queries:
+        assert "namespace" not in q
+
+    # Results must be flattened across both per-query response lists.
+    assert [r.id for r in results] == ["id1", "id2", "id3"]
+    assert results[0].score == 0.9
+    assert results[1].payload == {"name": "vector2"}