diff --git a/litellm/vector_stores/vector_store_registry.py b/litellm/vector_stores/vector_store_registry.py
index b71d6784873..40551894d46 100644
--- a/litellm/vector_stores/vector_store_registry.py
+++ b/litellm/vector_stores/vector_store_registry.py
@@ -32,8 +32,10 @@ else:
 
 
 class VectorStoreIndexRegistry:
-    def __init__(self, vector_store_indexes: list[LiteLLM_ManagedVectorStoreIndex] = []):
-        self.vector_store_indexes: list[LiteLLM_ManagedVectorStoreIndex] = vector_store_indexes
+    def __init__(self, vector_store_indexes: list[LiteLLM_ManagedVectorStoreIndex] | None = None):
+        self.vector_store_indexes: list[LiteLLM_ManagedVectorStoreIndex] = (
+            vector_store_indexes if vector_store_indexes is not None else []
+        )
 
     def get_vector_store_indexes(self) -> list[LiteLLM_ManagedVectorStoreIndex]:
         """
@@ -101,8 +103,8 @@ class VectorStoreIndexRegistry:
 
 
 class VectorStoreRegistry:
-    def __init__(self, vector_stores: list[LiteLLM_ManagedVectorStore] = []):
-        self.vector_stores: list[LiteLLM_ManagedVectorStore] = vector_stores
+    def __init__(self, vector_stores: list[LiteLLM_ManagedVectorStore] | None = None):
+        self.vector_stores: list[LiteLLM_ManagedVectorStore] = vector_stores if vector_stores is not None else []
         self.vector_store_ids_to_vector_store_map: dict[str, LiteLLM_ManagedVectorStore] = {}
 
     def _extract_tool_params(self, tool: dict) -> VectorStoreToolParams:
diff --git a/tests/test_litellm/vector_stores/test_vector_store_registry.py b/tests/test_litellm/vector_stores/test_vector_store_registry.py
index f19c3706845..ab274b41e5a 100644
--- a/tests/test_litellm/vector_stores/test_vector_store_registry.py
+++ b/tests/test_litellm/vector_stores/test_vector_store_registry.py
@@ -13,7 +13,10 @@ from unittest.mock import MagicMock
 import litellm
 from litellm.types.vector_stores import LiteLLM_ManagedVectorStore
 from litellm.vector_stores.main import search
-from litellm.vector_stores.vector_store_registry import VectorStoreRegistry
+from litellm.vector_stores.vector_store_registry import (
+    VectorStoreIndexRegistry,
+    VectorStoreRegistry,
+)
 
 
 @pytest.fixture(autouse=True)
@@ -182,3 +185,38 @@ def test_search_uses_registry_credentials():
             assert getattr(called_params, "aws_region_name") == "us-east-1"
     finally:
         litellm.vector_store_registry = original_registry
+
+
+class TestRegistriesDoNotShareDefaultState:
+    """#38874: a mutable default argument made every default-constructed registry share
+    one list, so upserting into one polluted every other instance and the class default."""
+
+    def test_vector_store_registries_do_not_share_a_list(self):
+        first = VectorStoreRegistry()
+        second = VectorStoreRegistry()
+        assert first.vector_stores is not second.vector_stores
+
+    def test_index_registries_do_not_share_a_list(self):
+        first = VectorStoreIndexRegistry()
+        second = VectorStoreIndexRegistry()
+        assert first.vector_store_indexes is not second.vector_store_indexes
+
+    def test_mutating_one_registry_leaves_the_next_one_empty(self):
+        first = VectorStoreRegistry()
+        first.vector_stores.append(
+            LiteLLM_ManagedVectorStore(vector_store_id="vs-leak", custom_llm_provider="bedrock")
+        )
+        assert VectorStoreRegistry().vector_stores == []
+
+    def test_mutating_one_index_registry_leaves_the_next_one_empty(self):
+        first = VectorStoreIndexRegistry()
+        first.vector_store_indexes.append({"index_name": "leak"})
+        assert VectorStoreIndexRegistry().get_vector_store_indexes() == []
+
+    def test_a_supplied_list_is_still_used_as_given(self):
+        supplied = [LiteLLM_ManagedVectorStore(vector_store_id="vs-1", custom_llm_provider="bedrock")]
+        assert VectorStoreRegistry(vector_stores=supplied).vector_stores is supplied
+
+    def test_a_supplied_empty_list_is_still_used_as_given(self):
+        supplied: list = []
+        assert VectorStoreRegistry(vector_stores=supplied).vector_stores is supplied