diff --git a/litellm/llms/vertex_ai/common_utils.py b/litellm/llms/vertex_ai/common_utils.py
index ccd4d4f293..9b23520dcd 100644
--- a/litellm/llms/vertex_ai/common_utils.py
+++ b/litellm/llms/vertex_ai/common_utils.py
@@ -710,14 +710,10 @@ def convert_anyof_null_to_nullable(schema, depth=0):
 
         if contains_null:
             # set all types to nullable following guidance found here: https://cloud.google.com/vertex-ai/generative-ai/docs/samples/generativeaionvertexai-gemini-controlled-generation-response-schema-3#generativeaionvertexai_gemini_controlled_generation_response_schema_3-python
+            # Empty `items: {}` on array branches is left in place; downstream
+            # process_items() converts it to {"type": "object"}, which Vertex
+            # requires whenever type == "array" (even inside anyOf).
             for atype in anyof:
-                # Remove items field if type is array and items is empty
-                if (
-                    atype.get("type") == "array"
-                    and "items" in atype
-                    and not atype["items"]
-                ):
-                    atype.pop("items")
                 atype["nullable"] = True
 
     properties = schema.get("properties", None)
diff --git a/tests/local_testing/test_amazing_vertex_completion.py b/tests/local_testing/test_amazing_vertex_completion.py
index 9070a9feab..3b4ecb82b1 100644
--- a/tests/local_testing/test_amazing_vertex_completion.py
+++ b/tests/local_testing/test_amazing_vertex_completion.py
@@ -3569,8 +3569,14 @@ def test_gemini_tool_calling_working_demo():
 
 
 def test_gemini_tool_calling_not_working():
-    load_vertex_ai_credentials()
-    litellm._turn_on_debug()
+    """
+    Regression test: tool params with anyOf containing both an empty-items
+    array branch and a null branch must serialize with items present on the
+    array branch (Vertex rejects array types missing `items`).
+    """
+    from litellm.llms.custom_httpx.http_handler import HTTPHandler
+    from litellm.llms.vertex_ai.vertex_llm_base import VertexBase
+
     args = {
         "messages": [
             {
@@ -3637,8 +3643,64 @@ def test_gemini_tool_calling_not_working():
         ],
         "vertex_location": "global",
     }
-    response = completion(model="vertex_ai/gemini-3-flash-preview", **args)
-    print(response)
+
+    client = HTTPHandler()
+    mock_response = MagicMock()
+    mock_response.status_code = 200
+    mock_response.headers = {"Content-Type": "application/json"}
+    mock_response.json.return_value = {
+        "candidates": [
+            {
+                "content": {
+                    "role": "model",
+                    "parts": [{"text": "Hello!"}],
+                },
+                "finishReason": "STOP",
+            }
+        ],
+        "usageMetadata": {
+            "promptTokenCount": 10,
+            "candidatesTokenCount": 5,
+            "totalTokenCount": 15,
+        },
+    }
+
+    with (
+        patch.object(client, "post", return_value=mock_response) as mock_post,
+        patch.object(
+            VertexBase,
+            "_ensure_access_token",
+            return_value=("fake-token", "fake-project"),
+        ),
+    ):
+        completion(
+            model="vertex_ai/gemini-3-flash-preview",
+            client=client,
+            **args,
+        )
+
+    sent_body = mock_post.call_args.kwargs.get(
+        "json"
+    ) or mock_post.call_args.kwargs.get("data")
+    assert sent_body is not None, "expected request body to be sent"
+    if isinstance(sent_body, str):
+        sent_body = json.loads(sent_body)
+
+    function_decl = sent_body["tools"][0]["function_declarations"][0]
+    callbacks_schema = function_decl["parameters"]["properties"]["config"][
+        "properties"
+    ]["callbacks"]
+    array_branches = [
+        branch
+        for branch in callbacks_schema["anyOf"]
+        if branch.get("type", "").lower() == "array"
+    ]
+    assert array_branches, "expected an array branch in callbacks anyOf"
+    for branch in array_branches:
+        assert "items" in branch and branch["items"], (
+            f"array branch in callbacks.anyOf must include non-empty items "
+            f"(Vertex rejects array types missing items). Got: {branch}"
+        )
 
 
 def test_vertex_ai_llama_tool_calling():
diff --git a/tests/test_litellm/llms/vertex_ai/test_vertex_ai_common_utils.py b/tests/test_litellm/llms/vertex_ai/test_vertex_ai_common_utils.py
index ef93375c3c..dc3be7114f 100644
--- a/tests/test_litellm/llms/vertex_ai/test_vertex_ai_common_utils.py
+++ b/tests/test_litellm/llms/vertex_ai/test_vertex_ai_common_utils.py
@@ -225,7 +225,11 @@ def test_build_vertex_schema():
                     "metadata": {"type": "object"},
                     "callbacks": {
                         "anyOf": [
-                            {"type": "array", "nullable": True},
+                            {
+                                "type": "array",
+                                "items": {"type": "object"},
+                                "nullable": True,
+                            },
                             {"type": "object", "nullable": True},
                         ]
                     },