diff --git a/litellm/llms/vertex_ai/common_utils.py b/litellm/llms/vertex_ai/common_utils.py
index ccd4d4f2934..8a6b5ddd70b 100644
--- a/litellm/llms/vertex_ai/common_utils.py
+++ b/litellm/llms/vertex_ai/common_utils.py
@@ -599,6 +599,8 @@ def process_items(schema, depth=0):
     if isinstance(schema, dict):
         if "items" in schema and schema["items"] == {}:
             schema["items"] = {"type": "object"}
+        elif schema.get("type") == "array" and "items" not in schema:
+            schema["items"] = {"type": "object"}
         for key, value in schema.items():
             if isinstance(value, dict):
                 process_items(value, depth + 1)
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 ef93375c3cd..5d8dec75456 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},
                         ]
                     },
@@ -288,6 +292,23 @@ def test_process_items_basic():
     process_items(schema)
     assert schema["properties"]["nested"]["items"] == {"type": "object"}
 
+    # Test array with no items field at all - Vertex requires items to be present
+    schema = {"type": "array"}
+    process_items(schema)
+    assert schema["items"] == {"type": "object"}
+
+    # Test array missing items inside anyOf branch
+    schema = {
+        "type": "object",
+        "properties": {
+            "callbacks": {
+                "anyOf": [{"type": "array"}, {"type": "object"}],
+            }
+        },
+    }
+    process_items(schema)
+    assert schema["properties"]["callbacks"]["anyOf"][0]["items"] == {"type": "object"}
+
 
 def test_vertex_ai_complex_response_schema():
     import json