diff --git a/src/google/adk/models/lite_llm.py b/src/google/adk/models/lite_llm.py
index e85772c5..dad5543f 100644
--- a/src/google/adk/models/lite_llm.py
+++ b/src/google/adk/models/lite_llm.py
@@ -1491,6 +1491,54 @@ def _message_to_generate_content_response(
   )
 
 
+def _enforce_strict_openai_schema(schema: dict[str, Any]) -> None:
+  """Recursively transforms a JSON schema for OpenAI strict structured outputs.
+
+  OpenAI strict mode requires:
+  1. additionalProperties: false on all object schemas (including nested/$defs).
+  2. All properties listed in 'required' (no optional omissions).
+  3. $ref nodes must have no sibling keywords (e.g., no 'description' next to
+     '$ref').
+
+  This function mutates the schema dict in place.
+
+  Args:
+    schema: A JSON schema dictionary to transform.
+  """
+  if not isinstance(schema, dict):
+    return
+
+  # Strip sibling keywords from $ref nodes (OpenAI rejects them).
+  if "$ref" in schema:
+    for key in list(schema.keys()):
+      if key != "$ref":
+        del schema[key]
+    return
+
+  # Ensure all object schemas have additionalProperties: false and list every
+  # property as required.
+  if schema.get("type") == "object" and "properties" in schema:
+    schema["additionalProperties"] = False
+    schema["required"] = sorted(schema["properties"].keys())
+
+  # Recurse into $defs (Pydantic's nested model definitions).
+  for defn in schema.get("$defs", {}).values():
+    _enforce_strict_openai_schema(defn)
+
+  # Recurse into property schemas.
+  for prop in schema.get("properties", {}).values():
+    _enforce_strict_openai_schema(prop)
+
+  # Recurse into combinators.
+  for key in ("anyOf", "oneOf", "allOf"):
+    for item in schema.get(key, []):
+      _enforce_strict_openai_schema(item)
+
+  # Recurse into array item schemas.
+  if "items" in schema and isinstance(schema["items"], dict):
+    _enforce_strict_openai_schema(schema["items"])
+
+
 def _to_litellm_response_format(
     response_schema: types.SchemaUnion,
     model: str,
@@ -1515,7 +1563,7 @@ def _to_litellm_response_format(
         and schema_type.lower() in _LITELLM_STRUCTURED_TYPES
     ):
       return response_schema
-    schema_dict = dict(response_schema)
+    schema_dict = copy.deepcopy(response_schema)
     if "title" in schema_dict:
       schema_name = str(schema_dict["title"])
   elif isinstance(response_schema, type) and issubclass(
@@ -1526,14 +1574,18 @@ def _to_litellm_response_format(
   elif isinstance(response_schema, BaseModel):
     if isinstance(response_schema, types.Schema):
       # GenAI Schema instances already represent JSON schema definitions.
-      schema_dict = response_schema.model_dump(exclude_none=True, mode="json")
+      schema_dict = copy.deepcopy(
+          response_schema.model_dump(exclude_none=True, mode="json")
+      )
       if "title" in schema_dict:
         schema_name = str(schema_dict["title"])
     else:
       schema_dict = response_schema.__class__.model_json_schema()
       schema_name = response_schema.__class__.__name__
   elif hasattr(response_schema, "model_dump"):
-    schema_dict = response_schema.model_dump(exclude_none=True, mode="json")
+    schema_dict = copy.deepcopy(
+        response_schema.model_dump(exclude_none=True, mode="json")
+    )
     schema_name = response_schema.__class__.__name__
   else:
     logger.warning(
@@ -1551,14 +1603,8 @@ def _to_litellm_response_format(
 
   # OpenAI-compatible format (default) per LiteLLM docs:
   # https://docs.litellm.ai/docs/completion/json_mode
-  if (
-      isinstance(schema_dict, dict)
-      and schema_dict.get("type") == "object"
-      and "additionalProperties" not in schema_dict
-  ):
-    # OpenAI structured outputs require explicit additionalProperties: false.
-    schema_dict = dict(schema_dict)
-    schema_dict["additionalProperties"] = False
+  if isinstance(schema_dict, dict):
+    _enforce_strict_openai_schema(schema_dict)
 
   return {
       "type": "json_schema",
diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index 39f6b540..8e353efb 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -26,6 +26,7 @@ import warnings
 
 from google.adk.models.lite_llm import _append_fallback_user_content_if_missing
 from google.adk.models.lite_llm import _content_to_message_param
+from google.adk.models.lite_llm import _enforce_strict_openai_schema
 from google.adk.models.lite_llm import _FILE_ID_REQUIRED_PROVIDERS
 from google.adk.models.lite_llm import _FINISH_REASON_MAPPING
 from google.adk.models.lite_llm import _function_declaration_to_tool_param
@@ -394,6 +395,145 @@ def test_to_litellm_response_format_with_dict_schema_for_openai():
   assert formatted["json_schema"]["schema"]["additionalProperties"] is False
 
 
+class _InnerModel(BaseModel):
+  value: str = Field(description="A value")
+  optional_field: str | None = Field(default=None, description="Optional")
+
+
+class _OuterModel(BaseModel):
+  inner: _InnerModel = Field(description="Nested model")
+  name: str
+
+
+class _WithList(BaseModel):
+  items: list[_InnerModel] = Field(description="List of items")
+  label: str
+
+
+def test_enforce_strict_openai_schema_adds_additional_properties_recursively():
+  """additionalProperties: false must appear on all object schemas."""
+  schema = _OuterModel.model_json_schema()
+
+  _enforce_strict_openai_schema(schema)
+
+  # Root level
+  assert schema["additionalProperties"] is False
+  # Nested model in $defs
+  inner_def = schema["$defs"]["_InnerModel"]
+  assert inner_def["additionalProperties"] is False
+
+
+def test_enforce_strict_openai_schema_marks_all_properties_required():
+  """All properties must appear in 'required', including optional fields."""
+  schema = _InnerModel.model_json_schema()
+
+  _enforce_strict_openai_schema(schema)
+
+  assert sorted(schema["required"]) == ["optional_field", "value"]
+
+
+def test_enforce_strict_openai_schema_strips_ref_sibling_keywords():
+  """$ref nodes must have no sibling keywords like 'description'."""
+  schema = _OuterModel.model_json_schema()
+  # Pydantic v2 generates {"$ref": "...", "description": "..."} for nested models
+  inner_prop = schema["properties"]["inner"]
+  assert "$ref" in inner_prop, "Expected Pydantic to generate a $ref property"
+  assert len(inner_prop) > 1, "Expected sibling keywords alongside $ref"
+
+  _enforce_strict_openai_schema(schema)
+
+  inner_prop = schema["properties"]["inner"]
+  assert list(inner_prop.keys()) == ["$ref"]
+
+
+def test_enforce_strict_openai_schema_handles_array_items():
+  """Array item schemas should also be recursively transformed."""
+  schema = _WithList.model_json_schema()
+
+  _enforce_strict_openai_schema(schema)
+
+  assert schema["additionalProperties"] is False
+  inner_def = schema["$defs"]["_InnerModel"]
+  assert inner_def["additionalProperties"] is False
+  assert sorted(inner_def["required"]) == ["optional_field", "value"]
+
+
+def test_enforce_strict_openai_schema_preserves_anyof_and_default():
+  """anyOf structure and default value for Optional fields must be preserved."""
+  schema = _InnerModel.model_json_schema()
+
+  _enforce_strict_openai_schema(schema)
+
+  opt_prop = schema["properties"]["optional_field"]
+  assert opt_prop["anyOf"] == [{"type": "string"}, {"type": "null"}]
+  assert opt_prop["default"] is None
+
+
+def test_to_litellm_response_format_dict_input_not_mutated():
+  """Passing a raw dict should not mutate the caller's original dict."""
+  schema = {
+      "type": "object",
+      "properties": {
+          "nested": {
+              "type": "object",
+              "properties": {"x": {"type": "string"}},
+          }
+      },
+  }
+  import copy
+
+  original = copy.deepcopy(schema)
+
+  _to_litellm_response_format(schema, model="gpt-4o")
+
+  assert schema == original, "Caller's input dict was mutated"
+
+
+def test_to_litellm_response_format_instance_input_for_openai():
+  """Passing a BaseModel instance should produce a valid strict schema."""
+  instance = _OuterModel(
+      inner=_InnerModel(value="test", optional_field=None), name="foo"
+  )
+
+  formatted = _to_litellm_response_format(instance, model="gpt-4o")
+
+  assert formatted["type"] == "json_schema"
+  schema = formatted["json_schema"]["schema"]
+  assert schema["additionalProperties"] is False
+  inner_def = schema["$defs"]["_InnerModel"]
+  assert inner_def["additionalProperties"] is False
+  assert sorted(inner_def["required"]) == ["optional_field", "value"]
+
+
+def test_to_litellm_response_format_nested_pydantic_for_openai():
+  """Nested Pydantic model should produce a valid OpenAI strict schema."""
+  formatted = _to_litellm_response_format(_OuterModel, model="gpt-4o")
+
+  assert formatted["type"] == "json_schema"
+  assert formatted["json_schema"]["strict"] is True
+
+  schema = formatted["json_schema"]["schema"]
+  assert schema["additionalProperties"] is False
+  assert sorted(schema["required"]) == ["inner", "name"]
+
+  # $defs inner model must also be strict
+  inner_def = schema["$defs"]["_InnerModel"]
+  assert inner_def["additionalProperties"] is False
+  assert sorted(inner_def["required"]) == ["optional_field", "value"]
+
+
+def test_to_litellm_response_format_nested_pydantic_for_gemini_unchanged():
+  """Gemini models should NOT get the strict OpenAI transformations."""
+  formatted = _to_litellm_response_format(
+      _OuterModel, model="gemini/gemini-2.0-flash"
+  )
+
+  assert formatted["type"] == "json_object"
+  schema = formatted["response_schema"]
+  # Gemini path should pass through the raw Pydantic schema untouched
+  assert schema == _OuterModel.model_json_schema()
+
+
 async def test_get_completion_inputs_uses_openai_format_for_openai_model():
   """Test that _get_completion_inputs produces OpenAI-compatible format."""
   llm_request = LlmRequest(