fix(models): forward tool output schema on the LiteLLM path

src/google/adk/models/lite_llm.py

@@ -2092,6 +2092,39 @@ def _schema_to_dict(schema: types.Schema | dict[str, Any]) -> dict[str, Any]:
   return schema_dict
 
 
+def _append_response_schema_to_description(
+    description: str,
+    function_declaration: types.FunctionDeclaration,
+) -> str:
+  """Appends a rendering of the function's output schema to its description.
+
+  OpenAI-compatible chat completions tool definitions have no standard field
+  for declaring the schema of a tool's result, so the schema is rendered into
+  the tool description, which is forwarded to the model. The description is
+  returned unchanged when the function declaration has no output schema.
+
+  Args:
+    description: The original tool description.
+    function_declaration: The function declaration to read the output schema
+      from. `response_json_schema` takes precedence over `response`.
+
+  Returns:
+    The description, with the rendered output schema appended when one exists.
+  """
+  response_schema: Optional[dict[str, Any]] = None
+  if function_declaration.response_json_schema:
+    response_schema = dict(function_declaration.response_json_schema)
+  elif function_declaration.response:
+    response_schema = _schema_to_dict(function_declaration.response)
+
+  if not response_schema:
+    return description
+
+  rendered_schema = json.dumps(response_schema, sort_keys=True)
+  suffix = f"Returns a JSON object conforming to this schema: {rendered_schema}"
+  return f"{description}\n{suffix}" if description else suffix
+
+
 def _function_declaration_to_tool_param(
     function_declaration: types.FunctionDeclaration,
 ) -> dict[str, Any]:
@@ -2129,7 +2162,9 @@ def _function_declaration_to_tool_param(
       "type": "function",
       "function": {
           "name": function_declaration.name,
-          "description": function_declaration.description or "",
+          "description": _append_response_schema_to_description(
+              function_declaration.description or "", function_declaration
+          ),
           "parameters": parameters,
       },
   }

tests/unittests/models/test_litellm.py

@@ -1711,6 +1711,88 @@ def test_function_declaration_to_tool_param_with_parameters_json_schema():
   assert _function_declaration_to_tool_param(func_decl) == expected
 
 
+def test_function_declaration_to_tool_param_with_response_json_schema():
+  """Ensure a raw response_json_schema is rendered into the description."""
+
+  func_decl = types.FunctionDeclaration(
+      name="fn_with_output",
+      description="desc",
+      parameters_json_schema={
+          "type": "object",
+          "properties": {"a": {"type": "string"}},
+      },
+      response_json_schema={
+          "type": "object",
+          "properties": {"result": {"type": "string"}},
+      },
+  )
+
+  tool_param = _function_declaration_to_tool_param(func_decl)
+
+  assert tool_param["function"]["description"] == (
+      "desc\nReturns a JSON object conforming to this schema:"
+      ' {"properties": {"result": {"type": "string"}}, "type": "object"}'
+  )
+  assert tool_param["function"]["parameters"] == {
+      "type": "object",
+      "properties": {"a": {"type": "string"}},
+  }
+
+
+def test_function_declaration_to_tool_param_with_response_schema():
+  """Ensure a types.Schema response is rendered into the description."""
+
+  func_decl = types.FunctionDeclaration(
+      name="fn_with_output_schema",
+      description="desc",
+      response=types.Schema(
+          type=types.Type.OBJECT,
+          properties={"result": types.Schema(type=types.Type.STRING)},
+      ),
+  )
+
+  tool_param = _function_declaration_to_tool_param(func_decl)
+
+  assert tool_param["function"]["description"] == (
+      "desc\nReturns a JSON object conforming to this schema:"
+      ' {"properties": {"result": {"type": "string"}}, "type": "object"}'
+  )
+
+
+def test_function_declaration_to_tool_param_without_response_schema():
+  """Ensure the description is unchanged when no output schema is declared."""
+
+  func_decl = types.FunctionDeclaration(
+      name="fn_without_output",
+      description="desc",
+      parameters_json_schema={"type": "object", "properties": {}},
+  )
+
+  assert (
+      _function_declaration_to_tool_param(func_decl)["function"]["description"]
+      == "desc"
+  )
+
+
+def test_function_declaration_to_tool_param_response_schema_without_description():
+  """Ensure an empty description yields only the rendered output schema."""
+
+  func_decl = types.FunctionDeclaration(
+      name="fn_no_description",
+      response_json_schema={
+          "type": "object",
+          "properties": {"result": {"type": "string"}},
+      },
+  )
+
+  assert _function_declaration_to_tool_param(func_decl)["function"][
+      "description"
+  ] == (
+      "Returns a JSON object conforming to this schema:"
+      ' {"properties": {"result": {"type": "string"}}, "type": "object"}'
+  )
+
+
 @pytest.mark.asyncio
 async def test_generate_content_async_with_system_instruction(
     lite_llm_instance, mock_acompletion