diff --git a/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py b/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
index 2f16e8ba87..83be3baf3c 100644
--- a/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
+++ b/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
@@ -392,6 +392,13 @@ async def call(
 
     # Attach parameters from auth into main parameters list
     api_params, api_args = self._operation_parser.get_parameters().copy(), args
+
+    # add any required arguments that are missing and have defaults: 
+    for api_param in api_params:
+      if api_param.py_name not in api_args:
+        if api_param.required and api_param.param_schema.default:
+          api_args[api_param.py_name] = api_param.param_schema.default    
+
     if auth_credential:
       # Attach parameters from auth into main parameters list
       auth_param, auth_args = self._prepare_auth_request_params(