diff --git a/api/core/tools/custom_tool/tool.py b/api/core/tools/custom_tool/tool.py
index f8467c54c5e24d..2d8c6bb5afd01c 100644
--- a/api/core/tools/custom_tool/tool.py
+++ b/api/core/tools/custom_tool/tool.py
@@ -105,10 +105,10 @@ def assembling_request(self, parameters: dict[str, Any]) -> dict[str, Any]:
         needed_parameters = [parameter for parameter in (self.api_bundle.parameters or []) if parameter.required]
         for parameter in needed_parameters:
             if parameter.required and parameter.name not in parameters:
-                raise ToolParameterValidationError(f"Missing required parameter {parameter.name}")
-
-            if parameter.default is not None and parameter.name not in parameters:
-                parameters[parameter.name] = parameter.default
+                if parameter.default is not None:
+                    parameters[parameter.name] = parameter.default
+                else:
+                    raise ToolParameterValidationError(f"Missing required parameter {parameter.name}")
 
         return headers