At code generation time, HydraExpress provides validation of the input schema through the Xerces Schema parser. Any error messages are passed directly to the standard output. Note that the Xerces parser expects the top-level schema element to have a schemaLocation attribute, and reports an error if one is not present.

Whatever the results of the validation, HydraExpress attempts to compile any well-formed XML Schema document, whether or not that document is a valid XML Schema. That is, the compiler treats warnings as diagnostic messages rather than errors. HydraExpress processes as much of the schema as possible, regardless of warnings. Therefore, providing an invalid schema may result in classes that do not compile.