Provided is a validation framework for modelling possible failures that might occur when an orchestrated transaction calls external services to ensure that error handling and reporting is robust and well designed. The disclosed techniques ensure that no changes are necessary to either the code making a call or the services that might be called. The techniques are not limited to web servers and REST APIs as they may be used to test and validate any kind of system that employs well defined APIs. The claimed subject matter, or “validation framework” may be added to an existing API or created as a new module that acts as a proxy server in a non-micro service type of system. Although described with respect to a gateway-API service, the claimed subject matter is equally applicable to other systems that process orchestrated transactions.