Handling sensitive data in an application using external processing

Methods and associated data processing system for handling sensitive data required by an application in a secure computer system. The secure computer identifies sensitive data in one or more data aspects of a request message. The secure computer system tokenizes the sensitive data in the one or more data aspects by replacing the sensitive data with tokenized data and stores a mapping between the sensitive data and the tokenized data in the secure computer system. The secure computer system sends the request message to an external computer system. After the request message is sent to the external computer system, the secure computer system receives a response message from the external computer system. The response message includes annotations for the tokenized data with transform instructions for the tokenized data. The secure computer system replaces the tokenized data with the sensitive data and performs the transform instructions on the sensitive data.