Method for operating a legacy software application, executed by running bytecode within a computing or execution environment, by means of using the legacy software application together with an interception-based or instrumentation-based agent functionality, running on or in the computing or execution environment of the legacy software application and being realized as a generic and configurable agent functionality, especially being agnostic to the legacy software application, wherein the agent functionality is able to intercept or instrument the execution of the legacy software application's bytecode, wherein by means of intercepting or instrumenting the legacy software application's bytecode the agent functionality is able to interact, dynamically and/or at runtime of the bytecode, with at least one procedure or procedure call as part of the legacy software application, wherein the method comprises the following steps: -- in a first step, the agent functionality is triggered to be operated together with the legacy software application running on or in the computing or execution environment of the legacy software application, -- in a second step, subsequent to the first step and at runtime of the agent functionality, the agent functionality intercepts or instruments or triggers the execution of the legacy software application's bytecode with regard to the at least one procedure or procedure call by means of the agent functionality being provided with program flow configuration data such that, dependent on a content of the program flow configuration data, the computing or execution environment executes the at least one procedure or procedure call of the legacy software application under the control of the agent functionality, wherein the agent functionality receives, from the legacy software application and upon intercepting the at least one procedure or procedure call during the execution of the legacy software application's bytecode, a request data object - related to the at least one procedure or procedure call of the legacy software application -, wherein in dependency of the content, at runtime of the agent functionality, of the program flow configuration data, the at least one procedure or procedure call is executed externally to the legacy software application, wherein a response data object - related to the at least one procedure or procedure call and accepted by the legacy software application - is generated and provided to the legacy software application. -- in a first step, the agent functionality is triggered to be operated together with the legacy software application running on or in the computing or execution environment of the legacy software application, -- in a second step, subsequent to the first step and at runtime of the agent functionality, the agent functionality intercepts or instruments or triggers the execution of the legacy software application's bytecode with regard to the at least one procedure or procedure call by means of the agent functionality being provided with program flow configuration data such that, dependent on a content of the program flow configuration data, the computing or execution environment executes the at least one procedure or procedure call of the legacy software application under the control of the agent functionality, wherein the agent functionality receives, from the legacy software application and upon intercepting the at least one procedure or procedure call during the execution of the legacy software application's bytecode, a request data object - related to the at least one procedure or procedure call of the legacy software application -, wherein in dependency of the content, at runtime of the agent functionality, of the program flow configuration data, the at least one procedure or procedure call is executed externally to the legacy software application, wherein a response data object - related to the at least one procedure or procedure call and accepted by the legacy software application - is generated and provided to the legacy software application.