Methods, systems, and computer program products are included for executing one or more instructions of a program in a debugging session; receiving a command at a debugger, the command comprising an expression for the debugger to evaluate in the debugging session; evaluating the expression, at least in part by the debugger, the evaluating including attempting to perform a write operation to write a data value to a target; preventing the data value from being written to the target; and outputting, by the debugger, a result of the evaluating.