A method, apparatus, and computer instructions for identifying unsafe synthetic transactions and modifying parameters for automated playback. Bytecode instrumentation is used to dynamically observe the behavior of application code directly and identify each universal resource locator traversed and parameters passed in a transaction. The bytecode instrumentation may determine, based on the parameters passed, that a parameter in the current transaction is an unsafe parameter, and that the unsafe parameter is associated with a test parameter in a previously overridden transaction. If the unsafe parameter has an associated test parameter, the bytecode instrumentation may dynamically override the unsafe parameter in the current transaction with the test parameter in order to make the transaction safe for synthetic playback. In this manner, transactions may be scheduled for playback as usual, and the bytecode insertion methods ensure that the unsafe parameters in the transactions are automatically overridden for the user.