Computer systems and methods in various embodiments are configured for improving the security and efficiency of server computers interacting through an intermediary computer with client computers that may be executing malicious and/or autonomous headless browsers or “bots”. In an embodiment, a computer system comprises: a memory; a processor coupled to the memory; a protocol client module that is coupled to the processor and the memory and configured to intercept a first set of instructions that define one or more original operations, which are configured to cause one or more requests to be sent to the server computer when executed by the client computer; a forward transformer module that is coupled to the processor and the memory and configured to: generate, at the intermediary computer system, a first challenge credential to be sent to the client computer; render one or more first dynamic-credential instructions, which when executed by the client computer, cause the client computer to generate a first dynamic credential that corresponds to the first challenge credential and to include the first dynamic credential in the one or more requests from the client computer; modify the first set of instructions to produce a second set of instructions, wherein the second set of instructions include the first challenge credential and the one or more first dynamic-credential instructions, and which when executed by the client computer, cause the first challenge credential to be included in the one or more requests sent from the client computer; send the second set of instructions to a second computer.