The disclosure describes a proxy server application that supports the dynamic modification of proxy rules implemented by a proxy server. The proxy rules implemented by the proxy server specify network behaviors to be performed at various points during the handling of requests from client applications. A proxy server implements the proxy rules by processing one or more user-generated network traffic filters for managing network traffic. In an embodiment, users generate network traffic filters by creating network traffic filter source code that specify processing steps to be performed by a proxy server relative to network messages the proxy server receives. In an embodiment, user-generated network traffic filters may be added, removed, reordered, or otherwise modified in a proxy server application at runtime in order to respond to current network conditions or to achieve other desired proxy configurations.