The present solution is directed towards a policy-based intermediary that dynamically and flexibly injects content in responses between a client and a server based on one or more policies. The present solution addresses the challenges of injecting content in a client-server transaction. The intermediary determines when and what content to inject into a response of a client-server transaction based on a request and/or response policy. The injected content may include timestamp and/or variable tracking of different events in a client-server transaction. For example, when an intermediary appliance is deployed in a system to accelerate system performance and improve user experience, the appliance may inject content based on policy to monitor the acceleration performance of the deployed appliance.