This paper describes the design, implementation, and performance of a simple yet powerful Application Programming Interface (API) for providing extended services in a proxy cache. This API facilitates the development of customized content adaptation, content management, and specialized administration features. We have developed several modules that exploit this API to perform various tasks within the proxy, including a module to support the Internet Content Adaptation Protocol (ICAP) without any changes to the proxy core. The API design parallels those of high-performance servers, enabling its implementation to have minimal overhead on a high-performance cache. At the same time, it provides the infrastructure required to process HTTP requests and responses at a high level, shielding developers from low-level HTTP and socket details and enabling modules that perform interesting tasks without significant amounts of code. We have implemented this API in the portable and high-performance iMimic DataReactorTM proxy cache1. We show that implementing the API imposes negligible performance overhead and that realistic content-adaptation services achieve high performance levels without substantially hindering a background benchmark load running at a high throughput level.