According to this disclosure, a proxy server is enhanced to be able to interpret instructions that specify how to modify an input object to create an output object to serve to a requesting client. Typically the instructions operate on binary data. For example, the instructions can be interpreted in a byte-based interpreter that directs the proxy as to what order, and from which source, to fill an output buffer that is served to the client. The instructions specify what changes to make to a generic input file. This functionality extends the capability of the proxy server in an open-ended fashion and enables it to efficiently create a wide variety of outputs for a given generic input file. The generic input file and/or the instructions may be cached at the proxy. The teachings hereof have applications in, among other things, the delivery of web content, streaming media, and the like.