This paper presents the design and implementation of Application-Aware Anonymity (A³), an extensible platform for deploying anonymity-based services on the Internet. A³ allows applications to tailor their anonymity properties and performance characteristics according to specific communication requirements. To support flexible path construction, A³ exposes a declarative language (A³LOG) that enables applications to compactly specify path selection and instantiation policies executed by a declarative networking engine. We demonstrate that our declarative language is sufficiently expressive to encode novel multi-metric performance constraints as well as existing relay selection algorithms employed by Tor and other anonymity systems, using only a few lines of concise code. We experimentally evaluate the A³ system using a combination of trace-driven simulations and deployment on Planet- Lab. Our experimental results demonstrate that A3 can flexibly support a wide range of path selection and instantiation strategies at low performance overhead.