Tachyon is a high-performance, cross-language IPC library that provides a lock-free, single-producer single-consumer (SPSC) ring buffer over POSIX shared memory. It achieves sub-100 nanosecond round-trip times by using zero-copy payloads, futex-based blocking, and a handshake protocol that transfers anonymous memory file descriptors via Unix domain sockets. The library exposes a C API and ships idiomatic bindings for Python, Rust, Go, Java, Kotlin, C#, and Node.js/TypeScript, all sharing the same core C++ implementation. It supports both unidirectional publish-subscribe and bidirectional RPC patterns, with batch operations, NUMA binding, and configurable polling modes. The codebase is production-grade: it includes extensive fuzz testing, architecture decision records, seccomp BPF profiles, a monitoring CLI tool, and a mature release pipeline publishing to all major package registries. The design is notable for its minimal syscall footprint on the hot path (only futex), careful cache-line alignment, and a no-serialization contract that lets consumers read payloads directly from shared memory.