Coz is a causal profiler for native code (C/C++, Rust, Go, Swift, Java) that uses virtual speedup experiments to identify which lines of code, if optimized, would most improve program throughput or latency. It operates by injecting a shared library (libcoz) via LD_PRELOAD or DYLD_INSERT_LIBRARIES, interposing synchronization primitives to track blocking and apply controlled delays on non-selected threads. The profiler outputs a JSON Lines profile that can be visualized with a built-in web viewer (D3.js scatter plots with loess smoothing) or analyzed via CLI. It includes an AI-assisted feature for suggesting progress point placement and optimization advice. The architecture is a hybrid: a core runtime library in C++ with language bindings for Rust, Go, Swift, and Java (JVMTI agent), plus a Python CLI that orchestrates profiling, visualization, and LLM integration. The project is a research artifact from UMass Amherst, published at SOSP 2015, and remains actively developed with modern features like streaming AI support.