Finicky is a macOS application that acts as a smart default browser handler. It intercepts URL open requests from the operating system and routes them to different browsers based on user-defined rules written in JavaScript or TypeScript, or via a simpler JSON rules file. The application is built as a hybrid of Go (using a custom Go-like language with Objective-C interop) and a Svelte-based web UI embedded via WKWebView. It includes a JavaScript virtual machine (goja) to evaluate user configuration scripts, supports URL rewriting, short URL resolution, browser profile detection, and provides a graphical configuration and testing interface. The codebase is well-structured with separate packages for the config API (TypeScript, compiled to JS), the UI (Svelte), and the core macOS app (Go/Objective-C). It demonstrates strong engineering practices including caching, debouncing, and comprehensive testing.