Kaitai Struct is a declarative language used to describe various binary data structures, laid out in files or in memory: i.e. binary file formats, network stream packet formats, etc.
The main idea is that a particular format is described in Kaitai Struct language (.ksy file) and then can be compiled with ksc into source files in one of the supported programming languages. These modules will include a generated code for a parser that can read the described data structure from a file or stream and give access to it in a nice, easy-to-comprehend API.
It’s been great at iterating on simple parsers for proprietary formats like InstallShield - I actually first heard about it from a blog post on Delivery Optimization’s wire protocol. The web IDE is really powerful until you need anything complex (like custom decryption for Installshield…). I ended up porting parts of it to VSCode’s Hex Editor extension. Lack of TypeScript support was annoying, Avery was working on it but nothing seems finished so far. And the maintainers seem to have a pretty high bar for PRs