Chibi-Scheme is a very small library intended for use as an extension and scripting language in C programs. In addition to support for lightweight VM-based threads, each VM itself runs in an isolated heap allowing multiple VMs to run simultaneously in different OS threads. The default repl language contains all bindings from R7RS small, available explicitly as the (scheme small) library. Support for additional languages such as JavaScript, Go, Lua and Bash are planned for future releases. Scheme is chosen as a substrate because its first class continuations and guaranteed tail-call optimization makes implementing other languages easy.