  This tool paper presents Caos: a methodology and a programming framework for
computer-aided design of structural operational semantics for formal models.
This framework includes a set of Scala libraries and a workflow to produce
visual and interactive diagrams that animate and provide insights over the
structure and the semantics of a given abstract model with operational rules.
Caos follows an approach in which theoretical foundations and a practical tool
are built together, as an alternative to foundations-first design ("tool
justifies theory") or tool-first design ("foundations justify practice"). The
advantage of Caos is that the tool-under-development can immediately be used to
automatically run numerous and sizeable examples in order to identify subtle
mistakes, unexpected outcomes, and unforeseen limitations in the
foundations-under-development, as early as possible. We share two success
stories of Caos' methodology and framework in our own teaching and research
context, where we analyse a simple while-language and a choreographic language,
including their operational rules and the concurrent composition of such rules.
We further discuss how others can include Caos in their own analysis and Scala
tools.
