You can cheaply and readably give a lot of clues to both agents and humans with some assertions at the start of a method:

  raise ArgumentError.new("...") unless ...

which can include type assertions but also a lot more. The agents seem to do well with this.

I've also had good results using agents to write Crystal https://crystal-lang.org/ which is Ruby-like but does have the static types and produces blazing fast static binaries. Might be a sweet spot for coding agents if you're building some backend services. But I'd still pick Ruby on Rails for a new full stack project.