Claude Code sessions can now message each other, and the design detail that matters is what they send. Summaries only, never history or files. That's a 50-year-old idea called the actor model, the same architecture that gave the global phone network nine nines of uptime. In 1973, Carl Hewitt proposed organizing computation as isolated actors that share no memory and communicate only through messages. In 1986, Ericsson turned it into Erlang to run telephone switches. One of those switches, the…