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 AXD301, reported 99.9999999% availability. Downtime measured in fractions of a second per year.
The reason it worked is the reason Anthropic copied it. Shared state is where systems die. When two processes touch the same memory, one bad write corrupts both. When they only exchange messages, a failure stays contained. You can kill a process, restart it, and the rest of the system never notices.
Map that to agents. Shared context windows would mean one hallucination poisons every session downstream, while burning the token budget to do it. A summary is a message. Small, inspectable, contained. The receiving session can act on it or ignore it, but its own context stays clean.
WhatsApp proved the ceiling. Built on Erlang, it served 450 million users with about 50 engineers when Facebook paid $19 billion for it. One engineer per 9 million users, running on message passing.
The AI industry keeps rediscovering distributed systems research from before most of its engineers were born. Microservices, event queues, now agents. Coordinating many unreliable workers is a problem telecom engineers solved in the 1980s. We renamed the workers.
ClaudeDevs (@ClaudeDevs)
New in Claude Code: your sessions can now message each other.
Instead of having to re-explain yourself in another session, you can now tell Claude to do it. It sends a summary (not your history or files), and the other session picks it up mid-task.
Video
— https://nitter.net/ClaudeDevs/status/2085817074816070014#m