Claude Code's cross-session messaging is a JSON file and a Unix socket

Claude Code's cross-session messaging, introduced in version 2.1.224, allows different sessions to communicate. This feature, though documented, was a quiet release and is underutilized. It operates using a JSON file and a Unix socket, with a signal example showing details like PID, CWD, and a messagingSocketPath. The transport work was originally done by PeterSR, supporting macOS and Linux under an MIT license.

This report is the first to detail the underlying JSON and Unix socket implementation of Claude Code's cross-session messaging, unlike the brief changelog entry that announced it.