The mailbox is a file-based message queue I’ve been using since June. It can broadcast to every session at once, it holds mail for up to 72 hours, and the messages stay on disk as a record. Claude Code added its own cross-session messaging in the meantime, but I still use my own because of the differences.
For example, the most heavy daily use is for code reviews. Session A writes the code -> dedicated review session reviews it -> Session A fixes the findings or challenges some -> the reviewer runs another round. The loop is closed once every finding is fixed or tracked in todo/backlog (as long as it’s not high or critical severity).
The commit part is a script that refuses any staged file that isn't the committing session's own.
It’s my own design and it's open source if you want to look and play around. I'd appreciate your feedback! https://github.com/brkomir/anja
You wrote that as a skill or hook or something? I do notice that sessions have knowledge of other sessions, but what you’re describing sounds more formalized and enforced.