Hideaki Takahashi, a security researcher at Columbia, released Agent Radio, a feature in his "AI-aware" Git tool h5i (v0.1.5) that lets two coding agents such as Claude Code and Codex coordinate by passing messages through the git repository they already share.
The mechanism is deliberately small. Under a protocol he calls i5h, each message is a single JSON object appended as one line to a messages.jsonl file on a dedicated ref, refs/h5i/msg, and a reply simply points back with a reply_to field. Because messages are immutable and keyed by id, two diverging clones merge by set union, producing a conflict-free, append-only log with no server, socket or schema registry. Pushing the ref shares the conversation; pulling merges it.
It is a personal experiment rather than a product, but it answers multi-agent coordination cleanly: reuse git's existing sync and history instead of standing up new infrastructure, and the audit trail comes for free.