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Mailbox atomicity — flock-based prepend and msg-id idempotency

Sessions communicate by prepending messages to flat-file mailboxes at .agent/inbox.md and .agent/outbox.md. Messages carry a YAML envelope (from, to, re, created, status, optional msg-id) followed by a free-text body. Newest messages live at the top so a Command Session reading an outbox via head sees the most recent activity first.

The atomicity problem: two sessions prepending to the same mailbox without coordination produce the classic read-modify-write race. Session A reads the current file, prepends its message, and writes back. Session B does the same simultaneously. Whichever write lands last wins; the other message is lost. This is reproducible with parallel AI-coding sub-agents; cross-user concurrency makes it inevitable.

bin/mailbox-prepend.sh solves this with flock. It takes the target mailbox path, derives a lock path under .agent/locks/ for archive-scoped mailboxes (or /tmp/foundry-mailbox-<sha>.lock for ad-hoc cases), and acquires an exclusive lock with a 30-second timeout before performing the read-modify-write. Two simultaneous calls serialise rather than collide.

A second safety: msg-id idempotency. If the message body contains a msg-id: field, the script scans the first 200 lines of the target for an existing entry with the same id and skips the prepend if found. This means a script that gets retried — a timer firing twice, an operator re-running, a hook misfiring — will not double-send. The 200-line window is generous enough to catch same-day duplicates without making the scan expensive on large archives.

This is two-line wrapping around flock and grep. The failure mode without it is silent — a lost message simply never arrives, and the sender has no indication. Building the discipline into a shared script that sessions and automation call is cheaper than building a more sophisticated queue.

See also

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