Send a brief/message to another agent's inbox. Writes a YAML-frontmatter markdown file to /.claude/inbox/-.md atomically. Caller identity comes from MAILBOX_MCP_AGENT_ID env var (default 'workspace'). Use this instead of writing files manually — it enforces naming, frontmatter, and audit.

Fetch a single mail by id from a given agent's folder. Returns full metadata + markdown body + absolute path.

List mails in a target agent's inbox or outbox. Filters: status, since (ISO date), labels (any-match), limit (default 100). Sorted newest-first.

Reply to a mail. Finds parent in the CALLER's own inbox or outbox (use mail_send if you need to reply across agents), sends the reply with to = parent.from by default, and auto-flips the parent's status to 'done'. One tool call instead of three.

Change a mail's status. Validates the state-machine transition (open ↔ holding → done/superseded; done and superseded are terminal). Atomic write + audit entry.

Walk the reply_to graph rooted at the given mail. Returns root + descendants in breadth-first order with depth annotation. Scans all known agents' folders — expensive but complete.

Backfill id: frontmatter into hand-written legacy briefs in a given folder. Scans .md files, generates an id from filename timestamp + nanoid suffix for any file missing one, rewrites atomically. Use dry_run: true first to preview.