Agent Letterbox turns separate coding agents into a live team. The bell creates the team in the moment; enveloped letters give that team durable memory.
A doorbell gets the right agent's attention. Each handoff, decision, ACK, and result is an enveloped letter: a Markdown-native upgrade with a human-readable body and a durable structured envelope for sender, recipient, identity, time, thread, and lifecycle. It survives scrollback, compaction, restarts, and changing terminals.
This is durable memory without a memory system: no hidden database or proprietary brain required. The record remains inspectable, searchable, and available to whatever memory or orchestration layer the team chooses later.
Choose the terminal your team already uses. Agent Letterbox is one shared protocol and durable letter store with four native doorbell adapters—not four separate products:
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cmux — primary entry point
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Agent Letter Bridge — extend the durable letter system to your phone while keeping the letter, not the chat service, as the source of truth.
- Awesome cmux — a curated guide to the cmux ecosystem for AI coding agents.
Each project has a safe manual install path and security guidance. Inspect the source, release notes, and automation before using it on your machine.