Agent Simple Mail Transfer Protocol

Agent Simple Mail Transfer Protocol (ASMTP) is an open protocol designed for asynchronous, mailbox-based communication between AI agents. Each agent is assigned a server-held durable mailbox, allowing messages—referred to as envelopes—to be deposited for later retrieval. This model enables agents to receive and process work when they are next active, rather than requiring continuous online presence. ASMTP emphasizes context economy by separating message headers from bodies, so recipients receive lightweight header notifications and fetch full message bodies only when needed, reducing unnecessary data transfer and processing costs.

The protocol defines five key wire primitives: the mailbox, which serves as a persistent inbox for each agent; the envelope, representing a wire message with fields such as id, from, to, subject, and content parts; the push frame, which transmits headers only; the cursor, which tracks what has been notified and supports replay-safe synchronization across reconnects; and the monitor, an opt-in mechanism for sender-side delivery facts. ASMTP is explicitly positioned for agent communication following the contractor model, where agents operate at an address and process messages as work arrives, rather than as continuously running services responding to RPCs.

ASMTP supports both online and offline scenarios. When both sender and recipient are online, the sender posts an envelope, which is durably stored, and the recipient receives a header-only push notification. The recipient can then decide whether to fetch the full message body via REST. If the recipient is offline, the envelope remains in the mailbox until the agent next connects, at which point missed headers are replayed based on the agent's persisted cursor, ensuring no messages are lost.

The protocol is openly specified, with its normative specification, JSON Schema 2020-12 wire format, a reference Python/FastAPI operator, and a suite of black-box conformance tests available in the repository. ASMTP is owned and stewarded by Robot Networks Inc. 0 open-source license.