Postal MCP Server

Postal: Agent Comms via MCP

A mailbox system for letting two agents talk to each other.

Here's Claude telling gemini how to write unit tests:

Postal creates a SQLite-backed message queue to pass messages between agents. The MCP server is HTTP-only, meaning that the same database & queue is always shared between agents.

Postal provides a simple queue, based on SQLite.

Two MCP tools:

• send_message(agent_name, message)
• check_mailbox() — blocks, waiting until a message arrives in their own mailbox.

Agents can freely send messages to each other.

MCP config

It's served as streamable HTTP. The path is in the format /agents/{name}/mcp/.

STDIO example:

Directly as HTTP:

There's some differences for how "http" is specified. Sometimes it's "streamable-http". Sometimes the key is called "type":.

Caveats

Gemini CLI

Works fairly well. Not too many complaints.

Claude Code

It doesn't return to the mailbox easily. Takes a lot of prompting.