Postal MCP

🚀 Postal: Agent Comms via MCP

Postal is a mailbox system that enables two agents to communicate with each other.

Here's an example of Claude instructing Gemini on how to write unit tests:

🚀 Quick Start

Postal creates a SQLite - backed message queue to facilitate message passing between agents. The MCP server only supports HTTP, which means that the same database and queue are always shared among agents.

graph LR
subgraph Claude
    ccli[CLI]--send-->csm["send_message()"]
    crec["check_mailbox()"]
end
subgraph Gemini
    gcli[CLI]--send-->gsm["send_message()"]
    grec["check_mailbox()"]
end
postal[(postal)]
csm-->postal-->crec-->ccli
gsm-->postal-->grec-->gcli

✨ Features

• Simple Queue: Postal offers a straightforward queue based on SQLite.
• Two MCP Tools:
  • send_message(agent_name, message)
  • check_mailbox() — This function blocks and waits until a message arrives in the agent's mailbox.
• Free Communication: Agents can freely exchange messages with each other.

💻 Usage Examples

Basic Usage

The following is an example of using the Gemini agent:

PROMPT="Notice that you have two tools, check_mail and send_to_agent. You should get **all** instructions from check_mail. And you should get **all** clarification by using send_to_agent to reply to the agent that gave you a task. Your goal is to keep completing tasks forever and never ask the user for anything. Only talk to agents. You are gemini"

gemini --yolo --prompt "$PROMPT"

📚 Documentation

MCP config

The MCP server is served as streamable HTTP, and the path follows the format /agents/{name}/mcp/.

STDIO example:

{
  "mcpServers": {
    "postal": {
      "command": "npx",
      "args": ["-y", "mcp-remote", "http://0.0.0.0:7777/agents/gemini/mcp", "--allow-http", "--transport", "http-only"]
    }
  }
}

Direct HTTP example:

{
  "mcpServers": {
    "postal": {
      "url": "http://0.0.0.0:7777/agents/gemini/mcp",
      "transport": "http"
    }
  }
}

Note that there are some differences in how "http" is specified. Sometimes it's "streamable-http", and sometimes the key is "type".

🔧 Technical Details

There are some caveats to be aware of when using Postal:

Gemini CLI

The Gemini CLI works fairly well, and there aren't too many issues reported.

Claude Code

The Claude code has a problem where it doesn't easily return to the mailbox and requires a lot of prompting.