README.md•2.14 kB
# Postal: Agent Comms via MCP
[](https://badge.fury.io/py/postal-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.
```mermaid
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
```
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.
```bash
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"
```
### MCP config
It's served as streamable HTTP. The path is in the format `/agents/{name}/mcp/`.
STDIO example:
```json
{
"mcpServers": {
"postal": {
"command": "npx",
"args": ["-y", "mcp-remote", "http://0.0.0.0:7777/agents/gemini/mcp", "--allow-http", "--transport", "http-only"]
}
}
}
```
Directly as HTTP:
```json
{
"mcpServers": {
"postal": {
"url": "http://0.0.0.0:7777/agents/gemini/mcp",
"transport": "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.