MCP server · Chat

postbag MCP server

Let your Claude Code and Codex sessions send each other messages on the same computer.

Chat Developer tools Coding For it and development Coding Agents

postbag is a small helper that lets two coding assistants on your machine talk to each other. You can ask Codex to review what Claude Code just wrote, or the other way round, and the reply shows up as a new turn in the other window. It is handy if you already use both Claude Code and Codex and want them to swap notes without you copying text back and forth.

What is an MCP server? The 30-second version

On its own, your AI can only chat with you. An MCP server is a small helper program that gives your AI a new skill or a connection to something else. This one connects your Claude Code session and your Codex session on the same computer, so they can send each other letters. When you ask one of them to message the other, this helper carries the note across.

What this MCP server does

You open Claude Code and Codex in the same repository and ask each one to join a named bag, which is just a shared conversation. When you ask one agent to send a letter, the helper passes it into the other agent's own input, so it arrives as a new turn. The other agent reads it, replies the same way, and the reply lands back in the first window. Every letter is also saved in one local log that either agent can page through later. You never copy and paste between windows.

What you can do with it

Ask Codex to review the change Claude Code just made
Have one agent write tests while the other writes the code
Get a second opinion when one agent is stuck on a bug
Read back the full history of letters in a bag
List the bags on your machine and who has joined them
Leave a bag when you are done, keeping the history

Try asking your AI

“Join postbag bag review as codex.”
“Join postbag bag review as claude, then ask codex to review my last commit.”
“Ask codex to look at the failing test in test_parser.py and suggest a fix.”
“Read the last few letters in the review bag.”

What it gives back to you

You get the other agent's reply as a normal new turn in its own window, so it reads like a message from a colleague. You can also ask either agent to page through the bag's history and it will show you the earlier letters. A send confirms that the letter was submitted and recorded, but it does not prove the other side read or acted on it.

Before you start

What you need

Python 3.10 or later
Claude Code installed
A recent Codex CLI that supports codex queue
Both sessions running on the same machine with the same postbag version

Good to know

The ledgers saved under your home folder hold Claude Code inbox tokens, so never commit or share them, and only connect sessions you trust with the task.

Install it with your AI

Add postbag MCP server to your AI, no technical skills needed

You don't install anything by hand. You copy one prompt, paste it into an AI that can work on your computer, and it checks, installs and connects the server for you, asking you when it needs something.

Sign in to get the install prompt

Members get a ready-made prompt that lets the Claude desktop app check postbag MCP server, install it and connect it for them, step by step. You don't need any technical skills: you copy, paste and answer a few questions. Your connected AI can also find and install any of the 4,361 MCP servers here for you.