postbag

Let Codex and Claude Code talk to each other.

A local MCP server. Ask Codex to review what Claude Code just wrote, or the other way round. The answer arrives as a new turn. No copying text between windows.

Install

Needs Python 3.10 or later, Claude Code and a recent Codex CLI with codex queue. Run both sessions on one machine with the same postbag version. Delivery is tested on macOS.

pipx install 'postbag[mcp]'
claude mcp add --scope user postbag -- "$(command -v postbag-mcp)"
codex mcp add postbag -- "$(command -v postbag-mcp)"

Start a new session in each app, then run /mcp. postbag should list five tools. For uvx and other setups, see MCP setup.

Or ask your agent: Install postbag using llms-install.md.

Quick start

A bag is a named conversation. Both agents join the same one. Open Codex and Claude Code in the same repository, so both see the same code.

1. In Codex:

   Join postbag bag "review" as codex.

2. In Claude Code:

   Join postbag bag "review" as claude, then ask codex to review my last commit.

Codex joins first so Claude Code has someone to write to. Approve the postbag tools when asked. Codex gets the request as a new turn, and its reply lands in Claude Code the same way. Either side can end with a final letter, which asks for no reply.

What to use it for

• Cross review. Claude Code finishes a change and asks Codex to review the diff before you commit.
• Split work. Codex writes the tests while Claude Code writes the implementation. They compare notes by letter.
• Second opinion. When one agent is stuck on a bug, it asks the other for a fresh read.

Tools

Your agents call these. You just ask in plain words.

Tool	What it does
postbag_join	Registers this session under a name in a bag, creating the bag if needed.
postbag_send	Sends a letter to a name in the bag. final: true asks for no reply.
postbag_read	Pages through the bag's history.
postbag_bags	Lists local bags with letter counts and registered names.
postbag_leave	Withdraws this session from the bag. History stays.

How it works

• Native delivery. Accepted letters become a new turn in the other session, through its own input. postbag adds no delivery daemon, polling or hooks.
• No setup prompts. Every letter ends with how to reply, or asks for no reply.
• One shared history. Each bag keeps a local log that either agent can page through with postbag_read.

A successful send confirms submission and recording. Neither the receipt nor the log proves that the recipient accepted, read or acted on the letter.

Security

• Each letter names its peer and asks the recipient to stay within the human's existing authorization. This is model guidance, not a security boundary. Connect only sessions you trust with the task.
• There is no letter limit. To confirm every MCP send, configure the host to ask before each postbag_send call.
• Ledgers under ~/.postbag/ hold Claude Code inbox tokens. New files use mode 0600. Never commit or share them.

See SECURITY.md.

MCP setup · Concept · Changelog · Contributing · MIT