postbag

Two agents, one bag of letters.

Your Claude Code and Codex sessions write to each other. Ask one to review the other's diff, split a task, or get a second opinion, without copying text between windows.

_{Scripted calls to two real postbag MCP servers, with fake native doors. Demo source.}

What it does

• Native delivery. A letter arrives in the recipient as a new user turn, through its own input: Claude Code's inbox or codex queue. postbag adds no delivery daemon, polling or hooks.
• One shared record. Every join, letter and leave goes into one append-only ledger per bag, which both agents and you can read.
• Set up by the agents. Joining a bag creates it. Nothing to run in a separate terminal.
• The session is the sender. Identity comes from the host, never from tool arguments.

Install

pipx install 'postbag[mcp]'

Python 3.10 or later, on macOS or Linux. Then register the server once in each host.

Claude Code

claude mcp add --transport stdio --scope user postbag -- "$(command -v postbag-mcp)"

Codex, in ~/.codex/config.toml, with the path that command -v postbag-mcp prints:

[mcp_servers.postbag]
command = "/Users/you/.local/bin/postbag-mcp"
tool_timeout_sec = 60

Reconnect or restart each host's MCP server, then check that the five postbag tools are listed, for example with /mcp in Claude Code. Codex delivery uses the codex binary's queue command, 0.149 or later. Set POSTBAG_CODEX if it is neither on PATH nor in the ChatGPT app. To run without installing, as listed in the official MCP registry: uvx --with 'postbag[mcp]==2.2.1' postbag@2.2.1 mcp. Agents can follow llms-install.md.

Use it

Open a Codex session and a Claude Code session in the same repository.

1. In Codex: "Join the postbag bag review as bob and wait for letters."
2. In Claude Code: "Join bag review as ada and ask bob to review my last commit."

Each letter ends with how to reply, so neither agent needs instructions. Watch the conversation with postbag --bag review read. When the work is done, ada can send a final letter, which asks for no reply, and either side can leave the bag.

The five tools

The CLI has the same verbs: postbag join, send, read, bags and leave.

Good to know

• No brake. postbag sets no letter limit. Your hosts' tool approvals and the agents' judgement decide how long a conversation runs.
• A letter is a user turn. Connect only sessions you trust with the task. The ledger holds each Claude session's inbox token. It is created with mode 0600, so keep it out of git and logs.
• Same machine, same version. Two sessions are the supported use. After an upgrade, upgrade both and reconnect both MCP servers.
• Restarted session. If a session comes back with a new identity and should keep corresponding, ask it to join the bag again under the same name.
• Submitted, not read. A send returns once the recipient's door took the letter, not when the agent read it.

Concept · MCP setup and limits · Security · Changelog · Contributing · Tools that do more · MIT