postbag

Two agents, one bag of letters. Any two Claude Code or Codex sessions on the same machine, of the same vendor or not, write to each other. Each letter reaches the other agent through its vendor's own wake-up door, lands in one ledger, and counts against a letter budget that only you can set.

Real commands, real output, fake doors: a temporary home and two throwaway sockets, so no session or token is shown. Tape: docs/demo.tape.

Use it for a review of the other agent's diff, to split a task and agree the interface by letter, or for a second opinion. Text travels by postbag, code by git.

Install

pipx install postbag
postbag --version

Python 3.10 or later, standard library only. Both sessions run the same postbag, since every command inside a letter is written for the version that sent it.

Each vendor in use brings its own door. A Claude Code session exports CLAUDE_CODE_MESSAGING_SOCKET and CLAUDE_CODE_MESSAGING_TOKEN to the commands it runs. A Codex session exports CODEX_SESSION_ID and has a codex binary with the queue command (0.149 or later). Set POSTBAG_CODEX if it is not in the ChatGPT app or on PATH. Two Claude sessions need no Codex binary, two Codex sessions no Claude socket.

Verified live on macOS with Claude Code 2.1.263 and Codex 0.153.4, across vendors, between two Claude sessions, and in a named bag. Linux passes CI, live delivery is unverified there. Windows is unsupported.

Quick start

1. Open two sessions on the same machine. Ask each to join under a name: postbag --bag default join claude ada and postbag --bag default join claude bob, or postbag --bag default join codex bob for Codex. Same-vendor pairs need distinct names.

2. In a terminal of your own, outside both sessions, run postbag --bag default open --limit 6. An exchange holds 12 by default.

3. Ask ada to send the first letter:

   postbag --bag default send @bob "Review my last commit. Reply with the top three findings."

   bob wakes with the letter: "Letter 1 of 6 from @ada to @bob via postbag (exchange 1, bag default)", how many letters are left, the body, and the one command that answers, postbag --bag default send @ada - with the reply on stdin. Neither agent needs instructions. The last letter says "do not send a reply", and the next send refuses and says stop.

4. Read the bag from anywhere with postbag --bag default read. Its first line names the bag, its names and the open exchange, then the records.

After a restart, rejoin the same bag under the same name. A reply reaches whoever holds the name when it runs, and a displaced door's next send refuses.

Bags

A bag is one ledger, and it has a name. default is ~/.postbag/ledger.jsonl. For a second conversation, open a second bag first, in your own terminal: postbag --bag acceptance open --limit 6. Each session then joins with the same flag, postbag --bag acceptance join claude ada and postbag --bag acceptance join claude bob, and ada sends with postbag --bag acceptance send @bob "...".

--bag goes before the verb and takes a name, kept in ~/.postbag/bags/<name>.jsonl, or an absolute path of printable characters. Only open creates a named bag, the other verbs refuse one that does not exist. Each command's output identifies its bag, and every command inside a letter or a refusal carries --bag, --bag default included, so a reply lands where the letter came from whatever the recipient's shell has set. ls ~/.postbag/bags lists them. Without --bag, POSTBAG_LEDGER selects a ledger by path.

How it works

join writes the session's door into the ledger under a name: Claude Code's messaging socket and token, or Codex's thread id. send knocks on that door, then appends the letter under a file lock, so two letters sent at once get distinct numbers and one budget. Each open starts the next exchange, and its budget is shared by everyone in the bag. Two sessions are the supported use, three or more is experimental. A bag is one ledger, the only state. No daemon, no polling, no hooks, no server, no config file, no bag index. CONCEPT.md is the whole specification in a page.

Security and limits

• A ledger holds every Claude session token and every letter in its bag. Writes keep the file 0600 and new state directories 0700. read hides the door fields, cat does not. Keep raw files out of git and logs.
• A letter becomes a user turn in the recipient session, so trust both with the task. postbag itself sends nothing off the machine, the vendor sessions forward the letter to their model services like any prompt.
• A name is an address, not authentication, and so is a bag. open refuses inside a session. Both checks read the vendors' session variables: a guardrail against mixed-up roles, not protection against another process.
• Unattended delivery to Claude was observed with bypass permissions on the tested versions, other modes may hold the letter for your approval. Codex needs permission to write the ledger and connect to the Claude socket.
• "Delivered" means submitted through the door, not read. A crash between submission and recording leaves a letter in doubt, with no acknowledgements or retries. Check the recipient before sending again.

postbag is a small bridge for two existing sessions. Tools that do more · Concept · Security · Changelog · Contributing · MIT