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. postbag submits each letter through its vendor's native door, records it in one ledger, and counts it 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. The CLI uses only the standard library. 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_THREAD_ID (older versions use 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.

MCP tools

The optional MCP interface lets agents call postbag_join, postbag_send, postbag_read, and postbag_bags directly. Sender identity comes from the host. The ledger, native delivery, and shared budget are the same as the CLI. Only the human can open or replenish an exchange.

Install the optional tools with:

pipx install 'postbag[mcp]'

Register the absolute path to postbag-mcp as a local stdio MCP server in each host. Setup, upgrades, and compatibility. The MCP SDK is required only for this interface.

Native delivery has been checked on macOS with Claude Code 2.1.285 and Codex 0.157.1 / desktop 0.158.0-alpha.2.1, including the installed 1.3.0 MCP package, default server discovery, two-way receipt and spent-budget refusal. Linux passes CI, but 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."
   

   If accepted, bob receives: "Letter 1 of 6 from @ada to @bob via postbag (exchange 1, bag default)", how many letters are left, the body, and the reply instructions: use postbag_send if MCP tools are available, or postbag --bag default send @ada - with the reply on stdin. 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. join, send and read refuse a missing one. Outputs identify their bags, 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. Without --bag, POSTBAG_LEDGER selects a ledger by path.

Run postbag bags for a count of paths found, remaining budgets, last letter times and registered names with vendors. It lists default, named and selected custom bags. Unselected external paths are omitted. Busy or unreadable bags are unavailable. Budgets do not expire. Registered names do not imply live sessions. Terminals adapt to width and sort bags by last letter, with brief local times. Pipes keep the plain table and full ISO timestamps.

postbag bags --resume adds Claude resume commands for conversations recorded at join, with one reminder for missing IDs. Rejoin after /clear or switching conversations. Resume needs saved history and opens a new process, not the old terminal.

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 delivery daemon, polling, hooks, or bag index. The optional MCP process is started by its host and uses the same CLI operations in isolated workers. CONCEPT.md is the whole specification in a page.

Tests

For a full development test run, install both test dependencies and the MCP extra:

python -m pip install -e '.[dev,mcp]'
python -m pytest -q

Without the mcp extra, the wire tests are skipped. Tests use private fixtures and fake native doors. See Contributing.

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 and bags hide door credentials. Keep raw files out of git and logs.
• An accepted letter is a user turn. Trust both sessions with the task. postbag itself sends nothing off the machine. 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.
• Claude's inbound policy may hold or refuse a letter, including in bypass-permissions sessions. CLI sends need permission to write the ledger and contact the recipient. MCP tools run with the server process permissions, outside the command sandbox; use host tool approvals for per-letter consent.
• "Delivered" means submitted through the door, not accepted or read. postbag does not wait for delivery notices or retry. A crash before recording leaves a submitted letter in doubt. Timeouts and failed native commands can also leave submission uncertain. Check both the bag and the recipient before sending again; an absent ledger record is not proof of failed delivery.

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