Letterbox

📌 Built for internal production use. Architecture proven across months of daily AI development. Open-sourced as a reference implementation. I use it almost daily.

If you use AI coding assistants in the terminal, you normally work with one at a time — and getting two of them to collaborate means copy-pasting messages between windows yourself. Letterbox lets two assistants (say, Claude and Gemini, or Gemini and Mistral's Vibe) talk directly to each other and work a task together, hands-free.

The result: one agent can plan while another reviews, or the two can split the work between them — collaborating on their own while you watch, instead of relaying every message by hand.

A small file-based comms protocol that lets two AI agents in separate terminals talk to each other in real time.

Letterbox lets two terminal coding agents — Claude Code, Gemini CLI, Antigravity, or Mistral's Vibe — hold a real-time conversation by passing message files through a shared directory. When one agent speaks, a 📬 notification is injected into the other's terminal and wakes it to read and reply. No network, no server, no shared memory: just JSON files in a folder and the OS's atomic-rename. It's the messaging layer that was built for an internal planning loop, extracted into a standalone, versioned tool in 2026. If you've ever wanted two CLI agents to collaborate on a task without you copy-pasting between windows, this is for you. It gets the occasional update at the author's whim (the launcher tells you when a newer one is out) — but it's currently unsupported small project, not a community one.

The bridge is cross-harness: Claude on one side, Gemini on the other, talking through the same channel, has been verified live. The one wrinkle is setup — Claude wires itself automatically, while Gemini and Antigravity load letterbox from their own settings. The Setup section walks through both.

Why it exists

I work with two AI collaborators every day — Claude and Gemini — each living in whatever terminal harness it runs in (Claude Code, Gemini CLI, Antigravity CLI, and now Mistral's Vibe). Letterbox is how I get them talking to each other instead of through me.

That happens in two modes. Sometimes it's manual: we're brainstorming and I want to loop the other model into the conversation. Sometimes it's automated — in the planning loop, Claude drafts a plan and each plan is routed to Gemini for review as a built-in stage. Letterbox carries both the same way.

It's harness-agnostic by design — Claude Code ↔ Gemini CLI ↔ Antigravity CLI ↔ Vibe in any combination — and same-model pairs work just as well: two Claude tabs, or two Gemini tabs, talking over one channel.

What it is

Each letterbox <harness> launch runs two coordinated processes inside one terminal:

  letterbox claude --channel demo --as alice
        │
        ├─ PTY-Parent  (the foreground letterbox process)
        │    • spawns the harness CLI as a PTY child
        │    • watches the channel directory for peer writes
        │    • injects 📬 notifications into the PTY on arrival
        │
        └─ the harness spawns:
             └─ letterbox mcp  (stdio MCP server, agent-spawned)
                  • send_message / check_messages / acknowledge
                  • check_latest_message / channel_info / list_channels

  Both sides coordinate ONLY through the filesystem:

        ~/.letterbox/channels/demo/
          msg-*.json           ← one file per message
          .read/alice.json     ← per-agent read markers
          .read/bob.json

There is no daemon, no IPC, no background service. The filesystem is the coordination medium — the PTY-Parent's watcher sees a new msg-*.json appear and renders a notification; the channel directory is durable, inspectable, and cat-able. Crash recovery is trivial because nothing valuable lives in memory.

How the agent gets the letterbox tools differs per harness, and it's the one thing you configure once:

• Claude Code takes a launch flag, so letterbox wires it automatically — it generates a temporary MCP config and passes --mcp-config to claude. Nothing for you to set up.
• Gemini CLI and Antigravity don't take that flag; they load MCP servers from their own settings file. You add a one-line, channel-agnostic letterbox entry there once, and the launcher hands each session its channel and identity through environment variables at launch — so you never edit settings per channel.
• Vibe loads MCP servers from ~/.vibe/config.toml. Its MCP subprocess inherits only a trimmed environment, so a one-time bridge script is needed to relay LETTERBOX_CHANNEL / LETTERBOX_SENDER from Vibe's own process env. Once that's in place, any channel works exactly like Gemini. See the Vibe setup section.

Who it's for

• People running terminal coding agents who want autonomous AI↔AI dialogue on one machine, without babysitting copy-paste between windows.
• People who value files as the source of truth — auditable, greppable, no opaque protocol, no magic.

Who it's NOT for

• Anyone wanting a hosted or networked chat service — the message protocol is filesystem-local and never touches the network. (The launcher makes one optional, best-effort version check at startup; disable it with LETTERBOX_NO_UPDATE_CHECK=1.)
• Anyone wanting a multi-user platform — it's a point-to-point bridge between agents on one machine, not a many-user hub (see Built for two).
• Windows-native users — v1 is POSIX-only (see What we don't support).
• Anyone wanting a supported product — letterbox is versioned and gets the occasional update at the author's whim (the launcher tells you when a newer one is out), but there's no roadmap, no SLA, and no commitment to take feature requests or keep maintaining it. Use it as-is; pull a newer version if it helps.

Built for two

Letterbox is a two-way bridge at heart — one peer talking to one peer is what it's designed and tuned for. Three or more agents can share a channel: directed addressing (send_message(to="<label>")) and the participants list make it workable, and same-channel broadcasts reach everyone. But a shared channel is a broadcast bus — every message wakes every participant. Without orchestration (turn-taking, a designated coordinator, or rules about who speaks when), an N-way room becomes a notification storm that can chew through a model's message/usage limits surprisingly fast. If you want three or more, bring your own conductor. The substrate is honest about who's in the room; the etiquette is on you.

Install

Available on PyPI and the MCP Registry.

pip install letterbox

Or from source:

pip install -e .          # or: pip install -e ".[dev]" for the test extras

This puts the letterbox command on your PATH. The command must resolve by name — each agent spawns letterbox mcp itself — so this is load-bearing. Confirm it:

which letterbox           # note this absolute path; Gemini/Antigravity setup needs it

You also need the harness you're launching (claude, gemini, antigravity, or vibe) installed, on your PATH, and logged in. Letterbox launches it for you.

Updating

Letterbox is versioned (letterbox.__version__, the single source of truth) and published to PyPI. On a human-facing launch the CLI makes one best-effort check (at most once a day, cached under ~/.cache/letterbox/) and prints a one-line notice if a newer version exists. To update:

pip install --upgrade letterbox

This is the only network call letterbox ever makes — the messaging protocol stays fully local. It runs with a tight timeout and is fully fail-silent: if it can't reach GitHub it simply prints nothing and never delays your launch. It is never run for letterbox mcp (the agent's stdio server). Disable it entirely with LETTERBOX_NO_UPDATE_CHECK=1.

Setup per harness

You only do this once per harness. Skip the harnesses you won't use.

Claude Code — nothing to do

Letterbox wires Claude automatically: at launch it writes a temporary MCP config (mode 0600) and passes --mcp-config <path> to claude. The letterbox tools appear in that session and nowhere else. There is no settings file to edit.

Gemini CLI — two one-time steps

1. Register the MCP server in ~/.gemini/settings.json (create the file if it doesn't exist). Use the absolute path to your installed letterbox (from which letterbox above), and pass only ["mcp"] — no channel, no identity:

{
  "mcpServers": {
    "letterbox": {
      "command": "/absolute/path/to/letterbox",
      "args": ["mcp"]
    }
  }
}