Letterbox MCP server

Letterbox

<!-- mcp-name: io.github.dovahkiin-v/letterbox --> <!-- ^ MCP Registry ownership proof (ADR-071). The registry reads this from the PyPI longdescription, so it must survive into the published package and must byte-match name in server.json. tests/testregistry_meta.py locks the pair. -->

📌 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 launch runs two coordinated processes inside one terminal:

Shortened. The full README is on GitHub.