# Letterbox (io.github.dovahkiin-v/letterbox@1.2.2)

> Source: <https://mcpindex.ai/server/io-github-dovahkiin-v-letterbox>
> Published: 2026-08-24 22:33:12+00:00

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# Letterbox

File-based comms protocol for AI-to-AI dialogue between terminal agents.

The current version, v1.2.2, was published to the official MCP registry on 2026-08-24. It is distributed as letterbox on PyPI, filed under Filesystem by this index, and declares 3 environment variables. Removals and unreachable sources are measured across every server this index tracks, contract drift across the servers that answer in consecutive snapshots; [the index's current counts](/stats) put this record in context.

## Using Letterbox in Claude, Cursor, Gemini CLI, Cline, or Zed?

MCP tool contracts can change remotely with no version bump. The mcpindex gate pins each contract and **HOLDs the call** when it drifts-before your agent acts. Zero credentials. This is not the package install for this server itself (use **Install this server** for that).

Rewrites your MCP host config so each server launches behind the gate. Inspect first: curl -fsSL https://mcpindex.ai/install.sh | less

```
uv tool install mcpindex-gate && mcpindex-config-wire
```

[method](/methodology)

Verdict not yet evaluated for this tool. The semantic screen takes adversarial cases first; coverage rolls out as the corpus expands (15/150 labels to graduation). The deterministic conformance probe is built but has not yet run on the public corpus, so a recorded verdict here is REVIEW or UNVERIFIED, never a clearing ALLOW. Until a verdict is recorded, an agent should treat this tool as not-yet-cleared and fall back to its own checks. Method: [the eval, four-state verdict, honest limits](/methodology).

Own this server? [Screen its description →](/screen)

## That verdict was true at screening time (snapshot 2026-08-24).

Contracts can change after screening, with no version bump. The gate pins Letterbox’s tool contracts on first sight and holds any silent change before your agent acts - the check that keeps being true on Tuesday.

[See your first HOLD in 2 minutes →](/guides/install-the-gate-first-hold)

Related: [how to trust an MCP server](/guides/how-to-trust-an-mcp-server) · [screen before install](/guides/screen-mcp-server-before-install) · [silent contract drift](/guides/mcp-silent-contract-drift)

`LETTERBOX_CHANNEL`

Channel name to open. Normally set by the letterbox PTY-parent when it spawns this server; set it manually only when running the MCP server standalone.

`LETTERBOX_SENDER`

This endpoint's identity on the channel — the name the peer agent sees on every message.

`LETTERBOX_INSTANCE_ID`

Per-launch instance id, the watcher/own-write join key that stops an agent seeing its own messages as incoming. Must be unique per launch.

[methodology](/methodology)

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