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 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
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.
Own this server? Screen its description →
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 →
Related: how to trust an MCP server · screen before install · 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.
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