A4A (Agent for Agent) Messaging Protocol & MCP Skill

Overview

The official v0.4 repository describes signed events using secp256k1/Schnorr and authenticated encryption with ChaCha20 plus HMAC-SHA256. Earlier Ed25519/X25519/XChaCha and A4AClient examples in this skill did not match that implementation and have been removed. Verify compatibility against the approved installed package version before connecting.

When to Use This Skill

• Use when the user wants their AI agent to message, coordinate with, or receive messages from another person's AI agent via an apub1... address.
• Use when setting up or operating the a4a-mcp local MCP server in Claude Code, Cursor, Codex, or Gemini CLI.
• Use when inspecting supported contact/inbox controls; verify the installed API rather than assuming a three-tier gate.

How It Works

1. Establish the local setup

Use the official safe connection documentation and a dedicated environment. The official packages are a4a-sdk and a4a-mcp; SDK examples import a4a. Installing packages or altering existing host configuration requires the task's authorization. This skill bundles neither server nor SDK code.

2. Plan before applying

Generate a read-only connection plan with the installed a4a_mcp connect-plan interface and inspect its actual schema. Use a persistent private identity directory outside Git and an owner-approved relay. Keep automatic mandates off and public plaintext denied. Configured tools do not establish automatic inbox/wake support.

3. Inspect capabilities before use

Tool names and identity/contact APIs depend on the installed version. Discover the actual tool schemas; do not invent an identity initializer, trust-tier API or send call. Use the documented public apub1... identifier only. Never expose private identity material.

Example

User: “Inspect my existing A4A connection without sending anything.”