Claude Code inter-session messaging ships - agent-to-agent coordination gets its first native primitive

Anthropic has quietly added cross-session messaging to Claude Code: running sessions can now pass messages directly to other active sessions, no external broker required.

In plain terms: Anthropic has shipped inter-session messaging for Claude Code - documented at code.claude.com with no standalone announcement. A test-runner session can now directly notify a deployment-checker session. No polling shared files, no external message queue, no human relay.

This fills a gap that anyone running parallel Claude Code agents has worked around. Until now, multi-agent coordination in Claude Code required external glue: shared state files, a Redis queue, or a webhook endpoint. The new API provides a first-class channel between sessions with each session identifiable and addressable.

[Under the hood] The protocol sits inside Claude Code's existing session infrastructure, inheriting its auth and context boundaries. Message delivery is asynchronous and tied to session lifecycle - the docs don't expose the transport layer, but the session-scoped design implies that a terminated session loses its pending messages.

So what: The coordination primitive completes the basic toolkit for multi-agent Claude Code workflows: you now have persistent memory (via MCP), tool-calling across contexts, and agent-to-agent messaging in a single environment. The practical win is eliminating the external scaffolding tax - teams running parallel agents can wire coordination without leaving the tool or managing a separate message broker.