If you run multiple Claude Code sessions at once, you know the tax: something breaks in one terminal, and you have to walk over to the other, re-explain the whole situation, and get it back on track. Cross-session messaging, shipping in Claude Code v2.1.224, removes that step entirely. Sessions can now find each other, send summaries, and coordinate without you acting as the middleman.

The problem it kills

Running parallel sessions was already possible, but when working on a codebase with multiple Claude Code sessions running simultaneously, each session was fully isolated with no ability to share context, findings, or coordinate work, leading to duplicated exploration, conflicting changes, and lost context when switching between sessions. The workaround was manual: copy-paste findings between terminals, re-explain decisions, and hope nothing got lost. The human became a message router between two AI agents that could coordinate directly.

What actually ships

Cross-session messaging lets Claude deliver a message from one of your Claude Code sessions to another. The key detail: a message is a piece of text one Claude writes to another, never conversation history or files. It is a targeted summary, not a context dump. Claude uses two new tools under the hood:

• ListAgents -- discovers which sessions are reachable and what they are named
• SendMessage -- delivers a plain-text message to a named session