Cross-session messaging enables Claude Code to deliver messages between independent sessions. When one session makes a change that breaks another's work, Claude can warn the affected session proactively. Similarly, if one session resolves a question blocking another, it can send the answer directly. A message is purely text written by one Claude to another, distinct from conversation history or files. To transfer entire conversations or context, users should resume the session instead.

Claude uses two tools automatically: List Agents discovers reachable agents, and Send Message delivers messages by name. This works for subagents, team teammates, and independent sessions. Common use cases include handing over findings, coordinating parallel worktrees, getting status updates from long-running tasks, and replying across machines.

Delivery depends on the receiving session's settings, resulting in three outcomes: Delivered, Held (requires approval), or Refused. Messages count toward usage like typed prompts. Permission boundaries remain per-session; sessions cannot approve actions blocked in their own context.

To see reachable sessions, run the /list-agents command. Sessions are named via /rename or --name flag. Same-machine delivery works when sessions share filesystem access. Container sessions cannot reach host sessions, but sessions within the same container can message each other. For cross-machine communication, sessions must be connected via Remote Control.