Adds POST /api/sessions/:id/input and a CLI command on top of it, so a script (or another agent) can hand an instruction to a running session without attaching to it. The text goes in as one write and the Enter follows 150ms later: agents that run a full-screen prompt coalesce a burst of input into a paste, and a newline inside that burst becomes a line break instead of sending the message. `--no-enter` skips the Enter and leaves the text in the prompt. The session argument takes an ID, a unique ID prefix, or a session name; names win over prefixes. A session whose pane has exited is refused (409) rather than silently accepted — unlike a stray keystroke on an attach socket, a send that goes nowhere is worth hearing about. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>