What probably matters to youSection of the release
What
The plugin system's internal dispatch table adds an agent.register action, which validates that a prompt is provided and that the description meets length rules, sitting alongside the existing agent.list action. The headless session's $ scripting surface also gains a refreshAgents() method next to the existing refreshCommands().
Separately, the turn-runner gains a takeSystemPromptDelivered option: a one-shot getter that reads and then clears an internal flag tracking whether the system prompt was delivered.
Why
This lets plugins register new agents programmatically and refresh the list of available agents, and lets other code check exactly once whether the system prompt has already been sent for a turn.
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed sinceAnthropic's documentation has since written up agent.register, on Claude Code changelog.* Fixed `$.agent.register` succeeding from a mod's hook that was still running after the mod was reloaded or removed; the call is now refusedchangelogsee the edit
How sure we are
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agreesAnthropic's documentation has since written up agent.register, on Claude Code changelog.
